As others have pointed out. It is important to remember that if you float your scope and then connect probe ground to a hot connection the entire case and terminals of the scope and anything else connected to it are now hot. You only need to touch any of those points and ground and you are fried. That is why you normally never float the scope but rather the DUT.
An alternative to be extra safe is to float both the device and the oscilloscope (using separate transformers!). Still you have to be mindful that your ground lead can energize your oscilloscope w.r.t. the device under test, so don't handle both at the same time in that case.
Well yeah that's pretty obvious from even basic electrical theory. It's more about parasitic components of circuits that actually become an issue sometimes!! But yes I do generally aggree with you of course!!
This was one of the shortest but absolute best videos I've seen of yours. Safety is number one, two, and three. Without proper safety knowledge, us techs can not do our job. Would love to see more videos like this. -Mr. Carlson's biggest fan.
@@MrDoneboy This particular design of isolation transformer only isolates the secondary from mains. There is a big difference between a bench test isolation transformer and this. All these video prove is there's a whole lot of people who have no business playing with electricity doing just that.
I have had Earth ground running through my isolation transformer for decades and never have had a problem. I never plug my test equipment into my isolation transformer only DUT. Many devices have components referenced chassis ground. Running the Earth ground all the way through to the DUT helps to eliminate noise while testing. That said, Mr Carlson is absolutely correct in his presentation and I am fully aware of how my bench is configured. Thanks Mr Carlson
Thank you Mr. Carlson. If I ever get an isolation transformer like yours I'll be sure to open it up and cut the ground connection to the secondary side.
Just a beginner here, but I thought the correct way was to hook up your oscilloscope normally, and to the ground and use an isolation transformer (with no ground) on the device you are testing.
Thank you so much for the info about these isolation transformers. I did not know they would connect the separate grounds. I can not believe it is called an isolation transformer. You may have saved some lives with this video. I honestly would assume an isolation transformer is ISOLATED.
A very interesting video Mr Carlson, it made me look up how houses are wired here in the UK. Once upon a time you either had a ground rod (some rural areas still do) or you had the house ground bonded to the sheath of the mains supply which went back to the substation before grounding out. The single phase domestic supply was one of the three phases of the national grid to live and the ground at the transformer for neutral so the earth and neutral were connected at that point. New builds and I think anything that is re-wired now has a system just like you describe with earth connected to neutral in the company fuse box at the distribution board. The other thought that comes to mind is that the earth connection on the secondary side of an isolation transformer seems to be absolutely pointless as it doesn't offer the safety that it appears to. The earth connection in an appliance is to provide a route to earth should anything fail internally and ground out the live causing the fuse to blow somewhere - either in the appliance, in the wall plug or at the distribution board. With more sophisticated systems with earth leakage or residual current detection that would trip out first. None of those things will happen if the live and neutral are isolated. But on the other hand you are unlikely to suffer much harm if you touch both the live and earth. When mine arrives I must test what potentials are there.
Thanks for that great tip Paul. It is always nice to be reminded of safety when it comes to electronics. You did a great job of explaining this.Have a safe and happy holiday and we will see you next year.
The isolation transformer I inherited from my uncle did not have a grounded outlet. As I recall, it had a 2 prong, non-polarized socket. Not sure when it was built, but I would guess in the 1950s, as most of the equipment I got from him seems to be from that era. Thanks for the great video!
For starters you would normally use the isolation transformer to isolate the device under repair, not the scope.My isolation transformer the test plug is only connected to the secondary, the ground prong is completely open on the transformer.Mine is a Hammond manufacturing co, made in Canada.
I have a Hammond model 171B isolation transformer. The ground prong of the output receptacle is definitely connected to the input cord ground and the transformer frame on mine (I just checked). In addition, Hammond's own data sheets for the product show the output ground terminal connected: www.hammfg.com/electronics/transformers/line/171 (My unit is an older model with no circuit breaker, and with an outlet instead of a pigtail for the output).
I just got a topaz isolation transformer. Heavy! I'm considering installing this in my van conversion as I am running audio gear. This video is eye opening. Thanks!
I made this exact mistake 2 days ago on my oscope. After the bright flash I started thinking about it and then realized -of course the ground clip is grounded - duh. Then I started looking into prices of variacs that have an isolated transformer and then found this video. BTW, my Oscope does not measure correctly anymore - both the voltages and wave forms are incorrect now. i have not yet tried to replace the probe so hopefully that will be the only issue. Just wanted to say I made this exact mistake and your video helped me understand and verify why. Totally makes sense now. I did not realized that the barrel was connected to earth ground and was treating it like a multimeter. Thanks for the Video!
Possible that the board foil to the input connector burnt and possibly you can fix that and with a little luck there will be no other damage. Good luck and God Bless!
I think this is a very under rated topic . I don't believe it has been addressed fully here which is understandable because this video highlights the difference between filter conditioning transformers and isolation transformer. We need to highlight links or create videos on practical techniques to isolate techs and equipment . More discussion is needed to increase safety when working on mains . There seems to be two topics a) human safety techniques while working on electrical equip and b) safely using test gear on high voltage without destroying equipment . Equipment requiring mentioning : isolation clothing , isolation transformers , differential probes.
Just a couple of comments: 1. You mentioned powering an oscilloscope from an Isolation Transformer (although, admittedly, you didn't actually suggest doing that). That should almost NEVER be done. (The only exception might be if you are actually repairing an oscilloscope, where the scope is the Device Under Test (DUT)) Only the DUT should be connected to an isolation transformer. 2. A standard Isolation Transformer can be converted to a "technician's isolation transformer" by internally disconnecting the transformer's secondary neutral from ground. This maintains the ground path all the way to the DUT (assuming that the DUT has a ground prong). It is much safer than using the cheater plug while also isolating the DUT from the mains. However, even with a "tech" Isolation Transformer, you still need to be careful not to do things like connect the secondary (output) to ground.
I won't name any names but there are 2 professional techs here on you tube that use 2 isolation transformers because it's safer. they plug both their scope and tv or whatever into an isolation transformer. They both work as professional techs servicing consumer electronics.
I fixed TV sets for 40+ years but never knew about this! Thanks for sharing this information. I always used the Sencore isolation variac with no problems but now in my home workshop I have a B&K TR-110 isolation transformer and it never occurred to me that I may be not be isolated like I assumed I was. I'll have to check this out. You may have just saved my scope.
Hi Mr. Carlson, thank you very much for this informative video! 1. Is it safe to disconnect the oscilloscope's earth wire at its plug, so that it only has a live an neutral connection? 2. Is it safe to disconnect the earth wire running from the ac mains supply to the isolation transformer's output plug, so that there is only a live and neutral connection available? 3. When do you need to use an isolation transformer? 4. Can you please do a video on how to connect an isolation transformer to a DUT, like a TV, amplifier etc. and also to instruments like oscilloscopes, signal generators etc. so that all of them can be used safely together to probe and repair a product. Thank you very much for your channel. I have learnt so much from your videos!
Balanced isolation transformers, which are sometimes used for audiophile and home theater systems, usually have a center-tap on the secondary which is sometimes connected to the ground connection on the output and chassis. Thus the output is not always isolated.
I've seen many comments here about only the DUT being isolated from ground, however, when I was a student learning troubleshooting and repair techniques, the instructors at the vocational school cautioned us to never have the test equipment grounded. They sent us out to the rest of the world having been taught the very same thing Mr. Carlson showed in this video. Every piece of test equipment in that shop had all of the ground pins on the power cords removed. Every one of them. Meanwhile in the electronics theory class down the hall, they blew up a brand new 540 series Tektronix scope because of said grounding issue! I don't remember anything else about the incident so I don't know what they were measuring, how it was hooked up or what- just that they mentioned it to one of our instructors and then we were told about that precaution. After that the electronics instructors were removing the ground pins from the power cords or using the adaptors as shown here. Anyone care to explain why it is bad to float your test equipment? Not looking of arguments or heated discussion, just good explanations.
I was taking an analog electronics class at the local community college back in the 90's. My lab partner was in the air force and she said usually stationed in an AWACS. I was taking notes and she was hooking up the circuit. so she had the power supply on and somewhere in the circuit she said lets see positive goes to negative and before i could say no!, she connected the connection. there was a bang and smoke came out the back of the power supply. The instructor walked over expressionless. Picked up the power supply, unlocked a closet. He exchanged the blown power supply for a new one, locked the closet and w/o a word put the new power supply down on the bench in front of us.
I knew about the problem with the ground, nice to know the reason why to. Good explanation. That said your demonstration with the bulb and the probe hovering so closely to exposed mains with your bare hands, made me shiver and that in a video about safety........ :)
+Bart Zuidgeest LOL, When I watched the video I chuckled, my hands look way closer to those open clips than they actually were. Ahh....the magic of video. Thanks for your comment Bart!
Excellent video! Too many folks don't understand this issue. It was beat into my head by an instructor, who used the example of a grounded scope and testing a component in an energized circuit, for example either to measure a voltage or to see a signal. If an inexperienced tech attempts to put the scope probes directly across a component in an energized circuit, the negative probe will short out all of the circuit "below" the component being tested. Bad things can happen! Thanks again for a great explanation.
Another informative video that reminds us to be safe! I had to go back to the schematic of my Heathkit IP-5220 Variable Isolated AC Supply to remind myself that it's configured just as you describe; always good to remember! Merry Christmas and Happy New Year Paul. 73 - Dino KL0S
Hi. George from Bulgaria here. Just wanted to show my appreciation. Your channel is THE MOST enjoyable content, regarding electronics and stuff on TH-cam. So calm and peaceful and in the meantime always super sharp and right on the topic. Not to mention your outstanding knowledge. Not everybody can achieve this. Respect. And, of course, im a subscriber right from the very first video. :)
Thank you Paul for bringing to our attention the need to consider the nature of the supply coming to,our "Isolation" transformer, and the way that itself is connected internally.You certainly made me reappraise my modelling activities wherein I replicate different types of supply and their earthing arrangements -Tierra-Tierra TT, Tierra y Neutro Separado TNS , Tierra y Neutro Combinado [Separado] TNC[S].I work mainly from a step-down transformer (240 V 50 Hz. Input to 110 V centre tapped output). This type of transformer is commonly empoyed on building sites to afford a measure of safety and are often referred to as isolation transformers when in fact, because of the earthed centre tap of the secondary winding are in fact in metallic connection with a current carrying conductor. The term Protective Extra Low Voltage is more appropriate, as for it to be a Safety Extra Low Voltage transformer, as according to the On Site Guide to BS 7671 Section 3.4.3,/ 414.3 " ... have no live part connected to earth or protective conductor of another system."This clearly means a double wound transformer with no part of the secondary circuit connected to earth.A common example found here in the UK would be the shaver socket outlet supplies for deployment in bath/shower rooms.Saludos.
