EEVblog 1622 - The BIG BEGINNER MISTAKE with Multimeters
ฝัง
- เผยแพร่เมื่อ 26 มิ.ย. 2024
- This is the biggest mistake beginners often make when using a multimeter ohms function.
A customer returned an EEVblog BM235 multimeter because it was "Faulty"
Was it? Let's find out what happens when you try and measuse a CAN bus with a multimeter.
Forum: www.eevblog.com/forum/blog/ee...
00:00 - Don't measure a CAN bus with a multimeter!
01:50 - Trying different multimeters
05:21 - Oscilloscope CAN bus measurement
06:30 - BEWARE measuring resistors in-circuit
08:16 - Power supply voltage example
If you find my videos useful you may consider supporting the EEVblog on Patreon: / eevblog
Or with crypto:
BTC: 33BsprBQNBtHuVzVwDmqWkpDjYnCouwASM
ETH: 0x68114e40ff4dcdd384750500501e20acf3875f8c
BCH: 35n9KBPw9T7M3NGzpS3t4nUYEf9HbRmkm4
USDC: 0x68114e40ff4dcdd384750500501e20acf3875f8c
LTC: MJfK57ujxy55su4XicVGQc9wcEJf6mAoXF
Web Site: www.eevblog.com
Store: eevblog.store/
Other channels:
EEVblog2: / eevblog2
EEVdiscover: / eevdiscover
T-Shirts: teespring.com/stores/eevblog
#ElectronicsCreators #Multimeter #Tutorial - วิทยาศาสตร์และเทคโนโลยี
![[UNCUT] ฝันรักห้วงนิทรา | EP.8 (4/4)](http://i.ytimg.com/vi/RDYJYiDwsj0/mqdefault.jpg)
Not for nothing, the guy may have thought the CAN bus was off when the key was turned off. I diagnose CAN faults fairly often and have helped other guys understand this same issue. They turn the key off, check the bus, and see some insane value on the meter, and start tearing their hair out looking for the culprit. Not knowing that since the batteries are still hooked up, the CAN bus is still energized, although not to its usual 2.3 - 2.7 V range (the trucks I work on typically have 100-200 mV still present on the bus with the key off). If they didn't know any better, they'd see 150mV or so, assume its phantom voltage or just a rounding error close enough to 0, and try to check resistance anyway.
Measure voltage first, always.
You’d like to think an auto-elec, or even a mechanic would know a lot of systems run with the key off. Seemingly not!
@@ammocraft Many electricians I know (or have encountered) don't know anything about electricity. They can pull cables and fit off switches and sockets but quiz them about electricity itself and they have no idea. It would not surprise me if auto-elecs and mechanics don't understand electronics as far as vehicle integration systems are concerned.
@@sw6188 Some of the smartest people make some of the dumbest mistakes. We're not robots, we're human. Thanks for posting this video, Dave.
@@ixfxi I was replying to the original comment, and my points stand - there are electricians out there who know jack sh*t about electricity. I have met them.
At the railways some collegues used a fluke meter to measure the resistance between two sections of rail that should be isolated from each other after I claimed the isolation was bad.
When their fluke DMM was showing OL they were smiling at me saying "You see, it's completely isolated". I asked them to reverse their probes and they started laughing even more at me, but I insisted and now their multimeter was showing a negative resistance ... they didn't understand what was happening until I explained them how a multimeter measures resistance.
After breaking the concrete arround the rails at that spot they found a red hot ball of steel connecting the 2 sections (clamp meter showed 80A).
PS : this was between a section that has a 3000V DC catenary and a maintenance workshop that is not electrified (rails are earthed in the workshop)
How these people got their job at all? Do they even know how isolation is actually tested? Have they heard of isolation tester?
@@mini_tamm It's not like they are stupid and know nothing about electricity, their main expertise is high voltage traction systems and I certainly learned a lot from them.
I wonder how you would have measured that knowing that you cannot disconnect anything.
The main problem here is that one section is earthed at the workshop and the outside section is grounded somewhere else.
There are specialised meters to measure this but they don't have those at the workshop because they have no use for those. Checking/maintaining the insulation between rail sections is the work of the network operator, so they had to call the network operator who came and fixed the problem.
At first I made a "lucky" guess (too long to explain what hapened to make that guess) but I was able to convince them by using an IR camera (thermal imaging).
