ฝัง
- เผยแพร่เมื่อ 15 ก.ค. 2024
- This video describes how I built my VARIAC, a device that can give you a variable AC voltage up to 270V and 1000W power. Of course I did not actually build the VARIAC, this is more about mounting and wiring a ready-made VARIAC transformer that I bought from RS-Components.
Carroll & Meynell 1 Phase 1kVA Variac, 1 Output, 240V
RS Stock no.:117-6014
This project uses mains voltage. If you follow along, you do so at your own risk.
0:00 What is a VARIAC
3:02 My VARIAC
5:44 Enclosure
6:19 Mounting bracket
8:01 Preparing the enclosure
10:55 Schematic
13:49 Wiring
15:17 First test
15:58 60W test
16:48 Overload test - วิทยาศาสตร์และเทคโนโลยี
A round, wheelie, rolling carbon brush? I NEVER! Really, that is the 1st time I've seen one. And it makes complete sense. I love me some Variacs :)
yes, totally agree. It is one of the reasons why I selected that one.
Nice construction. I made exactly the same execept I added some more weight by having an isolation transformer as well, this is quit handy when working with high voltage equipment
Hi Jens,
I could not agree more... but these things are expensive, even 2nd hand. I am still on the lookout. In the mean time, I am using a transformer from an old power supply as a poor-mans substitute. Using all its wingdings in series, it gets me a safe AC voltage close to 100V with about 50W power. With the VARIAC that becomes 0..100V which is quite usable. I may make a short video about it.
Was just rehabilitating a vintage Variac and had to give this a watch! A lot of the vintage ones weren't grounded at all interestingly enough. The vintage ones do use a double pole switch as well, but I'm not sure if there's much point in breaking the neutral at the same time other than maybe to prevent arcing if you are using it under a super high load maybe?
I've also see where they are configured to use a switch that sets whether live voltage is applied to the very end or part of the way in so that you can choose between over voltage and mains voltage as a maximum.
Assuming your varian is wired correctly, and the output was at 10VAC, how would touching it be harmful?
Where I think the danger lies is if live and neutral are swapped. In that case, then yes, touching the 10VAC output' is really touching the mains voltage in reference to ground. This could happen for two reasons - if the outlet you plug it into is wired incorrectly, or if the variac has a non-polarized plug and it's plugged in the wrong way.
Lastly, is there anything preventing you from applying your live to the center tap? Doing so I think would give you an output that is about double the mains voltage, though I don't really know what use you'd have for ~500V, but it could be useful with 120V sources to up convert to 240. My guess is that the downside would be that it could only handle half the current at the higher voltage
It is probably safe to touch it if its 10V but as you say, L and N could be swapped and anyway it is probably never a good idea. I don't think the wiring and isolation is designed for such high voltage that you are thinking of. I use an external isolation transformer if I need higher voltages. Much safer,
Great work, Heinz! As always... I have just ordered a BK Precision 1655A, for two reasons... One, AC mains is outside of my skill set and, two, I did not think that I could do better, financially, building an AC V Reg with Isolation. I will hang onto my DIY isolation transformer (aprox. 120VAC@3A) for extra uses, or share it, or sell it. I built it from all donor parts = two APC 1500 UPSs. I do have a ceramic socket to add a Dim Bulb current limiter, but I am encountering enough mains circuits to want very exacting control. I guess I just needed the extra sense of safety of having a professional engineered and built unit. Hopefully, it is worth the investment. Pardon me for being so rude as to inquire, but what is your approximate cost for this build? And, did you end up with better quality, or features, than a commercial unit?
The main cost driver is the VARIAC of course. The one I bought (RS stock is in the video description) is currently £105 (including VAT). Big steel enclosures are expensive too and I was lucky to have one that I could modify. The panel meter was about £13 delivered (Ebay). The rest are smaller items mostly from the parts bin. Is my VARIAC better than a commercial one? I think it may have more fuses and through the panel meter, I can see apparent power and power factor which is not something you find easily. And then there is the fun in designing, building and finally seeing it coming together and work as you hoped... priceless! (sorry, could not resist).
@@TheHWcave DIY priceless? Agreed!!! Indeed!
excellent auto-trans stuff but where did you get the ee blog meter:)? -I'm sure you did not have to travel all way to DJ's shop did you? and you could just have mounted the variac on the floor of the existing housing and controlled it from the top saving the extra material & weight! overall an impressive project though
on the EEVblog meter. I bought it quite some time ago in the UK. They are available. I was trying to find the place Simon's Electronics but the site is (no longer?) working. I think other places sell them as well. I did it because I wanted to support the EEVblog a bit (Dave gets a cut). Thanks on the suggestion. The enclosure with the VARIAC ways a ton and therefore sits directly on the floor. That place happens to be under a shelf. The control would not be accessible if it were not on the front. Of course I could have rotated the box if I followed your idea... Anyway, speaking of rotating, the only thing I would do different next time is to rotate the output socket 90 degrees to the right. Because the machine sits on the floor it is difficult to get a UK plug in with its lead coming out on the bottom. I probably will do this mod anyway once I get annoyed enough...