You have so much experience and knowledge in this field, and I trust you, so had to ask you about my next project: I'm thinking to make an a adjustable AC bench power supply. My concern is to have the AC stable, when something using it draws more than 1A (or more) of current. When I needed for my project AC, I just used 2 resistors, wired like voltage divider and also used 2 potentiometers - wire-wound style. It did the trick, but since I'm gonna play with more AC projects, I would like to make a nicer power supply that is stable. I do not need anything more than 10A (that would be at the most I can imagine for now). I have several transformers 120V to 30V, 16V, 12V outputs. One has both 12V and 30V in one. They came from a large UPS (Uninterruptible Power Supplies from 1200W and 1500W respectively). I'm not very experienced in electronics, just a new hobby after a 50+ years of other hobbies. I'm trying to rekindle something that started when I was a teenager. I do have 100Mgz Oscilloscope, 50Mgz Function Generator, several meters, 3D printer and lots of parts I salvaged from printers, UPSes ... I love to recycle and fix everything. So I'm not looking to make something that would require lots of knowledge that I lack of right now. I do play with Arduino's and simple projects..... I have several pictures of the transformers with their parts numbers and I know what voltage they produce. I could not find much about them on the Internet, not even how to connect the mains, but I figured it out - I think. What I like is a simple schema how to build the adjustable power supply and some protection for it; also what should I do to stabilize the output voltage.... I have these inexpensive meters from Harbor Freight that I can use to measure the output, so I don't have to even have a circuit for that. But if it has that, and it is simple and few parts, I am open to the idea also. You, or anyone else reading this, can contact me on my Gmail: czecht@gmail.com (Yes, I was born in Czech Republic), but I live here in the best country in the world! since 1981. Thank you Mr Carlson and thanks everyone else who would like to help me out!
Great Saftey Video. Comment on Isolation Transformers, the older ones came with a non-grounded plug and outlet. One did not have to worry about grounding saftey issue. Keep em comming, Greg
Thanks for this great video. I worked in the TV repair field in the 70's and then eventually went into the electronics field in the 80's to early 90's but I can't remember what we did back then to prevent this issue with scopes. Did scopes change in recent years regarding grounding? I used primarily Tektronix scopes. Maybe I just automatically used a ground adapter and never put much thought to it. Thanks for all you do to educate us.
An excellent explanation of something I was aware of (35 + years in semiconductor testing) but as you mentioned newer and less experienced techs may miss. I also want to commend you about the danger of just randomly defeating grounds as well. They serve a critical safety issue AND also a key point in shielding extraneous noise and signals in sensitive measurements. I'll share a story about a time in trying to measure picoamp level input bias currents on an op-amp and having massive 60Hz noise problems. I discovered(after a lot of time I need to add) the problem was on our very expensive computerized tester. A maintenance technician was tired of disconnecting and then reconnecting an elaborate bonding wire system to make sure all metal parts of the chassis were grounded and shielded every time service was required. So he decided not to reconnect the bonding wires..leading to large parts of the chassis acting like antenna and radiating 60Hz into sensitive measurement modules. Again enjoy your channel a great deal. I'm not too proud to say that even after years of experience in electronics, there is still plenty too learn. I'm now retired and starting to get into restoring some old tube powered radios, a new universe for me!
+GrandsonofKong Great to hear! Thanks for the story, and glad your enjoying the channel. Sounds like the tech was a little lazy. That sort of thing causes future techs a lot of issues. Merry Christmas!
Thank-you for another great lesson. I have a concern regarding your presentation of the autotransformer variac. Autotransformer by definition has a primary and a secondary except that the primary and secondary are connected together in an electrically common manner. That is what prevents isolation. Many autotransformer variacs allow for a secondary voltage above the line voltage 140 to 150 volts. That being accomplished by additional turns ratio on the secondary. Regarding the illustration you showed I believe I may have some 40 volt variable transformers like that. I haven't given much thought to this until your video. I'll have to inspect them again.
Paul, I want to first thank you, for all of your informative videos. Thank you, so much , for explaining about modern day ISOs. I own a VIZ isotap model WP-26A 400va. This was a gift, from an older tech. I seem to be unable to find any literature or manual, for the ISO. If by chance, you or your subscribers have any information, I would be rather grateful. If you have one of these older ISOs, maybe you could do a tutorial... Thank you, so very much and Happy New Years.. Cheers!
I was tought to always use TWO isolation transformers with no ground connection: One for load and the other for measuring device. Floating load can be measured with earthed scope but when ground lead is connected it becomes mains referenced. If you touch the device, you get shocked because it is no longer isolated from mains earth. Having also the scope floated via a second isolation transformer (with no earth), you avoid the problem. With that setup it is safe to touch any single point of a powered device. That is also safer than just isolating the scope from ground because in case anything leaks from "live" to scope chassis, it is isolated "live" ie. will not shock you. Never ever float only the scope and connect it to a mains connected load. Earth lead connected to eg. live wire makes scope chassis live which is very dangerous. As a kid I also made the classic mistake of connecting scope earth to live wire with all the sound and light effects assosiated. :) Never repeated that mistake.
Hi I have just come across this video I found it interesting as I have just this week been setting up my work bench with isolation transformer . You did a very good demonstration of what the problem is . But I am somewhat concerned about your proposed solution . What seems to be being disregarded is while you make your measurement if you have removed the earth wire from the scope power plug and you have connected the ground probe clip to the hot side At this point the steel case of your scope has become live in relation to anything else that is in your area that is connected to ground if you touched this case while adjusting a knob you would likely get a shock . If you had a data lead from your scope to a pc then you would then be putting high voltage down that lead on the ground into other equipment which you may damage . In the event that you are using as a demonstration I would use a DVM to measure the voltage between the conductor and the ground clip and ensure that it is a very low value possibly a volt or lower before making the connection with the ground clip I would not need to remove the earth from the scope . If one gets used to doing these basic checks before making a connection then it can save a lot of damaged equipment . I am in the UK so all out power is 240 / ref to earth regards bill
Thanks, I now have a better understanding. But it seems like you didn't finish the topic on Isolation Transformers. It's good to be aware that the secondary might be linked to Earth. But then, how should we setup a proper isolation system? Should we break the secondary connection? I want to be able to use my scope to test the long list of devices I have.
Francois, The best way to ensure total isolation is to remove any ground connection at the output of your IT. Some manufacturers ground the metal container that boxes the IT as a matter of safety. Depending on the type of outlet socket this can be problematic as the mounting screws can become grounded and defeat the purpose. For example, in the UK and Ireland, all socket mountings are connected to the earth pin and when mounting them as I had to do , I literally had to cut the copper connection to isolate the output otherwise the mounting screws would have made the output grounded. These are small things that need to be considered whilst making sure your IT is suitable for bench work.
My understanding was that you should pretty much never float your scope, either through an isolation transformer, or especially from ground. Keep the scope plugged into the wall, and pit the device under test on the isolation transformer.
PelDaddy that was my first thought, put the scope i the wall, and put the device under test into the iso.... if you have an older piece of equipment (like 98% or so of stuff on this channel) it probably only has a 2 prong plug anyway.... so the iso-ground issue isn't an issue....
Teb Tengri the reason is that with a floating earth scope the BNC connectors are still attached together and to the chassis of the scope. So if you would hook mains voltage to the ground clip of a probe, all the other probes/BNCs and the case would be live. Since you normally treat the ground clips etc. as safe to touch, and often clip it straight onto the nearest ground of a DUT, you make it very easy for yourself to mess up and ruin your day. If you want the convenience of not having to worry where you stick your probes, there is no substitute for a proper isolated probe.
Modern isolation transformers often have the neutral and ground tied together at the outlet on the secondary side of the transformer. It is a clip that can usually be removed.
Mr Carlson's Lab Hello and thanks for your videos! I am new to repairing electronics can you please explain why it's so important to keep the ground for safety. I keep hearing this but don't understand why it makes it more safe? Why is it more safe for the BNC connectors to be grounded or the chassis of the isolation transformer to be grounded? I am sorry for asking such a noob question I just don't see anyone explaining this... Thank you
The earth connection provides an alternative path for the power should a fault arise. Since it is always connected it will protect even with the appliance turned off or more importantly beleived to be turned off. The permanent connection also continues to work if the active and neutral lines become reversed as can happen when untrained people wire up plugs. A well made appliance will often switch both active and neutral lines to also protect against reversed plugs. Many modern products do not have metal exterior chassis and will employ what is called double insulation where the usually plastic chassis acts as a second insulating medium. Virtually all power tools are made this way these days and will not require an earth connection.
I'm only new to this stuff and this is something that scares me because (although I've not had to use one yet) I know it's going to bite me in the ass some day... God I'm glad I watched this video I was going to go to bed but desided to watch one last video. Thank you Mr Carlson.🙏👏
Great video. I have a variac and isolation transformer (no ground pin on the secondary of the isolation transformer) but most of the time I use my Sencore PR-57.
Excellent video, I have learned tons from you over time. A long time listener, first time caller so to speak. I'm kinda scared to hook my expensive o'scope up to mains now. Anyhow, my question is such, I use a variac and just plug it in, but is it polarity conscious? Meaning, the hot lead always goes thru the carbon wiper brush you talked about. Is it essential that the hot lead do this? Reason I asked, mine is a cheapo from "over there", and has no ground pin, and both leads are the same, no big spade lug to polarize it to neutral. Can be plugged in either way. Should I make sure that the hot line is going to the wiper, and the neutral is being passed thru. Or does it all really matter? Also, whats the best way to read my 120vac input line power in my house? Use a 2 prong old style plug adapter as you do? Can I put the 100w bulb in series with the o-scope leads so the magic smoke stays in if there is a problem, what do you think about that idea? As always, love your channel. see you on the next on.