@@ppdan Yeah of course you can't measure if you have voltage on the lines. But as you said they didn't understand even how multimeter works. That wonders me. But I have no idea where or how or what you measured, but interesting anyways. Thanks for sharing.
@@mini_tamm They didn't know nor wondered how it exactly works, but after being taught they completely understood what is going on.
Their title is "electrotechnician rolling stock" then there was a very small group which I was part of "electrotechnician electronics".
Might sound weird but electronics is a recent "phenomenon" at the railways. Before that it was "simple" thyristor choppers or camshafts driven by a servo activating/deactivating resistors on series traction motors.
Negative Ohns should mean I get more current out than I put in, yeah?! Free energy!
Also known, of course, as "mhO" before your boring Siemens were adopted!
It just means you get have a voltage opposite the polarity of the current flowing through it mate.
Such as for example, on every power supply 😂.
Not necessarily.. I don't know how it works, but you get negative resistance in fluorescent lights if I remember correctly. An increase in the voltage across the terminals results in a decrease in the current going through it.
Again, I don't know how it works, but it's not free energy unfortunately 😡.
negative debt
@@phizcThe opposite. Less current, more voltage
I was taught to disconnect the battery from vehicle prior to checking CAN termination resistor integrity.
How often do problems with CAN bus termination happen?I am curious since I never measure automotive CAN bus systems.
@@PyroShimnot so much the termination resistor giving problems, but a break anywhere in the CAN circuit on one side can cause an issue, first check is to grab the can lines and measure between them with battery disconnected and see if there's 60 ohms. If you measure 120 there's a break in the circuit as both termination resistors are in parallel.
At that point you start splitting the circuit by unplugging modules and measuring circuit integrity..
@@mattyb7736 Thank you for the explanation!
-306 ohms. Thats better than a superconductor, it's a superduperconductor
I shall call it LK100, I'll be rich.
Free energy finally!
No, its measuring in the mirror universe 😂
Maybe somebody snuck one of the new super batteries into the circuit being hyped by Matt “Fraudal”.
Who knew the automotive CAN bus was so advanced!
Old multimeter works probably because it gives a high current on low resistance, and then the bus drivers are unable to load it much to change the display, and also likely there is a low pass filter on the ohms input to damp the changing voltage it is already getting, and averaging it out. Likely will be as fast as a wet weekend when autoranging as well, and also take a good few seconds to settle to a steady value with the plain resistor attached.
My good old trusty Fluke 87 True RMS works great to this day using a 9V battery. hehe
After something close to a decade of following the channel I almost always learn something new here.
Dave using the oppotunity to show off his huge collection of Multimeters 😁
Yeah. That made me chuckle...
Can never have enough multimeters.
IKR, I'm so bloody jealous. I don't even have a decent one newer than 40 years old. (cry)
I know right? I was thinking he would stop after 2 or 3 just to make the point... but noooo.
@@marcfruchtman9473 probably (and rightly) rubbing it in for the guy who returned a meter that he clearly had no idea how to work.
Like @fredfred2363 said, it made me chuckle as well, funny!
Negative resistance just means you're connected to a power source (current out instead of current in hence the "-")
*Hard work beats talent when talent doesn’t work hard.*
As of today I "knew" that you cannot measure resistance on live circuits and also not in circuit, but now I know! Thanks!
It may be possible that your customer had no idea he was working on a live can bus. Sometimes just the presence of the key in the vicinity of the car is enough to wake up the network.
I think he was just confused because the multimeter does not react the same way as some others. Diagnosing can is fairly simple but if a meter would show 0 or some hundreds of ohms I would be thrown off as well because I am not used to the meter reacting that way to live bus
No. When Dave explained you can’t measure resistance in live circuit, the customer replied ‘he knows how to measure it correctly. Suggested Dave Googles it if he lacks that knowledge. Stop assuming it’s someone else’s fault. That’s not how customer service works.’
Maybe the cheap meter overwhelmed whatever currents existed in the bus and thus came up with a reasonably accurate number
I always love videos like this. It really helps challenge your concept of "obvious". I always use this kind of mistake as self reflection, especially if I am in a position of teaching people material that I consider obvious or relies on material that I consider obvious.
Neophytes in the automotive world often reach for resistance testing as a surrogate for voltage drop testing when assessing wiring integrity. Problem is, you may not know the details and complexity of the circuit. With active drop testing you better assess the circuit under its working load, you don't have to disconnect it all from power, and you don't need to know as much about what else is connected.