Nice job, have just subscribed. Do you have a link to the brackets, or did you weld them yourself? I can't see anything similar on eBay
I go mine ready made from the ebay seller vds_fabrications
www.ebay.co.uk/itm/Heavy-Duty-Shelf-Brackets-Handmade-Rustic-Industrial-Design-Modern-very-strong-H/264529809676?hash=item3d9734910c:g:Cp4AAOSwmepcCsLJ
@@TheHWcave Excellent, many thanks for the quick reply. I was looking at the cheap Chinese variacs on eBay but after seeing your video I think I will buy a proper one from Farnell and do it myself. Now to find a good steel case that doesn't cost the earth...
You could probably just go to a steel shop and pay $40-80 and have them weld you a custom one for almost nothing in terms of material cost and just an hour or 2 of labor.
Great video, I am looking to do the same as I currently (no pun intended) have an open frame Variable transformer I'd like to put in a case, are you able to let me know where you sourced yours from.
That case is 40+ years old and I can't remember where I got it from. Originally I had put a self-built bench power supply in (with analog V and A meters, hence the large holes). I used that for many years, but then switched to smaller bench power supplies so this thing ended in the garage until I repurposed it for the VARIAC. My advice is not to try to buy new enclosures (outrageous expensive) but look for not working old power supplies or vintage valve based RF equipment and use those cases. Much cheaper and you may find useful components inside. I managed to score quite a number of useful cases and components that way for very low cost.
I have a 2.5kw 240v transformer.
However I am in the US and for 240v we use two live wires each roughly 180 degrees out of phase.
Does it still wire up normally?
I am no expert in US wiring but if your transformer is indeed for 240V you would connect one end of the primary to live of phase 1 and the other end to live of phase 2. Of course earth needs to be connected to the chassis as well. Also make sure you use a double pole power switch to disconnect both live wires when turning power off. I am not sure what the regulations are for fuses in this situation. Intuitively I would put one (rated for half the max current) in each live wire
@@TheHWcave Thanks
Hi. Right now I'm winding a coil for my own variac and I have a question: can I connect one wire from the outlet to the load directly and other wire through the slider of a variac and to the load? If I'm correct, if I connect it like this, it should operate like a rheostat (potentiometer). Will it work like I described or will I need to connect it using schematics from the internet? Thanks!
You are brave winding your own variac and I hope you know what you are doing. A variac is basically wired like you would a rheostat but it works very differently (as a transformer). You can get more volts out than in with a variac but not with a rheostat. Not sure what schematics you follow but the basic wiring is really quite simple. The coil of the variac is connected between neutral and live. The load is connected between neutral and the slider. Its mains voltage, so be careful.
@@TheHWcave okay, thank you very much for your answer! Regarding schematics: I basically googled "variac pinout".
what happens if your input is less then 240v lets say 20v? what happens to the out put load.
The VARIAC allows selecting anything from 0 to 115% of the input. If you feed 240VAC it will allow selecting 0 to 276VAC output. If you feed 20VAC it will allow selecting 0 to 23VAC for the output. It can produce slightly more output voltage than you input because its a transformer and because of the way the coils are wired (its an option you can chose when you wire it up). Its a different story for the panel meter. That one needs at least 60V AC or something like it, to be able to work. It can show 20VAC at the output but it can't work with 20VAC at the input. So while the VARIAC will happily work with 20VAC input, the panel meter will stay dark.
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No, I have never worked with these unfortunately
@@TheHWcave thanks
und keinen trenntrafo mit der gleichen va leistung mit eingebaut???
There is an easy explanation: I did not had the money for buying an isolation transformer at that time. Also, these are big! It would have been very tricky to fit both the VARIAC and the isolation transformer into the same enclosure.
what about inrush current?
If you are asking if the VARIAC itself has an inrush current, the answer is probably yes, but it is not significant. I have not noticed it and the fuses are still fine. If you are wondering how the VARIAC deals with inrush currents of devices connected at the output, my normal usage is to have the VARIAC turned to the lowest setting (4V or so) and then turn power on. Then increase voltage slowly. So there is actually no inrush current. Most of the time I use the VARIAC to test new builds, or repairs, so going slowly and stop immediately if something looks wrong before any damage is done is key. Occasionally I set the VARIAC to 115V and then turn power on to see how well devices behave when used in 115V countries. I'm using small devices, not massive transformers or motors, so I have not found inrush current to be a problem.