I made my own box with an isolation transformer, some bulbs which can be individually engaged and disengaged and a volt/current meter. However, I have a question. The upside of the isolation transformer is that if you touch the live coming out of it, you don't get electrocuted. However if you touch both live and neutral, you do and the RCD won't cut power as it's located before the transformer. At least this is my understanding. Do we feel that the main advantage of having an isolation transformer is worth giving up the RCD protection? I am honestly asking, not questioning.
As a note, in the UK our supply typically includes a separate earth that is handled way back at the generator, so earth and neutral have no connection within our premises.
Gary Richardson : Attaching one of the two wires of the mains generating alternator physically to the earth ensures that this wire is at zero potential (relative to the earth, which you are also touching) and thus safe to touch by you. You could not touch the other wire from the alternator or your body would complete the circuit to the earth, with risk of electrocution. If neither of the two wires from the alternator were attached to the earth, there would in effect be no continuously neutral or live wire, as both of the wires would take it in turns to be either live or neutral (for a duration which is relative to the alternation frequency of the alternator). It is safe to touch both the neutral wire and the frame of any metal cased and earthed machine (supply alternator also connected to the earth), simply because it is at the same potential as you, relative to the (elected) live wire, so if the live wire inside the machine should break and then touch the metal frame of the machine, the frame of the machine would then conduct the live current to the earth, rendering the metal frame essentially harmless to the touch. This is due to the very large voltage drop that the earth imparts to the circuit and hence machine case. It must be said that this condition would normally result in a fuse blowing due to the inevitably large current flow now taking place, assuming the earth wire is substantially large enough to carry the now elevated current flow.
Me Carlson, I’ve seen this warning/explanation many times but what I haven’t seen is a proper fix for the problem. I can disconnect the “earth” lead on the output side of my isolation transformer but I don’t think that’s the proper fix. What is?
Thanks for the reminder! Safety is #1. As a 45+ year HVAC/R service tech, I’ve been “poked” by 24v, 120v, 575v, and even lightning (100’ underground behind Niagara Falls)-and I wouldn’t wish the experience on my worst enemy. Is the methodology of using “balanced input” (where you use 2 probes, with their ground leads clipped together and not connected to anything else, and their signal probes connected to the AC line) a safe alternative?
Thank you so much for this and for all your videos. I understand that, when you restore old test gears, you usually prefer to avoid connecting a cable with ground pin. I understand the concern expressed in this video, but I wonder if there are other situations in which the ground connection could be counterproductive (or very dangerous). Thank you for your attention.
Hi Paul, wow I don't understand why that transformer is officially out in the market as certified isolation transformer. In Europe this would never ever get the declaration of confirmatory to be sold as isolation transformer. Following VDE/ EN norm the secondary side never is attached to protective ground/ earth. But anyway what ever norms are telling at all it doesn't make any sense...., or is there is something so much substantially different in Canada''s power distribution compared to Europa I don't know?? Anyway very interesting video ! Thank you. 73 Peter
+TRXBench There seems to be a lot of confusion about this even in europe. Mostly an "isolation transformer" has a earth connection on the secondary and a "safety transformer" has not, but has this clearly marked with a warning next to the outlet (and warns you not to hook up more than one device). But I have seen a "safety isolation transformer" (made by peaktech) that has a earth connection on the secondary, completely defeating its purpose (even false advertising imho). But I'd be very interested, if there are codes in Canada, that prohibit safety transformers without mains earth reference. After all these things are only for full galvanic separation (thus preventing any current flow out of the DUT into any earth around) during repairs under power. Nobody in their right mind would use them for anything else. But a well made and very important video nontheless. Thank you for it.
+Róbert Valdimarsson Hi Robert, yes very important issue and you are right a lot of confusion all over. I'm talking only about a galvanic protective separation transformer (Schutztrennung) according VDE 0570 Part 2-4, DIN EN 61558-2-4, EN 61558-2-4, IEC 61558-2-4
+TRXBench I agree fully, but politics and bureaucracy gets in the way of sense once again. If they want to include that "Earth" they need to re-label the transformer, and remove the word "Isolation" Merry Christmas to you and yours Peter!
+TRXBench Yes, you pretty much have to rely on the norm numbers here. I have a propper one here for up to 1500VA. I got it very cheaply second hand and quickly found out why: It does not have a inrush current limiter. So every time i switch it on, I have to make a trip to the switch box and reset the breaker :D But a limiter module is not too expensive. Still the best money I have spent so far, since it might very well save my life some day. Mery christmas and a happy new year to all of you too.
Great video. I work on a lot of tube guitar amps and I use a variac and isolation transformer. Do you recommend plugging the unit/amp under test AND the O-scope into the isolated (and earth/ground lifted) power source or only the unit / amp under test? I realize this could be a tricky question because when the scope is earth grounded and the probe clip is on the chassis, the chassis is once again grounded to earth as if the test unit was earthed at the receptacle, even though the mains AC voltage is "isolated." Thanks for any response you can offer. I'm in Mission and I refurbished a cabinet for a vintage amp you later did a video on the electronics rebuild.
Good video! Even some test equipment containing a built-in isolation transformer and variac has the ground pin in the receptacles bonded to the ground in the line cord. My VectorViz ISO-V-AC II WP-30 is set up this way. The other option besides using a 3 to 2 prong adapter is to use a high voltage differential probe such as the Caltest CT2593-1. I use one of these when working on offline switching power supplies. The chassis will still be hot, and you will need to be careful, but you won't blow your scope or probes up as the differential probe is good to 1000V common mode.
Another great video, I am very interested in these devices but currently cannot afford. However, I built a "Dim Bulb Tester" with an array of different bulbs for their wattages. Recently, I purchased a Peavey "Flight Case Mx", VTX Series Amp Head" 130 watts. The pre amp is solid state, into (4) GL6'S. I bought it from the local pawn shop for $120 I did play it at the shop before I did the deal. But because I did buy it from a pawn shop, I am suspicious of why the amp was there. It isn't hot, been on their shelf for two years. So did somebody pawn because it was breaking g down? or did they need the money? So I have the amp now, I have opened on my bench. I am curious if there is a fail while testing. I know this can indicate a short, My question is about resistors going out of spec, will that also cause the dim bulb tester to fail too? If you don't mind, I have another question. I have read that some guitar player hook up a variac to their amps. I am guessing they might be doing this to play their amp at a lower volume. But still getting the crunch. My curiosity is: if I found the right bulb if I could duplicate this effect with the dim bulb tester?
How good the connection is between earth and neutral depends on the earthing system used. In my country there is three systems in use, those are IT, TT and TN-C-S. Thank you for an informative video. I just restored the grounding on my second hand Tektronix 475A, the previous owner had just broken of the grounding prong.
I suggest you check whether your isolation transformer and\or variac (my "variac" is isolating) have fuses. One of mine did not have a fuse, so I had to install one. I blew it often since. The other even had two fuses, one on each primary lead, but they were 3.15A good to keep the office powered but too much to limit the damage to the DUT.
My neutral rail is earthed with a screw into the concrete wall in my fusebox. When my house was built only the cooker and the fridge were even attatched to that earth and the wall sockets had no earth panel in them.
If you remove the ground from the isolation transformer on the secondary side, can you plug the variac into the IT and use it and still maintain isolation? This would be useful with a 240v IT when needing 117v for USA guitar amps like a champ etc.
Paul, thanks for yet another informative and well-presented video. Maybe you can help me, either with my thinking or with my technique. I recently came into two identical transformers scrounged from backup UPSs and experimented with wiring the two 24VAC secondaries back-to-back, thus creating, at least in my mind, a 1:1 transformer. I grounded the receptacle ground lug and the metal cabinet to the line cord ground conductor. So far, so good: 120VAC input, 118VAC output at the receptacle. In my mind I now have "floating" 120VAC at the receptacle and continuous earth ground back to the breaker panel, but the voltmeter says otherwise. When I measure from the "hot" receptacle lug I read 41VAC to ground, and 19VAC from the "neutral" lug to ground. I don't quite understand how a supposedly "isolated" transformer secondary is referencing to ground. All this is predicated on my assumption that by wiring the two secondaries back-to-back, I created a 1:1 transformer. Here's hoping my explanation is clear enough and you or your readers can shed some light. Thanks again.
So far loving the Tech tips Tuesdays. The Tektronix manuals discuss this now in the warnings. I had a run in once with thinking I was isolated and being told I was isolated but actually wasn't due to this same condition. My experiment didn't match your nice light bulb example. No, I had the blue flash once I got the alligator near the DUT. Lucky for me I wasn't hurt and I was holding the rubber shell. I now double check with a voltmeter first for any potential difference. The probe and scope were saved by a semiconductor going into escape velocity.
Thanks for a great video on this topic and addressing the safety ramifications off making mistakes in this area. This did breed one question. I have a gfci that's feeding power to my bench. I believe it would not work if a fault is generated on the secondary of the isolation transformer. Am I correct?
I have that exact same model of variac. I also own a device called a one-AC line viewer, which is a transformer you plug in the wall and has isolated BNC outputs to hook to your scope to view Hot, Neutral, and Ground. More recently I purchased a fluke scope meter, model 99B I believe, but mostly because I needed a portable and battery operated scope but the isolation is icing on the cake as I work on high power broadcast transmitters. Anyway thanks for the informative videos, nothing here I didn't already know but many others don't and it could save a life, or a scope at least...
i think im gonna always use a battery operated scope even if its wimpy. you can get some good longevity out of a stack of 18650 batteries and its clean too :)
That looks like a medical grade isolation transformer Paul. That is typical wiring for these I believe. I use a Sencore PR-57 isolation/variac for my bench work. I bought it on ebay years ago and had to open it up to repair the switch bank for the different display modes. I honestly didn't even look to see how the thing was electrically wired for the output. I think I'll do that now. But I don't see why Sencore would wire their unit as your example is. But who knows... Thanks for the heads up and Merry Christmas to you and your family! Love your videos. Tom
This video should be viewed in electronics schools today. I knew of this problem 20 years ago and didn't know how to get around without floating the scope. I think the should be followed up with a GFCI video.