Agreed. Doesn't help when the service manuals show resistance checks on massive swaths of harness. Having said that, the CAN resistance check is a valid preliminary diagnostic when performed properly.
Also agree. Doesn't help the procedure writers love to add resistance tests in the first few steps of their pinpoint tests. They also expect you to disconnect the 12v and unplug difficult to reach connectors to do this.
Yep, Bob Andersen (bandersentv YT channel) got bit by this in a recent video where he was measuring resistances in an old tube-type TV. He had dipped the focus coil (which had a rusty steel case) in Evap-o-rust, and it caused a chemical reaction between the steel case and the copper wire, which turned it into a battery, which led to wildly inaccurate readings in ohms mode. Never measure resistance/continuity, if voltage present in the circuit! 😊
Thanks for typing this, I saw the video and this was a valuable lesson. I was thinking it was like there was a battery in the circuit somewhere, but never made the connection with electrochemical corrosion.
In IT I deal with this all the time, the problem of low level tool vs high level tool. Now you could make a tool that would measure this bus across many scenarios - yet it would be specific and useless in undocumented cases. Then you can also make a low level tool like meter that requires the thinking and process at the side of the user. Since most of IT is abstract, the outcomes are even less obvious than a mistake when handling ignition vs battery.
Hate to admit it but this trap has caught me as well, particularly when the circuit you're interested in is turned off but there's still something downstream turned on interfering with your measurements with like a ground loop or something, a bus is a great example, you only have to forget to turn off 1 device on it.
But I thought for sure you would have demo'ed this on a bench DMM where you can adjust the NPLC in resistance mode. When the NPLC is very small the meter can read the resistance of the bus termination resistors in between bus packets. But when the NPLC is set to a long duration the packet transmission interferes with the measurement. I would guess the cheap-o meter has a really short (or no) time constant RC filter in the measurement circuit?
Also I think the biggest risk trying to measure resistance in a live circuit is actually damaging the device you're probing. I was probing a PCB I was bring up once, got distracted, forgot to unplug it, probed a node in the gate driver of a transistor in continuity mode and *BANG!* straight through short in the H-bridge (was only 12 V).
The NPLC bench meter stuff is interesting but beyond the scope of a beginner video like this I think. And most people don't use a bench meter.
-Ohms display is really handy! Then you instantly know something is wrong. I'd be upset to have a meter with zero reading in cases like this!
Apprenticeships always used to teach the basics to kids aged 17+ both theory and practice combined. You were taught how all the tools worked and how to care for them and how to use them. I still draw from the knowledge I gained 40+ years ago during my apprenticeship. Measureing resistance on a live circuit with a multimeter was a simple 'no-no' period. So many engineers I meet don't know the basics let alone logical fault finding techniques. It's very disappointing.
The one that gets me every fucking time is when ohms/continuity mode turns on a transistor some where in the circuit
That whole "in circuit measurement" thing strikes again. :)
Ohms range on my old Fluke 77 won't turn on silicon junctions. Not so sure about germanium.
When I was young, there was no diode test range in the multimeters we had then, so we used the ohm range for transistor and diode tests. Actually very useful.
I got caught out on an even worse one, using diode check to measure a mosfet. A mosfet with a very low gate threshold voltage.
I unknowingly charged up the gate by measuring it first, then noticed the mosfet was shorted drain-source. Switched the meter to resistance, and it measured 0 ohms.
As it was out of circuit, the gate capacitance kept a charge, which kept the mosfet in its on state.
But then I discharged the gate by touching all three terminals at once with my fingers, and the short disappeared.
Of course this is all expected behavior knowing what was going on, but I didn't initially think about it and made myself real confused for a bit!
I guess some meters limit the lower bounds to 0, but the advantage of showing negative values is it gives you a good idea somethings wrong.
Gotta admit I've never seen negative resistance measured before!
Same thing would happen if you try measuring a diode in forward direction: different multimeters will measure different "resistance". And this will not be any fault of any multimeter. And all those resistance values will be a pure nonsense due to the forward voltage of the diode interfering with multimeter measuring the voltage drop. Same goes about measuring any semiconductor with ohm meter. I always cringe when I see a video of some laptop repair, where the author is measuring "resistance" of this and that on the motherboard... (sometimes though it may make sense when checking for difference between things that should be similar)
I watch those videos every single day and it works for them. Maybe you could elaborate more in the comments section of those guys
Isn't to first rule in electronics class to never measure resistance on a live circuit since it's the best way to risk frying your meter, the circuit or both? This even counts for low voltages.