@@TheHWcave Thanks for the fast reply. Yes I was asking about the inrush current of the variac itself. I suppose if you turn on the power while the variac is set low, the line voltage sees a large inductance, so the in rush current is small. And using small loads on the output simulates a open circuit at the output, limiting the current. So i suppose you are correct that the way you use the variac is what limits the in-rush current.
I just saw Marco Reps' new video about needing a soft-start for his 3KW Variac because the inrush routinely trips his home circuit-breaker. Check-it out if you have not seen it. Well, mine is just 1KW and the DC resistance of the winding is about 12 Ohm vs Marco's 2.something. I suppose that explains why I don't have an inrush problem
Less "building" a variac and more "sticking a variac in an enclosue". :P I want to build my own from scratch.
If you’ve got a source for this, please let me know
Price
currently £121 (inc VAT)
When you had it connected to a heater and got up to the 3.5 A that was only 682 Watt that I saw on your meter because of the lower voltage at 189 so it really is not the 1000 Watt you have in your video title... Is it capable of putting out the 1000 Watt or come anywhere close to that?
I calculated what it should be if you could get the 1000 Watt at 189 Volt, you should have 5.29 Ampere.
At 270 Volt you should get 3.7 Ampere to get the 1000 Watt rating, because of how a Variac is constructed using a single wire with the same diameter I can understand why you can't draw more than the maximum current of 3.7 Ampere at any voltage, still a bit strange that it doesn't follow the law of energy when the voltage goes down the current goes up.
And why doesn't it go down to zero volt?
Hi Ricardo, good points and maybe I should have added an explanation.
The specs of the VARIAC are here: carroll-meynell.com/variable-transformers/item/variable-transformer-mvt1k-3-5a
It is rated for 3.5A. The current capacity of wire before it gets too hot or even melts is independent of the voltage. If you look at the AWG charts or equivalent to see how much current a wire of a given diameter and material can carry. This transformer can handle 3.5A, or 240V or 1000VA (not Watt !). Its the limit you reach first that counts. Even if the voltage selected is just 24V instead of 240, I can't draw more than 3.5A current or it will get too hot. Similar at 240V and 700W active power the current is only about 3A but at a power factor of 0.7, the apparent power is 1000VA. This is one of the reasons why I like having that panel meter in there which shows power factor and apparent power.
The reason that it does not go down to zero is simply a mechanical limit. The pickup wheel is designed to always stay on the wire coil. The rotating arm has a spring which presses the wheel against the coil with quit a force. To go down to zero, the wheel would have to leave the coil and drop a millimetre or so down onto the carrier. Rotating back to non-zero means it would have to mount the coil again. Its a bit like a bike going from pavement down to street and then back up the pavement. These bumps would over time damage the coil and wheel, so there is simply a mechanical stop that prevents the pickup wheel from ever leaving the coil even if that means there are always a few turns that cannot be reached on either end.
@@TheHWcave Okay thanks for the explanation and the link to the specs site.
I would add an extra indicator lamp or an extra switch or both to make it visible so you can see when there's an output because it's not isolated from the mains which makes even the 5 volt dangerous to touch, I really recommend you to make some sort of dead man switch or an extra indicator that is very visible to you when there's output just to help yourself to not make the mistake of touching the live wires, believe me you can better prevent any accidents from happening and take it from me that if you work enough with it you'll get careless eventually and you may get killed because of it.
Another option is to get yourself an isolation transformer and use that to isolate yourself from the mains.
I appreciate your concern but the power switch has a light and the panel meter's red LED 7-segment digits are also quite conspicuous. Also I purposely installed only a normal UK power socket as output and not 4mm banana sockets.
I do have a make-shift isolation transformer next to it (there is a video about it) which used to be more of less permanently connected to the VARIAC. I say used-to because I now have a proper isolation transformer that took its place. Maybe I make a video about that one as well.
@@TheHWcave as long as you're aware of what you're doing it's okay, just saying that even the best of us make mistakes so be prepared for it.
Very true. Mains is always dangerous and even an isolation transformer can still have 240V or more.
dude its a great project and executed very well but you didnt build a variac. you simply bought one and put it in a pretty box! im not dissing you by any means but your title was to me clickbait and very misleading. for example look up a scariac and you will see what i mean. peace!!
All you did was change the case, you didn't build a variac.