This a great subject and I can relate to it. I was working on a project in the military and was using a variac and a scope. I blow up the ground and I should have know better.
Hi, Mr. Carlson. I am a fan of your videos and you are doing a great job with sharing your experiences. By the way, I am really surprised about the connection of the earth and the neutral at the breaker panel. In Turkey, it is strictly forbidden to connect these two lines. Instead of this method, every apartment or house have their ground line connected to the soil via a copper plate covered with galvanize.
you forget that the secondery side of the transformer delievering the Voltage to your appartement is most likely grounded so in your case the connection would be directly at the transformer and not in your breaker panel i don't know much about Elektronic Installations in Turkey but not grounding the secondery side of the transformer would propose a lot of Risks and pretty much makes the ground line you are talking about useless
Basically this is about the differences between the 4 kinds of system grounding listed in the IEC electrical installation standard(s). In a TT system, the Neutral is grounded at the power company distribution transformer, while the Protective Earth is grounded at your house, thus the two are connected only via the ground itself plus any leaking equipment in your house. In a TN-S system, the Neutral is grounded at the power company, plus there are mechanical and contractual guarantees this connection is reliable. In your fuse panel *before* the RCD/GFI, the Protective Earth wire is connected to this guaranteed grounded neutral, but is kept separate in the rest of your house so any excess current leaking from Live to PE can still trip the RCD protection, this is what Mr Carlson described as being standard in his area (except his lab has no working RCD protection or that lightbulb would have tripped it instantly). In a pure TN system there is no distinction, the PE and N are the same wire (called PEN) all the way to your outlets. This is considered dangerous as it leaves no way to detect deadly ground leakage going through someone's heart. Finally there is the so called IT system where the power source has no solid connection to ground, there is no Neutral and your outlets have two Live wires and an optional PE connected to ground. In an IT system there should be an alarm system constantly measuring if either of the Live wires becomes accidentally grounded, a "first fault" which must be fixed before any "second fault" occurs between one of the other Live wires and ground. IT systems are used in some places such as parts of Norway, but are regarded as something special/exotic in most of the world. The white isolation transformer demonstrated in the above video converts any of the 4 systems to a separate IT system without the fault monitoring. (Hope I remembered all these abbreviations correctly, it's been a few years since I last looked at the subject).
Hi Paul. Great video. Do you think you can do a video specifically on best practices for slowly bringing up some old tube based electronics with a Variac/isolation transformer? What do you check for (other than things going up in smoke)? How quickly do you bring it up, etc? I know for amps you often remove all the tubes except for the rectifier tube. I know every situation is unique but maybe there are some general procedures to follow?
I worked at a very small electronics store that had been around for 50 years. The hardest thing to get, vacuum tubes. None are make in the US. Some made in China, Russia, Poland. Some surplus found and tested. I've used a tube tester in my younger years. My local store was the same family that started it. Government surplus was its first product stock.
I had recently bought a small "isolated transformer" for safety reasons per advise here (TH-cam) and in books on testing circuits with scopes and other earthed equipment. Concerned, I tested and was surprised and confused to discover the ground line connection on the IT. Thanks to your video and the "EEVblog #279 - How NOT To Blow Up Your Oscilloscope!", I now understand. VERY VALUABLE! Thx!! Using the light bulb also provided a safety check method. +Thx. Now, I need to understand why AC circuits/devices don't blow up since hot and ground (via neutral) are connected 60-times a second (USA). ;-)
Surging Circuits neutral does not go positive. The hot is alternating between positive and negative 60 times a second. The neutral is the reference potential. It does not change.
I'm always surprised that people, (trades people I'm talking about) that work with mains/supply wiring, do not realize that neutral is referenced to ground. There are many issues that can creep up, especially with circuit boards/ICs, with this problem. Great video
Another thing to keep in mind, often newer isolation transformers will also Earth the Neutral coming off the SECONDARY of the transformer, When working on an AA5 with a hot chassis this can be VERY dangerous
Hey Paul, maybe it's worth mentioning, that also many variacs exist, that ARE insulation transformers. Toroidal transformers with primary winding and a secondary winding, that comes with the brush... I do have one of those transformers in use in my lab since about 10 years now, and it works great. Also, beside the risk of accidently connecting ground level to your DUT via a scope, it could also easily happen via other units, e.g. sine generators etc... For testing my guitar amps, I selected all my necessary units to be NOT ground connected - which is an oscilloscope, a sine wave generator, a loop station with an instrument signal, a dummyload that includes a wattmeter and that has an insulated output that is connected to my recording studio (for recording the result of 1 - 1,5 hrs test-runs of amps that I repaired, to confirm that they are working 100% correctly).
Hi Lieselotte. In all my time, I have never come across a "Variac" that has any isolation. I encourage you to test your isolation with an OHM meter to verify any resistance. If you get any resistance reading from the input to output, your device is not isolated. If there is isolation, it's not a "Variac."
@@MrCarlsonsLab Hey Paul! My Transformer is an insulated variac without any doubt! The schematic next to the terminals shows a primary winding plus a separated insulated secondary, that has got the variable tap. Also I confirmed this with an ohm-meter and, on top, with a specific device tester (it shows >20MOhm between primary and any connection in the secondary in the mode for "protective insulation", measured under 500V). So, there is definitely absolutely no connection between primary and secondary... If you are interested, I can send you some photos of this device. Also, at the tube-amp workshop I used to work years ago, we had such devices (insulated Variacs) at every workbench, though I never ever opened one of these units, so there might have been 2 separate transformers (one for insulation plus an extra variac without insulation) in them...
I enjoy your videos and you are very knowledgeable. I was very surprised however to see this video on safety and then your demonstration using the light bulb and the exposed line voltage of 120 volts. I only hope new people (hobbyist) to the field don't follow that example. Maybe at least a warning like: "Don't try this at home kids". Again I do enjoy most of your videos.
It's disputable whether the output socket of an isolating transformer should have an earth pin. Some say yes, some no. I am inclined to prefer no, but not everyone agrees. However, (In the UK anyway) there is a far worse gotcha to beware of, in that there are transformers sold for building site tools which used to be called 'isolating transforners' but which are actually autotransformers. I believe the practice of calling them this has now been banned, but you may still see some old hands in the building trade referring to them as isolating transformers. If one of these finds its way onto the electronics testbench it's an accident waiting to happen. On any unknown transformer, do a resistance check between the live and neutral output and all three input pins, especially the earth. If there is a low resistance to earth OR to the the input supply, it is NOT an isolating transformer, and could cause an accident on the bench.
so veriac and scope can be pluged into just common and hot using the 3 prong adaptors to the isolation transformer at the same time like the 3 in one plug cut the ground lug off as well for extra safty? or just the scope?
![แล้วแต่เวรแต่กรรม - ครูศรีหนุ่ม [OFFICIAL MV]](http://i.ytimg.com/vi/Xide0fkAS8g/mqdefault.jpg)
As others have pointed out. It is important to remember that if you float your scope and then connect probe ground to a hot connection the entire case and terminals of the scope and anything else connected to it are now hot. You only need to touch any of those points and ground and you are fried. That is why you normally never float the scope but rather the DUT.
An alternative to be extra safe is to float both the device and the oscilloscope (using separate transformers!). Still you have to be mindful that your ground lead can energize your oscilloscope w.r.t. the device under test, so don't handle both at the same time in that case.
Well yeah that's pretty obvious from even basic electrical theory. It's more about parasitic components of circuits that actually become an issue sometimes!! But yes I do generally aggree with you of course!!
yep all metal on the scope
This was one of the shortest but absolute best videos I've seen of yours. Safety is number one, two, and three. Without proper safety knowledge, us techs can not do our job. Would love to see more videos like this.
-Mr. Carlson's biggest fan.
¹¹¹1😅
In my book, any isolation transformer that feeds ground through to the secondary side is NOT an isolation transformer.
It's still considered isolated, just not from ground.
You are exactly correct, Sir!
@@NunYa953 Actually... From the neutral bar, which is connected to ground, in the electrical panel!
@@MrDoneboy
This particular design of isolation transformer only isolates the secondary from mains. There is a big difference between a bench test isolation transformer and this. All these video prove is there's a whole lot of people who have no business playing with electricity doing just that.
@@NunYa953 It is your life, sir...you only have one!
The creativity you have to show your points is simply amazing. So grateful for the way you help us.
I have had Earth ground running through my isolation transformer for decades and never have had a problem. I never plug my test equipment into my isolation transformer only DUT. Many devices have components referenced chassis ground. Running the Earth ground all the way through to the DUT helps to eliminate noise while testing. That said, Mr Carlson is absolutely correct in his presentation and I am fully aware of how my bench is configured. Thanks Mr Carlson
Thank you Mr. Carlson. If I ever get an isolation transformer like yours I'll be sure to open it up and cut the ground connection to the secondary side.
Just a beginner here, but I thought the correct way was to hook up your oscilloscope normally, and to the ground and use an isolation transformer (with no ground) on the device you are testing.
Thanks Paul. You explained it very well. People need to be careful with the ground of USB plugs from their laptops and devices also.
+Robert Calk Jr.
Thanks for your input Robert!
+Robert Calk Jr.
Good point about USB. Check out the USB port isolator at Adafruit (search for product #2107)
+Ken Cosi
Looks like a handy tool, thanks.
I love this comment. A USB isolator is less than 19 dollars on Amazon. No reason not to use one!
USBs being dangerous... really lol
Best demonstration of potential ground loop hazards re scopes etc on
You Tube, clear and simple.
Thank you so much for the info about these isolation transformers. I did not know they would connect the separate grounds. I can not believe it is called an isolation transformer. You may have saved some lives with this video. I honestly would assume an isolation transformer is ISOLATED.