I thought the first rule is to not clap you hands behind someone as they are probing around :)
@@dimitriapproved😂
or let a door slam shut, which in my innocent youth I once did in a switch room as 2 electricians were trying to isolate some fault 😱
I remember when we did our finals my co-student asked me why he can't measure a resistor in a live circuit. I was so embarrassed that he didn't know the answer after 3 years of EE.
When I was in a senior EE class, we were going over the test answers and a student asked how we were supposed to know that a op amp input offset current was DC.
Well as long as you know exactly what you are doing it's not impossible to measure resistance in a "live" circuit - as long as the switching noise in your circuit is fast enough and has a constant average DC offset you can still get a decent measurement by averaging the voltage with and without a current applied - but if the averaged resistance you get by this calculation is actually what you wanted to measure is of course a different question 😊. An even simpler "trap" is trying to measure a resistor in circuit which happens to be parallel to another resistance...
I tutored Physics II, which was mostly electricity & magnetism. A problem involved drawing a schematic and describing what each component did. One student drew it with a dead short across a capacitor.
I did 2 years of EE degree at Portsmouth university in UK before I dropped out.
My lab partner who had successfully passed the first year with me didn’t know that a resistor was high resistance and a copper wire was low resistance (compared to each other).
So I spent our lab time teaching him the basics. I think he didn’t like it but for some reason it didn’t register in my mind at the time… or maybe I was unable to proceed till he understood more.
@@davadoff First, I should say, my degree is in chemical, not electrical, engineering. But I'm an experienced electronics technician, "Radio/TV Engineer," and have taken some EE classes. I find it inexplicable that many EE students, many "upper division," don't know things I understood when I first studied electronics, as a teenager.
This needs not only a RTFM, but become literate enough for reading to mean something.
Not mentioned in this video is how some DVM's are low voltage ohms ranges, in order to measure passive resistance without exceeding silicon junction conductance levels, whereas others are higher voltage in order to get more precise measurements of low resistance paths ... that need to be free of diode junctions that may forward bias.
Certain models may mix those traits, and run low voltage on k & M ranges, and high voltage on low ohms just as on diode test functions.
A good lab for EE students might be to test DVM's in such a way that they can predict measurements of live circuits with certain source or sink impedences and signal waveforms, or with a diode (including as part of a transistor junction" in certain placements relative to distributed resistors.
MOSFETs, Triacs, or SCR's, wouldn't be fair to include, as they can have less linear or more extreme triggered state conditions.... unless they were on the advanced exam, and students were expected to show comprehension and awareness by stating those test conditions could not be evaluated as shown.
When checking the resistance of a component in-circuit, I like to check the voltage across it before checking the resistance. That way, I'll know if I didn't properly turn the power off or if there's a capacitor nearby that hasn't fully discharged. It doesn't guarantee parallel components won't mess up the reading, but it makes it less likely there'll be an issue.
And even if the capacitor is discharged, the meter will charge it, making it seem a lower resistance that's slowly rising as its charge rate decreases.
@@gblargg Yeah, it's not a perfect method, but it would've prevented the situation in the video. If I measure a resistive component this way and it's close to what I expect, I figure the component is almost certainly good. If I measure it and it's off, then I'll consider what I need to do to get a more confident reading, such as desoldering at least one side of the component.
Dave’s knowledge is only surpassed, by his vast collection of meters! 😊
Or ability to talk fast! (TIC!)
@@martyyoung598 This is funny! 👍
High end bench top multimeters will specify the test current applied when making resistance measurements. Some will even have a way to configure it for a reduced test current mode for lower self-heating in the resistor under test.
The CAN bus has either dominant ( when the signals are actively driven ) and recessive state ( when the bus is not being driven ). Even with a powered bus, when there is no activity and the bus is in a recessive state, you should be able to measure the termination.
It could be that the old multimeter doesn't produce a high enough current that the bus actually thinks that something on it tries to communicate OR produces a high enough current that the rest of the devices think that they have lost arbitration and stay in a recessive state. Pick your poison, bet's on the second.