Thank you Paul, I did not know this you quite possibly saved me from an injury
Thank you. I needed the reminder about isolation transformers with a common ground, and hence common neutrals. Glen
Great safety video, and as always, very understandable. Hope you and your family have a fantastic Christmas and New Year. Regards Bob (UK)
+Bob Mellor
Thanks Bob! Merry Christmas to you and yours as well!
A very interesting video Mr Carlson, it made me look up how houses are wired here in the UK. Once upon a time you either had a ground rod (some rural areas still do) or you had the house ground bonded to the sheath of the mains supply which went back to the substation before grounding out. The single phase domestic supply was one of the three phases of the national grid to live and the ground at the transformer for neutral so the earth and neutral were connected at that point. New builds and I think anything that is re-wired now has a system just like you describe with earth connected to neutral in the company fuse box at the distribution board.
The other thought that comes to mind is that the earth connection on the secondary side of an isolation transformer seems to be absolutely pointless as it doesn't offer the safety that it appears to. The earth connection in an appliance is to provide a route to earth should anything fail internally and ground out the live causing the fuse to blow somewhere - either in the appliance, in the wall plug or at the distribution board. With more sophisticated systems with earth leakage or residual current detection that would trip out first. None of those things will happen if the live and neutral are isolated. But on the other hand you are unlikely to suffer much harm if you touch both the live and earth. When mine arrives I must test what potentials are there.
Thanks for that great tip Paul. It is always nice to be reminded of safety when it comes to electronics. You did a great job of explaining this.Have a safe and happy holiday and we will see you next year.
+The Radio Shop
Thanks, and Merry Christmas!
The isolation transformer I inherited from my uncle did not have a grounded outlet. As I recall, it had a 2 prong, non-polarized socket. Not sure when it was built, but I would guess in the 1950s, as most of the equipment I got from him seems to be from that era. Thanks for the great video!
For starters you would normally use the isolation transformer to isolate the device under repair, not the scope.My isolation transformer the test plug is only connected to the secondary, the ground prong is completely open on the transformer.Mine is a Hammond manufacturing co, made in Canada.
I have a Hammond model 171B isolation transformer. The ground prong of the output receptacle is definitely connected to the input cord ground and the transformer frame on mine (I just checked). In addition, Hammond's own data sheets for the product show the output ground terminal connected: www.hammfg.com/electronics/transformers/line/171
(My unit is an older model with no circuit breaker, and with an outlet instead of a pigtail for the output).
I just got a topaz isolation transformer. Heavy! I'm considering installing this in my van conversion as I am running audio gear. This video is eye opening. Thanks!
I made this exact mistake 2 days ago on my oscope. After the bright flash I started thinking about it and then realized -of course the ground clip is grounded - duh. Then I started looking into prices of variacs that have an isolated transformer and then found this video. BTW, my Oscope does not measure correctly anymore - both the voltages and wave forms are incorrect now. i have not yet tried to replace the probe so hopefully that will be the only issue. Just wanted to say I made this exact mistake and your video helped me understand and verify why. Totally makes sense now. I did not realized that the barrel was connected to earth ground and was treating it like a multimeter. Thanks for the Video!
oh mate. how did you get on? did you fix the scope?
Did you get your Oscope fix yet.
@@carl112466 Dude is probably dead because he didn't know about current limiters.
Possible that the board foil to the input connector burnt and possibly you can fix that and with a little luck there will be no other damage. Good luck and God Bless!
Two of Ed
Thanks Paul for a very clear explanation on this very serious matter.
I think this is a very under rated topic . I don't believe it has been addressed fully here which is understandable because this video highlights the difference between filter conditioning transformers and isolation transformer. We need to highlight links or create videos on practical techniques to isolate techs and equipment . More discussion is needed to increase safety when working on mains . There seems to be two topics a) human safety techniques while working on electrical equip and b) safely using test gear on high voltage without destroying equipment . Equipment requiring mentioning : isolation clothing , isolation transformers , differential probes.
I agree this is a huge topic that seems to catch out even the best of them.
Just a couple of comments:
1. You mentioned powering an oscilloscope from an Isolation Transformer (although, admittedly, you didn't actually suggest doing that). That should almost NEVER be done. (The only exception might be if you are actually repairing an oscilloscope, where the scope is the Device Under Test (DUT)) Only the DUT should be connected to an isolation transformer.
2. A standard Isolation Transformer can be converted to a "technician's isolation transformer" by internally disconnecting the transformer's secondary neutral from ground. This maintains the ground path all the way to the DUT (assuming that the DUT has a ground prong). It is much safer than using the cheater plug while also isolating the DUT from the mains. However, even with a "tech" Isolation Transformer, you still need to be careful not to do things like connect the secondary (output) to ground.
+Louis Wilen
Thanks for your comment Louis!
yes , you are so right here .....
sometimes silly and basic things goes so under the radar , right ??
I won't name any names but there are 2 professional techs here on you tube that use 2 isolation transformers because it's safer. they plug both their scope and tv or whatever into an isolation transformer. They both work as professional techs servicing consumer electronics.
Robert Slackware Yep. At least a very low Ω path.
Lots to learn here, important stuff. Your doing a fine job.
I fixed TV sets for 40+ years but never knew about this! Thanks for sharing this information. I always used the Sencore isolation variac with no problems but now in my home workshop I have a B&K TR-110 isolation transformer and it never occurred to me that I may be not be isolated like I assumed I was. I'll have to check this out. You may have just saved my scope.
And potentially, your life.
Hi Mr. Carlson, thank you very much for this informative video!
1. Is it safe to disconnect the oscilloscope's earth wire at its plug, so that it only has a live an neutral connection?
2. Is it safe to disconnect the earth wire running from the ac mains supply to the isolation transformer's output plug, so that there is only a live and neutral connection available?
3. When do you need to use an isolation transformer?
4. Can you please do a video on how to connect an isolation transformer to a DUT, like a TV, amplifier etc. and also to instruments like oscilloscopes, signal generators etc. so that all of them can be used safely together to probe and repair a product.
Thank you very much for your channel. I have learnt so much from your videos!
Balanced isolation transformers, which are sometimes used for audiophile and home theater systems, usually have a center-tap on the secondary which is sometimes connected to the ground connection on the output and chassis. Thus the output is not always isolated.
super good video, very easy to understand, great safety tips, thank you Paul, have a great christmas and a happy new year!!!
+jeff b
Thanks Jeff! Merry Christmas and a Happy New Year to you and yours!
I've seen many comments here about only the DUT being isolated from ground, however, when I was a student learning troubleshooting and repair techniques, the instructors at the vocational school cautioned us to never have the test equipment grounded. They sent us out to the rest of the world having been taught the very same thing Mr. Carlson showed in this video. Every piece of test equipment in that shop had all of the ground pins on the power cords removed. Every one of them. Meanwhile in the electronics theory class down the hall, they blew up a brand new 540 series Tektronix scope because of said grounding issue! I don't remember anything else about the incident so I don't know what they were measuring, how it was hooked up or what- just that they mentioned it to one of our instructors and then we were told about that precaution. After that the electronics instructors were removing the ground pins from the power cords or using the adaptors as shown here.
Anyone care to explain why it is bad to float your test equipment? Not looking of arguments or heated discussion, just good explanations.
I was taking an analog electronics class at the local community college back in the 90's. My lab partner was in the air force and she said usually stationed in an AWACS. I was taking notes and she was hooking up the circuit. so she had the power supply on and somewhere in the circuit she said lets see positive goes to negative and before i could say no!, she connected the connection. there was a bang and smoke came out the back of the power supply. The instructor walked over expressionless. Picked up the power supply, unlocked a closet. He exchanged the blown power supply for a new one, locked the closet and w/o a word put the new power supply down on the bench in front of us.
I knew about the problem with the ground, nice to know the reason why to. Good explanation. That said your demonstration with the bulb and the probe hovering so closely to exposed mains with your bare hands, made me shiver and that in a video about safety........ :)
+Bart Zuidgeest
It made me shiver too.
+Bart Zuidgeest
LOL, When I watched the video I chuckled, my hands look way closer to those open clips than they actually were. Ahh....the magic of video. Thanks for your comment Bart!
Excellent video! Too many folks don't understand this issue. It was beat into my head by an instructor, who used the example of a grounded scope and testing a component in an energized circuit, for example either to measure a voltage or to see a signal. If an inexperienced tech attempts to put the scope probes directly across a component in an energized circuit, the negative probe will short out all of the circuit "below" the component being tested. Bad things can happen!
Thanks again for a great explanation.
+Clay Nicolsen
Thanks Clay!
Another informative video that reminds us to be safe! I had to go back to the schematic of my Heathkit IP-5220 Variable Isolated AC Supply to remind myself that it's configured just as you describe; always good to remember! Merry Christmas and Happy New Year Paul. 73 - Dino KL0S
+Dino Papas
Merry Christmas to you and yours as well Dino!
learn something new every day, i think a variac is what this gadget is found in an old house is, and the basics of how they work, Big Thumbs Up
Hi. George from Bulgaria here.
Just wanted to show my appreciation.
Your channel is THE MOST enjoyable content, regarding electronics and stuff on TH-cam.
So calm and peaceful and in the meantime always super sharp and right on the topic.
Not to mention your outstanding knowledge.
Not everybody can achieve this.
Respect.
And, of course, im a subscriber right from the very first video. :)
+Joko Tash
Thanks for the very kind words Joko!
Thank you Paul for bringing to our attention the need to consider the nature of the supply coming to,our "Isolation" transformer, and the way that itself is connected internally.You certainly made me reappraise my modelling activities wherein I replicate different types of supply and their earthing arrangements -Tierra-Tierra TT, Tierra y Neutro Separado TNS , Tierra y Neutro Combinado [Separado] TNC[S].I work mainly from a step-down transformer (240 V 50 Hz. Input to 110 V centre tapped output). This type of transformer is commonly empoyed on building sites to afford a measure of safety and are often referred to as isolation transformers when in fact, because of the earthed centre tap of the secondary winding are in fact in metallic connection with a current carrying conductor. The term Protective Extra Low Voltage is more appropriate, as for it to be a Safety Extra Low Voltage transformer, as according to the On Site Guide to BS 7671 Section 3.4.3,/ 414.3 " ... have no live part connected to earth or protective conductor of another system."This clearly means a double wound transformer with no part of the secondary circuit connected to earth.A common example found here in the UK would be the shaver socket outlet supplies for deployment in bath/shower rooms.Saludos.
+Hassan Burrows
Thanks for taking the time to write Hassan! I'm glad your finding the videos helpful.
Thanks for covering the dimmer switch question in place of a Variac. :)
Excellent explanation for safety.
You have so much experience and knowledge in this field, and I trust you, so had to ask you about my next project: I'm thinking to make an a adjustable AC bench power supply. My concern is to have the AC stable, when something using it draws more than 1A (or more) of current. When I needed for my project AC, I just used 2 resistors, wired like voltage divider and also used 2 potentiometers - wire-wound style. It did the trick, but since I'm gonna play with more AC projects, I would like to make a nicer power supply that is stable. I do not need anything more than 10A (that would be at the most I can imagine for now). I have several transformers 120V to 30V, 16V, 12V outputs. One has both 12V and 30V in one. They came from a large UPS (Uninterruptible Power Supplies from 1200W and 1500W respectively). I'm not very experienced in electronics, just a new hobby after a 50+ years of other hobbies. I'm trying to rekindle something that started when I was a teenager. I do have 100Mgz Oscilloscope, 50Mgz Function Generator, several meters, 3D printer and lots of parts I salvaged from printers, UPSes ... I love to recycle and fix everything. So I'm not looking to make something that would require lots of knowledge that I lack of right now. I do play with Arduino's and simple projects..... I have several pictures of the transformers with their parts numbers and I know what voltage they produce. I could not find much about them on the Internet, not even how to connect the mains, but I figured it out - I think. What I like is a simple schema how to build the adjustable power supply and some protection for it; also what should I do to stabilize the output voltage.... I have these inexpensive meters from Harbor Freight that I can use to measure the output, so I don't have to even have a circuit for that. But if it has that, and it is simple and few parts, I am open to the idea also. You, or anyone else reading this, can contact me on my Gmail: czecht@gmail.com (Yes, I was born in Czech Republic), but I live here in the best country in the world! since 1981. Thank you Mr Carlson and thanks everyone else who would like to help me out!
Great Saftey Video. Comment on Isolation Transformers, the older ones came with a non-grounded plug and outlet. One did not have to worry about grounding saftey issue.
Keep em comming, Greg
+Gregory West
Thanks for your comment Greg!
Thanks for this great video. I worked in the TV repair field in the 70's and then eventually went into the electronics field in the 80's to early 90's but I can't remember what we did back then to prevent this issue with scopes. Did scopes change in recent years regarding grounding? I used primarily Tektronix scopes. Maybe I just automatically used a ground adapter and never put much thought to it. Thanks for all you do to educate us.
*Merry Christmas Mr Carlson* Thank You so much for your videos.
+MrMac5150
Merry Christmas to you an yours as well!
An excellent explanation of something I was aware of (35 + years in semiconductor testing) but as you mentioned newer and less experienced techs may miss. I also want to commend you about the danger of just randomly defeating grounds as well. They serve a critical safety issue AND also a key point in shielding extraneous noise and signals in sensitive measurements.
I'll share a story about a time in trying to measure picoamp level input bias currents on an op-amp and having massive 60Hz noise problems. I discovered(after a lot of time I need to add) the problem was on our very expensive computerized tester. A maintenance technician was tired of disconnecting and then reconnecting an elaborate bonding wire system to make sure all metal parts of the chassis were grounded and shielded every time service was required. So he decided not to reconnect the bonding wires..leading to large parts of the chassis acting like antenna and radiating 60Hz into sensitive measurement modules.
Again enjoy your channel a great deal. I'm not too proud to say that even after years of experience in electronics, there is still plenty too learn. I'm now retired and starting to get into restoring some old tube powered radios, a new universe for me!
+GrandsonofKong
Great to hear! Thanks for the story, and glad your enjoying the channel. Sounds like the tech was a little lazy. That sort of thing causes future techs a lot of issues. Merry Christmas!
You are always the best teacher to learn electronics
Thank-you for another great lesson. I have a concern regarding your presentation of the autotransformer variac. Autotransformer by definition has a primary and a secondary except that the primary and secondary are connected together in an electrically common manner. That is what prevents isolation. Many autotransformer variacs allow for a secondary voltage above the line voltage 140 to 150 volts. That being accomplished by additional turns ratio on the secondary.
Regarding the illustration you showed I believe I may have some 40 volt variable transformers like that. I haven't given much thought to this until your video. I'll have to inspect them again.
Paul, I want to first thank you, for all of your informative videos. Thank you, so much , for explaining about modern day ISOs. I own a VIZ isotap model WP-26A 400va. This was a gift, from an older tech. I seem to be unable to find any literature or manual, for the ISO. If by chance, you or your subscribers have any information, I would be rather grateful. If you have one of these older ISOs, maybe you could do a tutorial... Thank you, so very much and Happy New Years.. Cheers!
+scotland770
Glad your enjoying the video's. Thanks for your comment!
I was tought to always use TWO isolation transformers with no ground connection: One for load and the other for measuring device. Floating load can be measured with earthed scope but when ground lead is connected it becomes mains referenced. If you touch the device, you get shocked because it is no longer isolated from mains earth. Having also the scope floated via a second isolation transformer (with no earth), you avoid the problem. With that setup it is safe to touch any single point of a powered device. That is also safer than just isolating the scope from ground because in case anything leaks from "live" to scope chassis, it is isolated "live" ie. will not shock you.
Never ever float only the scope and connect it to a mains connected load. Earth lead connected to eg. live wire makes scope chassis live which is very dangerous.
As a kid I also made the classic mistake of connecting scope earth to live wire with all the sound and light effects assosiated. :) Never repeated that mistake.
Hi I have just come across this video I found it interesting as I have just this week been setting up my work bench with isolation transformer . You did a very good demonstration of what the problem is . But I am somewhat concerned about your proposed solution . What seems to be being disregarded is while you make your measurement if you have removed the earth wire from the scope power plug and you have connected the ground probe clip to the hot side At this point the steel case of your scope has become live in relation to anything else that is in your area that is connected to ground if you touched this case while adjusting a knob you would likely get a shock . If you had a data lead from your scope to a pc then you would then be putting high voltage down that lead on the ground into other equipment which you may damage . In the event that you are using as a demonstration I would use a DVM to measure the voltage between the conductor and the ground clip and ensure that it is a very low value possibly a volt or lower before making the connection with the ground clip I would not need to remove the earth from the scope . If one gets used to doing these basic checks before making a connection then it can save a lot of damaged equipment .
I am in the UK so all out power is 240 / ref to earth regards bill
Another excellent program! Thank you!
Your welcome!
Great video and advice for beginners. Blew the end off a scope probe once! Have a great Christmas and look forward to more great videos in 2016.
+rupert handford
Thanks, and Merry Christmas to you as well Rupert!
Thanks, I now have a better understanding. But it seems like you didn't finish the topic on Isolation Transformers. It's good to be aware that the secondary might be linked to Earth. But then, how should we setup a proper isolation system? Should we break the secondary connection? I want to be able to use my scope to test the long list of devices I have.
Francois, The best way to ensure total isolation is to remove any ground connection at the output of your IT. Some manufacturers ground the metal container that boxes the IT as a matter of safety. Depending on the type of outlet socket this can be problematic as the mounting screws can become grounded and defeat the purpose. For example, in the UK and Ireland, all socket mountings are connected to the earth pin and when mounting them as I had to do , I literally had to cut the copper connection to isolate the output otherwise the mounting screws would have made the output grounded. These are small things that need to be considered whilst making sure your IT is suitable for bench work.
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My understanding was that you should pretty much never float your scope, either through an isolation transformer, or especially from ground. Keep the scope plugged into the wall, and pit the device under test on the isolation transformer.
+PelDaddy
Thanks for your comment!
PelDaddy that was my first thought, put the scope i the wall, and put the device under test into the iso....
if you have an older piece of equipment (like 98% or so of stuff on this channel) it probably only has a 2 prong plug anyway.... so the iso-ground issue isn't an issue....
I agree but there are 2 techs here on you tube that float their scopes thinking it makes things safer.
What's the reasoning not to float the scope?
Teb Tengri the reason is that with a floating earth scope the BNC connectors are still attached together and to the chassis of the scope. So if you would hook mains voltage to the ground clip of a probe, all the other probes/BNCs and the case would be live. Since you normally treat the ground clips etc. as safe to touch, and often clip it straight onto the nearest ground of a DUT, you make it very easy for yourself to mess up and ruin your day. If you want the convenience of not having to worry where you stick your probes, there is no substitute for a proper isolated probe.
Modern isolation transformers often have the neutral and ground tied together at the outlet on the secondary side of the transformer. It is a clip that can usually be removed.
+theflinx
Thanks for your comment!
Mr Carlson's Lab Hello and thanks for your videos! I am new to repairing electronics can you please explain why it's so important to keep the ground for safety. I keep hearing this but don't understand why it makes it more safe? Why is it more safe for the BNC connectors to be grounded or the chassis of the isolation transformer to be grounded? I am sorry for asking such a noob question I just don't see anyone explaining this... Thank you
The earth connection provides an alternative path for the power should a fault arise. Since it is always connected it will protect even with the appliance turned off or more importantly beleived to be turned off. The permanent connection also continues to work if the active and neutral lines become reversed as can happen when untrained people wire up plugs. A well made appliance will often switch both active and neutral lines to also protect against reversed plugs.
Many modern products do not have metal exterior chassis and will employ what is called double insulation where the usually plastic chassis acts as a second insulating medium. Virtually all power tools are made this way these days and will not require an earth connection.
Merry Christmas Mr Carlson and a happy New Year.
+Neil Robert Turner Guitars
Same right back at ya Neil, Merry Christmas!