-302 Ohms! Holy crap, you've discovered something even better than room temperature superconductors. This perpetual motion like effect will mean free power for everybody! Hooray!
Hey, if that guy doesn't want his multimeter back I'll buy it and pay for shipping today. Just let me know if you send the information and the money. I kicked myself for not getting one when you first brought them out.
You can often get away with measuring resistors in circuit in vacuum tube circuits. When the filaments are cold, the tubes are open circuit. Just make sure the capacitors are discharged first.
R A Penfold's two books on using multimeters include hints about making sure circuits are not powered when measuring ohms, and being cautious about semiconductors getting switched on. It also includes a warning not to deliberately overload a meter - mainly referring to analogue meters and a friend of his who worked at a supplier who were receiving meters with overload damage. Some customers had tried, reading they could withstand 250V AC, connecting them across the mains switched to various ranges. The protection only worked on voltage ranges, not resistance and current - the specification made this clear but the customers did not read it. Of course, your tests with overloading are part of reviews - making sure things are up to standard, but the ordinary user should not overload a meter deliberately. One of Penfold's rules I still follow on the rare occasion I am working with an analogue meter - if it has no clear "off" setting and leaving it with the leads attached, set to a high voltage range to minimise the chance of damage if someone connects it without checking.
Not even an electrical engineer, and as soon as he said the circuit was powered on I knew *exactly* what the problem was....
....Ohm's law, people. Learn it, love it, live it.
I can't imagine ANY technician or engineer trying to measure resistance with the circuit "live"! I don't know what kind of protection circuits various modern meters are likely to have, BUT...In the old days, you not only knew you'd get an erroneous reading, but you could expect a "snap, crackle, pop!" sound from the meter, as gray smoke wafted up from out of the banana jacks!
The customer insist he knows what, here's his response after I explained how in-circuit measurements work:
"Thanks for your information. I am auto electrician, I know how to measure CAN resistance correctly. You can google the right way to measure CAN resistance if you lack this knowledge.
Before you understand the whole thing, please stop assuming it is someone else's fault. That's not how customer service works."
@@EEVblog Okay Dave, now you know how customer service is supposed to work. No winner winner, no chicken dinner for you!.
@@EEVblog Nothing feels more frustrating than fighting stupidity and then losing....
@@EEVblog Stupid people don't know when to stop. They know everything that needs to be known and will beat you with their knowledge.
Damn I hate people like that. He messed up and should own it
Likely the tech taking the resistance reading isn't aware of burden voltage/current either. It's just another trap for young players.
Negative resistance!!! Dude! You've discovered free energy!! ⚡⚡🎶🎶🎶👍👍😁😜
We need to talk about your DMM hoarding disorder
Im hoping its very contageous!
New to electronics and learned a valuable lesson. Thanks.
Not only does live voltage on a line interfere with the meter's ability to measure resistance, the meter might mess up the circuit it's testing. Particularly delicate instruments can even react to the change in capacitance of the circuit from adding the probe wire. If you don't already know pretty well how the system works, live probing it is usually a bad idea. For an example of the kind of odd and difficult to predict interaction, look up "MIT More Magic".
I work in the field of access control systems. And when checking multi-apartment intercom systems, without knowing the specifics of the system, you can measure a lot of different things with a multimeter. Especially when you are looking for a current leak or voltage coming from nowhere where it shouldn’t be. And only with experience do you understand that it is not enough to measure resistance, you need to carry out the entire cycle of measurements: voltage, voltage in LowZ mode, only then, making sure that there is no dangerous high voltage in the circuit, measure the voltage drop and measure at high resistance limits. And only then will it become clear whether the circuit is intact or not.
CAN bus (or any other 'in-circuit') voltage offsets what the Wheatstone bridge uses for it's reference voltage (if it uses one but how else?) in the meter, which alters the meters 'accuracy' (calibration?).
Thanks Dave for all your efforts and sharing!
It's easy to avoid this, always "park" your volt meter on a "volts" setting. That way it'll be safe.
We had a similar rule with analog meters, though we went one better--always park on highest range ac volts. With AVO 8 meters , it was the "off" position instead.
@@bryanwheeler1608 If an analog meter has an "off" position you should use it. The off position puts a short across the needle which dampens movement and makes it more resistant to mechanical shock.
Bravo!!
Somebody is going to learn something today.