I'm only new to this stuff and this is something that scares me because (although I've not had to use one yet) I know it's going to bite me in the ass some day... God I'm glad I watched this video I was going to go to bed but desided to watch one last video. Thank you Mr Carlson.🙏👏
Another informative video, thanks and merry Christmas..
+Jim McDowell
Merry Christmas to you too Jim!
The best video on isolation transformer
Great video.
I have a variac and isolation transformer (no ground pin on the secondary of the isolation transformer) but most of the time I use my Sencore PR-57.
+Michael Lloyd
Thanks Michael!
Excellent video, I have learned tons from you over time. A long time listener, first time caller so to speak. I'm kinda scared to hook my expensive o'scope up to mains now. Anyhow, my question is such, I use a variac and just plug it in, but is it polarity conscious? Meaning, the hot lead always goes thru the carbon wiper brush you talked about. Is it essential that the hot lead do this? Reason I asked, mine is a cheapo from "over there", and has no ground pin, and both leads are the same, no big spade lug to polarize it to neutral. Can be plugged in either way. Should I make sure that the hot line is going to the wiper, and the neutral is being passed thru. Or does it all really matter? Also, whats the best way to read my 120vac input line power in my house? Use a 2 prong old style plug adapter as you do? Can I put the 100w bulb in series with the o-scope leads so the magic smoke stays in if there is a problem, what do you think about that idea? As always, love your channel. see you on the next on.
Very informative. Thanks so much,
Kevin
+Kevin Gray
Thanks Kevin!
I made my own box with an isolation transformer, some bulbs which can be individually engaged and disengaged and a volt/current meter.
However, I have a question. The upside of the isolation transformer is that if you touch the live coming out of it, you don't get electrocuted.
However if you touch both live and neutral, you do and the RCD won't cut power as it's located before the transformer. At least this is my understanding.
Do we feel that the main advantage of having an isolation transformer is worth giving up the RCD protection? I am honestly asking, not questioning.
Great video. So important.
As a note, in the UK our supply typically includes a separate earth that is handled way back at the generator, so earth and neutral have no connection within our premises.
Gary Richardson : Attaching one of the two wires of the mains generating alternator physically to the earth ensures that this wire is at zero potential (relative to the earth, which you are also touching) and thus safe to touch by you. You could not touch the other wire from the alternator or your body would complete the circuit to the earth, with risk of electrocution. If neither of the two wires from the alternator were attached to the earth, there would in effect be no continuously neutral or live wire, as both of the wires would take it in turns to be either live or neutral (for a duration which is relative to the alternation frequency of the alternator).
It is safe to touch both the neutral wire and the frame of any metal cased and earthed machine (supply alternator also connected to the earth), simply because it is at the same potential as you, relative to the (elected) live wire, so if the live wire inside the machine should break and then touch the metal frame of the machine, the frame of the machine would then conduct the live current to the earth, rendering the metal frame essentially harmless to the touch. This is due to the very large voltage drop that the earth imparts to the circuit and hence machine case.
It must be said that this condition would normally result in a fuse blowing due to the inevitably large current flow now taking place, assuming the earth wire is substantially large enough to carry the now elevated current flow.
Me Carlson, I’ve seen this warning/explanation many times but what I haven’t seen is a proper fix for the problem. I can disconnect the “earth” lead on the output side of my isolation transformer but I don’t think that’s the proper fix. What is?
Use the adapter, contrary to Mr. Carlson it's still safe.
Man, i´m always impressed by that opener in total view....:)
+TubiCal
Thanks!
As always, interesting, and well explained, thanks.
Merry Christmass, from Denmark. :-)
Morten
+Morten Rolsing
Thanks, Merry Christmas Morten!
Awesome video ! thank you and Merry Christmas :)
+Navpers 47
Merry Christmas to you too!
Thanks for the reminder! Safety is #1. As a 45+ year HVAC/R service tech, I’ve been “poked” by 24v, 120v, 575v, and even lightning (100’ underground behind Niagara Falls)-and I wouldn’t wish the experience on my worst enemy.
Is the methodology of using “balanced input” (where you use 2 probes, with their ground leads clipped together and not connected to anything else, and their signal probes connected to the AC line) a safe alternative?
Thank you so much for this and for all your videos.
I understand that, when you restore old test gears, you usually prefer to avoid connecting a cable with ground pin.
I understand the concern expressed in this video, but I wonder if there are other situations in which the ground connection could be counterproductive (or very dangerous).
Thank you for your attention.
Another great job. Thank you
Hi Paul, wow I don't understand why that transformer is officially out in the market as certified isolation transformer. In Europe this would never ever get the declaration of confirmatory to be sold as isolation transformer. Following VDE/ EN norm the secondary side never is attached to protective ground/ earth. But anyway what ever norms are telling at all it doesn't make any sense...., or is there is something so much substantially different in Canada''s power distribution compared to Europa I don't know?? Anyway very interesting video ! Thank you. 73 Peter
+TRXBench There seems to be a lot of confusion about this even in europe. Mostly an "isolation transformer" has a earth connection on the secondary and a "safety transformer" has not, but has this clearly marked with a warning next to the outlet (and warns you not to hook up more than one device). But I have seen a "safety isolation transformer" (made by peaktech) that has a earth connection on the secondary, completely defeating its purpose (even false advertising imho).
But I'd be very interested, if there are codes in Canada, that prohibit safety transformers without mains earth reference. After all these things are only for full galvanic separation (thus preventing any current flow out of the DUT into any earth around) during repairs under power. Nobody in their right mind would use them for anything else.
But a well made and very important video nontheless. Thank you for it.
+Róbert Valdimarsson Hi Robert, yes very important issue and you are right a lot of confusion all over. I'm talking only about a galvanic protective separation transformer (Schutztrennung) according VDE 0570 Part 2-4, DIN EN 61558-2-4, EN 61558-2-4, IEC 61558-2-4
+TRXBench
I agree fully, but politics and bureaucracy gets in the way of sense once again. If they want to include that "Earth" they need to re-label the transformer, and remove the word "Isolation"
Merry Christmas to you and yours Peter!
+TRXBench Yes, you pretty much have to rely on the norm numbers here. I have a propper one here for up to 1500VA. I got it very cheaply second hand and quickly found out why: It does not have a inrush current limiter. So every time i switch it on, I have to make a trip to the switch box and reset the breaker :D But a limiter module is not too expensive. Still the best money I have spent so far, since it might very well save my life some day.
Mery christmas and a happy new year to all of you too.
+Róbert Valdimarsson
Thanks Robert, Merry Christmas!
Great video. I work on a lot of tube guitar amps and I use a variac and isolation transformer. Do you recommend plugging the unit/amp under test AND the O-scope into the isolated (and earth/ground lifted) power source or only the unit / amp under test? I realize this could be a tricky question because when the scope is earth grounded and the probe clip is on the chassis, the chassis is once again grounded to earth as if the test unit was earthed at the receptacle, even though the mains AC voltage is "isolated." Thanks for any response you can offer. I'm in Mission and I refurbished a cabinet for a vintage amp you later did a video on the electronics rebuild.
Good video!
Even some test equipment containing a built-in isolation transformer and variac has the ground pin in the receptacles bonded to the ground in the line cord. My VectorViz ISO-V-AC II WP-30 is set up this way.
The other option besides using a 3 to 2 prong adapter is to use a high voltage differential probe such as the Caltest CT2593-1. I use one of these when working on offline switching power supplies. The chassis will still be hot, and you will need to be careful, but you won't blow your scope or probes up as the differential probe is good to 1000V common mode.
+Steve Rodgers
Thanks for your comment Steve!
Another great video, I am very interested in these devices but currently cannot afford. However, I built a "Dim Bulb Tester" with an array of different bulbs for their wattages.
Recently, I purchased a Peavey "Flight Case Mx", VTX Series Amp Head" 130 watts.
The pre amp is solid state, into (4) GL6'S. I bought it from the local pawn shop for $120 I did play it at the shop before I did the deal. But because I did buy it from a pawn shop, I am suspicious of why the amp was there. It isn't hot, been on their shelf for two years. So did somebody pawn because it was breaking g down? or did they need the money?
So I have the amp now, I have opened on my bench. I am curious if there is a fail while testing. I know this can indicate a short,
My question is about resistors going out of spec, will that also cause the dim bulb tester to fail too?
If you don't mind, I have another question. I have read that some guitar player hook up a variac to their amps. I am guessing they might be doing this to play their amp at a lower volume. But still getting the crunch. My curiosity is: if I found the right bulb if I could duplicate this effect with the dim bulb tester?
Great video. thanks.. One question. could u please two good brands to buy for isolation transformer and variac?
Life saving (probably) information. Thank you.
How good the connection is between earth and neutral depends on the earthing system used. In my country there is three systems in use, those are IT, TT and TN-C-S. Thank you for an informative video. I just restored the grounding on my second hand Tektronix 475A, the previous owner had just broken of the grounding prong.
There is one.
There are three.
I suggest you check whether your isolation transformer and\or variac (my "variac" is isolating) have fuses. One of mine did not have a fuse, so I had to install one. I blew it often since. The other even had two fuses, one on each primary lead, but they were 3.15A good to keep the office powered but too much to limit the damage to the DUT.
Nice video Mr Carlson. Going to check my pr57. Thanks!
+Richard Cunningham
Thanks Richard!
My neutral rail is earthed with a screw into the concrete wall in my fusebox. When my house was built only the cooker and the fridge were even attatched to that earth and the wall sockets had no earth panel in them.
If you remove the ground from the isolation transformer on the secondary side, can you plug the variac into the IT and use it and still maintain isolation? This would be useful with a 240v IT when needing 117v for USA guitar amps like a champ etc.
Paul, thanks for yet another informative and well-presented video. Maybe you can help me, either with my thinking or with my technique. I recently came into two identical transformers scrounged from backup UPSs and experimented with wiring the two 24VAC secondaries back-to-back, thus creating, at least in my mind, a 1:1 transformer. I grounded the receptacle ground lug and the metal cabinet to the line cord ground conductor. So far, so good: 120VAC input, 118VAC output at the receptacle. In my mind I now have "floating" 120VAC at the receptacle and continuous earth ground back to the breaker panel, but the voltmeter says otherwise.