For measuring Ohms on a resistor in (not live) circuit there's a simple rule by which you can filter out wrong results, because a faulty resistor has really almost never a lower resistance: when you measure a lower resistance as stated on the colour code, you cannot trust the result, and you have to dé-solder one side and measure again. When the result is equal or higher than the colour code states, you can trust the result.
I have always thought that it would be nice if every meter came with an explanation of how each kind of measurement is performed. Indeed, this seems like essential information, as the instrument always has the potential to interact with the circuit being tested. They don't have to reveal secrets.. just show us a simple model of the measurement. Otherwise, I have to assume that the meter is a magic box and its readings should *always* be reliable.
You can also get a negative resistance measurement by reversing the 4 wire current with the voltage sense.
I would love to hear you explain CAN watchdog circuits Dave ! It would be another GREAT learning tool for many
Some high end 3D printers also use CAN bus for the various circuit boards to talk to each other. But they borrowed the protocol from automotive.
Horses for courses as specialists meters will be set up to give readings you really want.
This is why I have 4 multimeters in the van for different tasks.
So have Automotive, electronics (accurate), industrial & high voltage as working on automation including lorry & industrial.
So many people don't realise these issues as they buy the multimeter from the industrial wholesaler which only sells limited amounts or the company they work for supplying so they make sure it does not come back!
So most people are totally ignorant but this will become an issue as these dedicated meters are expensive against buying online!
I learned this by having my meter on the wrong setting before they had the fancy saftey feature. Used to have to tear apart the rig and hope you only blew a fuse! I did not know the new meters protected us bumble heads.
As soon as i saw the meter readings I knew 😂 - trying to do a resistance test on a live circuit.
Catches a lot of people out because they dont know how dmm's work.
They work by magic, right? You put the magic switch to ohms, and then tell it which object to measure by pointing at it with the probes. Pointing has always been part of casting spells.
@@johnpublic6582 It's magic, definitely all magic. I'm amazed more people don't know 😂
Also, ghost phantom voltage can give false and odd troubleshooting results. 😘 Thank you for the great video.
FIrst learned this stuff on the old Simpson 260 when going through 'A' school in the service. More than one meter fuse blown in our class, lol
A common problem with measuring the terminator resistance in a vehicle is that the CAN bus can be live even when the vehicle is off. One fault that causes “parasitic draw” is that some module does not go to sleep at all after key off. You have to make sure the CAN bus is totally dead before doing the measurement. They make a breakout box with LEDs on it to help with this. Usually disconnecting the battery will kill the bus comms, but then it requires a drive cycle to prep it for a state inspection in the USA, so disconnecting the battery is sometimes not desired. The little oscilloscopes are very handy for many tasks, and chasing CAN bus problems is one of them.
Great video.
Auto sparky here, and yeah its always important to ensure all devices on the network were shut down before checking the network integrity
thanks
So basically, there is still a missing feature on multimeters - clear indication of foreign voltage in ohms mode (which should be easy to implement - just key off the measuring current periodically and check if there is voltage :) ).
DMMs usually inject a constant current and measure the voltage hence it jumping all over the place trying to measure the output voltage. To be fair that's mentioned but it then turns into the voltage the meter is outputting for most of the video.
I think the cheap multimeter put a high voltage on the datalines of the bus which caused the other components to disconnect themselves from the CAN bus for damage control. It is probably powered by a 9V battery. That way the cheap multimeter could measure the correct termination resistance. Good brand multimeters do ohms reading by supplying a much lower voltage, which does not shut down the other components on the CAN-bus (so they interfere with the measurement).
I always wished for a multimeter that would be able to measure volts and ohms simultaneously (series resistance to the voltage). So you could measure if a pin is active high or just has the pull-up resistor enabled, in that case voltage only measurement would measure the same in both cases.
I made the mistake of measuring a resistor in circuit.
When I was 13 years old and still learning.
CAN bus > CAN't bus
You can measure resistors in circuit to see if they are burnt out or not. If it says 10k, and you measure practicaly anything higher than that, there is good chance that resistor is dead. Not the most precise method, but most of the time it works.
If you're sure there's no voltage and the in-circuit '100Ω' resistor measures 1433Ω, then you can be pretty sure the resistor is scuppered.
I didn't know. I bet this saved me quite a few hours of struggle some time in the future. Unless I forget, will try not to!
I am a block of noob so I like this. Thanks mate!
The amount of fuses I’ve popped in my fluke meters by switching to ohms and measuring live mains power while forgetting to kill the circuit. I just keep a box of fuses on the van now.
The tech probably thought that the bus was down because the key was off. Best to disconnect the negative battery cable before this test.
''I have too many multimeters.''
SAID NO ONE EVER!
It is so simple. Do not try to measure resistance when the circuit is powered or signals are present, there might be voltage across the resistor that will affect the reading
Moral of the story: if you get an unexpected and unreasonable reading while checking a resistance, switch to both AC and DC volts ranges to check if voltage is present. That is especially true when trouble shooting power supplies where capacitors may hold a few tenths Volt charge for quite some time, not behaving like the effectively open circuit you’d expect a capacitor to be.
Isn't that taught in schools anymore?????? Damn, this world is being turned into a mess....!!!
Thank you.
Didn't know most meters have that protection.
Doesn't mean I'm going to put it to the test though......😊
Lucky enough both my brother and my sister' husband were in electronics, one at Marconi and the other had a municipal position. They made sure I was acquainted with that stuff when I was 14.
A multimeter could perhaps be made to sense any voltage in the circuit when in ohms range, showing "1d107" at the user.
Protek 608 low Voltage Ohm range .25V is very handy for use on un powered circuit.
It’s the same when measuring an IO pin on a microcontroller. You get a funny number depending on what’s going on and if it’s hi low or tri state high impedance, etc.
I learned this the hard way 💥 😅
@EEVblog you should upgrade that Fluke 87V to a Fluke 88VA. ;) I use it daily in car diagnostics. It doesn't replace an oscilloscope, but it is still fast. Great video on measuring live lines! I learned the hard way...
Some semi's will power up if you apply miniscule voltages only to their GPIO pins, TI micros spring to mind.
You did it! Room temp superconductor 🎉
I call it LK100, I'll be rich!
If the ohms reading is strange put it on DC and AC volts to see what that is, if its not zero don't trust the ohms reading.
I noticed too that CAN BUS must be off in order to measure correctly. I have PeakTech multimeter.
When the CAM operates, it has a Digital signal above 0 volts with pulses Above or Below the no-signal line.
I wonder if negative Ohms means the electrons are falling out?
I like those probes at 7:18.
This(canbus/auto) really is the killer use case for the cheapy DMM/Scope combos...
Well d'uh. Try measuring the resistance of the mains power supply while it's supplying power...
My 34461A measures negative resistance on larger inductors if it it's in auto-range mode, it causes some funny oscillation there and that messes up the reading. Haven't found anything about limitations like that in the manual and it sure confused me a good bit when I wanted to make, what I thought to be, a trivial measurement.
Thumbing up 👍
Continuity on a live circuit? Exactly my thought, no one ever uses multimeters on ohms/continuity on a live circuit
I had a false result when I measured a possible conection betwen two plates, the multimeter indicated a negative ohms result, I puted in milivolts scale and comproved that was a galvanic pile effect.
Maybe a video about CAN buses?
yep, that's the load impedance of an active circuit.
Damn. I was hoping that -ve resistance was an amazing over unity discovery 🙂
I think you are correct, he must have turned the power on or turned the key on and didn't realize it.
I remember having an old analog meter that you MUST turn off ohms when measuring voltage, because it would blow a fuse if you didn't.
Or not. I can use my key fob to talk to the wireless module when the car is off off. The wireless module then talks down the CANbus to the door modules and unlocks and puts the windows down. The only "off" in the car is with all batteries removed and caps drained.
@@johnpublic6582 Doors usually run on a low speed CAN or LIN bus.. The high speed CAN bus at the DLC should be off if the key is off. But even the low speed CAN for the doors will go to sleep after a few minutes.. Wireless fob and remote start will usually be on it's own dedicated BUS.. He could have turned on the key, or sometimes just opening the door will trigger modules to turn on. It all depends on what he was actually testing. Some vehicles have several more different networks.. and some do everything on the same network. It all depends
@@calholli Exactly. It depends. Up to the tech to actually know what is going on, how to use the tools, and how to understand the test results.
I think if you know the principle of a analog multimeter you know that you can't do a measurement if the circuit is powered. With an analog multimeter you have to adjust the needle if the internal battery loses it's power.If you understand this than you know. But this is my opinion.
Multimeter 101 for sure.