When I measure from the "hot" receptacle lug I read 41VAC to ground, and 19VAC from the "neutral" lug to ground. I don't quite understand how a supposedly "isolated" transformer secondary is referencing to ground. All this is predicated on my assumption that by wiring the two secondaries back-to-back, I created a 1:1 transformer. Here's hoping my explanation is clear enough and you or your readers can shed some light. Thanks again.
4 years on and still no reply , humph...
So far loving the Tech tips Tuesdays. The Tektronix manuals discuss this now in the warnings. I had a run in once with thinking I was isolated and being told I was isolated but actually wasn't due to this same condition. My experiment didn't match your nice light bulb example. No, I had the blue flash once I got the alligator near the DUT. Lucky for me I wasn't hurt and I was holding the rubber shell. I now double check with a voltmeter first for any potential difference. The probe and scope were saved by a semiconductor going into escape velocity.
+W3AMD US
LOL "escape velocity" That's a great term. Glad to hear all was ok in the end. Thanks for the kind words, and glad your enjoying the series.
Thanks for a great video on this topic and addressing the safety ramifications off making mistakes in this area. This did breed one question. I have a gfci that's feeding power to my bench. I believe it would not work if a fault is generated on the secondary of the isolation transformer. Am I correct?
I have that exact same model of variac. I also own a device called a one-AC line viewer, which is a transformer you plug in the wall and has isolated BNC outputs to hook to your scope to view Hot, Neutral, and Ground. More recently I purchased a fluke scope meter, model 99B I believe, but mostly because I needed a portable and battery operated scope but the isolation is icing on the cake as I work on high power broadcast transmitters.
Anyway thanks for the informative videos, nothing here I didn't already know but many others don't and it could save a life, or a scope at least...
+Nathaniel Steele
Thanks for your comment Nathaniel!
i think im gonna always use a battery operated scope even if its wimpy. you can get some good longevity out of a stack of 18650 batteries and its clean too :)
Great work as always!
+Dave Mckim
Thanks Dave, Merry Christmas!
Thank you. This was valuable information.
Glad you enjoyed James!
That looks like a medical grade isolation transformer Paul. That is typical wiring for these I believe.
I use a Sencore PR-57 isolation/variac for my bench work. I bought it on ebay years ago and had to open it up to repair the switch bank for the different display modes. I honestly didn't even look to see how the thing was electrically wired for the output. I think I'll do that now. But I don't see why Sencore would wire their unit as your example is. But who knows...
Thanks for the heads up and Merry Christmas to you and your family!
Love your videos.
Tom
+AntiqueRadioandTV
Thanks for your comment Tom! Merry Christmas to you and yours as well!
This video should be viewed in electronics schools today. I knew of this problem 20 years ago and didn't know how to get around without floating the scope. I think the should be followed up with a GFCI video.
+Dan Todd
Thanks for your comment Dan!
This a great subject and I can relate to it. I was working on a project in the military and was using a variac and a scope. I blow up the ground and I should have know better.
+Howard B
Thanks for your comment Howard!
Hi, Mr. Carlson. I am a fan of your videos and you are doing a great job with sharing your experiences. By the way, I am really surprised about the connection of the earth and the neutral at the breaker panel. In Turkey, it is strictly forbidden to connect these two lines. Instead of this method, every apartment or house have their ground line connected to the soil via a copper plate covered with galvanize.
you forget that the secondery side of the transformer delievering the Voltage to your appartement is most likely grounded so in your case the connection would be directly at the transformer and not in your breaker panel i don't know much about Elektronic Installations in Turkey but not grounding the secondery side of the transformer would propose a lot of Risks and pretty much makes the ground line you are talking about useless
Basically this is about the differences between the 4 kinds of system grounding listed in the IEC electrical installation standard(s). In a TT system, the Neutral is grounded at the power company distribution transformer, while the Protective Earth is grounded at your house, thus the two are connected only via the ground itself plus any leaking equipment in your house. In a TN-S system, the Neutral is grounded at the power company, plus there are mechanical and contractual guarantees this connection is reliable. In your fuse panel *before* the RCD/GFI, the Protective Earth wire is connected to this guaranteed grounded neutral, but is kept separate in the rest of your house so any excess current leaking from Live to PE can still trip the RCD protection, this is what Mr Carlson described as being standard in his area (except his lab has no working RCD protection or that lightbulb would have tripped it instantly). In a pure TN system there is no distinction, the PE and N are the same wire (called PEN) all the way to your outlets. This is considered dangerous as it leaves no way to detect deadly ground leakage going through someone's heart. Finally there is the so called IT system where the power source has no solid connection to ground, there is no Neutral and your outlets have two Live wires and an optional PE connected to ground. In an IT system there should be an alarm system constantly measuring if either of the Live wires becomes accidentally grounded, a "first fault" which must be fixed before any "second fault" occurs between one of the other Live wires and ground. IT systems are used in some places such as parts of Norway, but are regarded as something special/exotic in most of the world. The white isolation transformer demonstrated in the above video converts any of the 4 systems to a separate IT system without the fault monitoring. (Hope I remembered all these abbreviations correctly, it's been a few years since I last looked at the subject).
Hi Paul. Great video. Do you think you can do a video specifically on best practices for slowly bringing up some old tube based electronics with a Variac/isolation transformer? What do you check for (other than things going up in smoke)? How quickly do you bring it up, etc? I know for amps you often remove all the tubes except for the rectifier tube. I know every situation is unique but maybe there are some general procedures to follow?
Thanks for your input Kurt!
I worked at a very small electronics store that had been around for 50 years. The hardest thing to get, vacuum tubes. None are make in the US. Some made in China, Russia, Poland. Some surplus found and tested. I've used a tube tester in my younger years. My local store was the same family that started it. Government surplus was its first product stock.
I had recently bought a small "isolated transformer" for safety reasons per advise here (TH-cam) and in books on testing circuits with scopes and other earthed equipment. Concerned, I tested and was surprised and confused to discover the ground line connection on the IT.
Thanks to your video and the "EEVblog #279 - How NOT To Blow Up Your Oscilloscope!", I now understand. VERY VALUABLE! Thx!!
Using the light bulb also provided a safety check method. +Thx.
Now, I need to understand why AC circuits/devices don't blow up since hot and ground (via neutral) are connected 60-times a second (USA). ;-)
Glad you enjoyed Stephen.
Ground and hot are *not* connected "60 times a second". It's ground and *neutral* that are connected.
+davecc0000 Thx. Being AC, when the neutral goes +, and neutral is connected to ground (-), isn't there a conflict?
Surging Circuits neutral does not go positive. The hot is alternating between positive and negative 60 times a second. The neutral is the reference potential. It does not change.
I'm always surprised that people, (trades people I'm talking about) that work with mains/supply wiring, do not realize that neutral is referenced to ground. There are many issues that can creep up, especially with circuit boards/ICs, with this problem.
Great video
+Phil Woodland
Thanks Phil! Merry Christmas!
Another thing to keep in mind, often newer isolation transformers will also Earth the Neutral coming off the SECONDARY of the transformer, When working on an AA5 with a hot chassis this can be VERY dangerous
Question. In a lab or test bench what is the proper order of the iso xformer v the variac? Which plugs into the wall first?
Hey Paul, maybe it's worth mentioning, that also many variacs exist, that ARE insulation transformers. Toroidal transformers with primary winding and a secondary winding, that comes with the brush... I do have one of those transformers in use in my lab since about 10 years now, and it works great.
Also, beside the risk of accidently connecting ground level to your DUT via a scope, it could also easily happen via other units, e.g. sine generators etc... For testing my guitar amps, I selected all my necessary units to be NOT ground connected - which is an oscilloscope, a sine wave generator, a loop station with an instrument signal, a dummyload that includes a wattmeter and that has an insulated output that is connected to my recording studio (for recording the result of 1 - 1,5 hrs test-runs of amps that I repaired, to confirm that they are working 100% correctly).
Hi Lieselotte. In all my time, I have never come across a "Variac" that has any isolation. I encourage you to test your isolation with an OHM meter to verify any resistance. If you get any resistance reading from the input to output, your device is not isolated. If there is isolation, it's not a "Variac."
@@MrCarlsonsLab Hey Paul! My Transformer is an insulated variac without any doubt! The schematic next to the terminals shows a primary winding plus a separated insulated secondary, that has got the variable tap. Also I confirmed this with an ohm-meter and, on top, with a specific device tester (it shows >20MOhm between primary and any connection in the secondary in the mode for "protective insulation", measured under 500V). So, there is definitely absolutely no connection between primary and secondary... If you are interested, I can send you some photos of this device.
Also, at the tube-amp workshop I used to work years ago, we had such devices (insulated Variacs) at every workbench, though I never ever opened one of these units, so there might have been 2 separate transformers (one for insulation plus an extra variac without insulation) in them...
I enjoy your videos and you are very knowledgeable. I was very surprised however to see this video on safety and then your demonstration using the light bulb and the exposed line voltage of 120 volts. I only hope new people (hobbyist) to the field don't follow that example. Maybe at least a warning like: "Don't try this at home kids". Again I do enjoy most of your videos.
+jwrtiger
Thanks for your comment!
@Robert Slackware Wow, that's clever.
It's disputable whether the output socket of an isolating transformer should have an earth pin. Some say yes, some no. I am inclined to prefer no, but not everyone agrees.
However, (In the UK anyway) there is a far worse gotcha to beware of, in that there are transformers sold for building site tools which used to be called 'isolating transforners' but which are actually autotransformers. I believe the practice of calling them this has now been banned, but you may still see some old hands in the building trade referring to them as isolating transformers.
If one of these finds its way onto the electronics testbench it's an accident waiting to happen.
On any unknown transformer, do a resistance check between the live and neutral output and all three input pins, especially the earth. If there is a low resistance to earth OR to the the input supply, it is NOT an isolating transformer, and could cause an accident on the bench.
so veriac and scope can be pluged into just common and hot using the 3 prong adaptors to the isolation transformer at the same time like the 3 in one plug cut the ground lug off as well for extra safty? or just the scope?