EEVblog 1622 - The BIG BEGINNER MISTAKE with Multimeters

[@Breakstuff455khz]

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.

[@nikize]

Measure voltage first, always.

[@ammocraft]

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!

[@sw6188]

@@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.

[@ixfxi]

@@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.

[@sw6188]

@@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.

[@ppdan]

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)

[@mini_tamm]

How these people got their job at all? Do they even know how isolation is actually tested? Have they heard of isolation tester?

[@ppdan]

@@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).

[@mini_tamm]

@@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.

[@ppdan]

@@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.

[@SeanBZA]

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.

[@BillAnt]

My good old trusty Fluke 87 True RMS works great to this day using a 9V battery. hehe

[@kirkhamandy]

I think you maybe right. If you look at the image of the old multimeter it says "Auto Range" meaning, I guess, it's made for car mechanics who might well use it to measure the terminating resistors. It may in fact be designed that way with the manufacturer knowing exactly who is using the product. If so, that's pretty smart ;)

[@SnabbKassa]

Negative Ohns should mean I get more current out than I put in, yeah?! Free energy!

[@Xaelloss]

Also known, of course, as "mhO" before your boring Siemens were adopted!

[@islandfireballkill]

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 😂.

[@phizc]

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 😡.

[@calholli]

negative debt

[@jaakkolehto1487]

⁠@@phizcThe opposite. Less current, more voltage

[@clintondekock9935]

Dave using the oppotunity to show off his huge collection of Multimeters 😁

[@fredfred2363]

Yeah. That made me chuckle... Can never have enough multimeters.

[@marcdraco2189]

IKR, I'm so bloody jealous. I don't even have a decent one newer than 40 years old. (cry)

[@marcfruchtman9473]

I know right? I was thinking he would stop after 2 or 3 just to make the point... but noooo.

[@marcdraco2189]

@@marcfruchtman9473 probably (and rightly) rubbing it in for the guy who returned a meter that he clearly had no idea how to work.

[@georgedavall9449]

Like @fredfred2363 said, it made me chuckle as well, funny!

[@spelunkerd]

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.

[@radman999]

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.

[@adaycj]

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.

[@chitlitlah]

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.

[@gblargg]

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.

[@chitlitlah]

@@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.

[@darranrowe174]

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.

[@Joetechlincolns]

I was taught to disconnect the battery from vehicle prior to checking CAN termination resistor integrity.

[@PyroShim]

How often do problems with CAN bus termination happen?I am curious since I never measure automotive CAN bus systems.

[@mattyb7736]

​​@@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..

[@PyroShim]

@@mattyb7736 Thank you for the explanation!

[@Broken_Yugo]

Its a step a lot of people like to skip.

[@RichardCorongiu]

​@@PyroShimyes they do. The ones I've seen...someone tapping into a CAN wire for power !!!... fairly often there is one 120 Ohm in a module and the other external. ..be careful if you read 120 Ohm ..DONT conclude it's a resistor (at either end) ...more likely wiring break You should follow wiring down CAN High and CAN Low separately...you might suddenly jump from zero Ohms to 120. You've zeroed in on the break. Wiring diagrams are pretty useful if not required It saves time I where to start...and splices etc.

[@williamsquires3070]

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! 😊

[@ericrawson2909]

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.

[@p_mouse8676]

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.

[@dimitriapproved]

I thought the first rule is to not clap you hands behind someone as they are probing around :)

[@dreamok732]

​@@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 😱

[@RK-kn1ud]

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.

[@gearloose703]

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

[@davadoff]

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.’

[@rexsceleratorum1632]

Maybe the cheap meter overwhelmed whatever currents existed in the bus and thus came up with a reasonably accurate number

[@WizardTim]

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).

[@EEVblog]

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.

[@shaunclarke94]

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!

[@mysterycoolman198]

Any energy storage can also create negative resistance. For example, measure a capacitor and then flip the polarity.

[@ResonantFreak1]

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.

[@motivatedpeon]

As of today I "knew" that you cannot measure resistance on live circuits and also not in circuit, but now I know! Thanks!

[@natecontarino1748]

-306 ohms. Thats better than a superconductor, it's a superduperconductor

[@EEVblog]

I shall call it LK100, I'll be rich.

[@gjmi72]

Free energy finally!

[@eeknud]

No, its measuring in the mirror universe 😂

[@bwobbles2368]

Maybe somebody snuck one of the new super batteries into the circuit being hyped by Matt “Fraudal”.

[@chrisd1746]

Who knew the automotive CAN bus was so advanced!

[@MMMMMMarco]

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.

[@russellhltn1396]

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.

[@robstamm60]

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...

[@Pootycat8359]

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.

[@davadoff]

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.

[@Pootycat8359]

@@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.

[@ТарасКорж-г4т]

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)

[@silviuguseila2552]

I watch those videos every single day and it works for them. Maybe you could elaborate more in the comments section of those guys

[@timturner7609]

The one that gets me every fucking time is when ohms/continuity mode turns on a transistor some where in the circuit

[@johnpublic6582]

That whole "in circuit measurement" thing strikes again. :)

[@bryanwheeler1608]

Ohms range on my old Fluke 77 won't turn on silicon junctions. Not so sure about germanium.

[@MitzaMaxwell]

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.

[@iamdarkyoshi]

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!

[@nigeltrueman5931]

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.

[@d0wnboy]

Logical fault-finding is a disappearing skill. I work in IT and it’s now filled with a population of test-takers that couldn’t figure a thing out unless the answer is right in front of them. Ugh.

[@mc.the_machine]

Always a CAN-do attitude. Especially being down under with Uncle Bob, when his parts are flapping in the breeze. Salutes from America (USA)!

[@arenaengineering8070]

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.

[@marc_3808]

After something close to a decade of following the channel I almost always learn something new here.

[@SamiJumppanen]

-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!

[@chrisguli2865]

Negative resistance just means you're connected to a power source (current out instead of current in hence the "-")

[@mckidney1]

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.

[@yellingintothewind]

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".

[@rocketman221projects]

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.

[@----.__]

I was lucky enough to work with one of the original Bosch CAN bus designers at a robotics company years ago. Very knowledgeable and funny German fellow who used to design a lot of the robotic control systems for us. Once in a while we'd hear a loud pop from his corner of the office and then in a jovial German accent it would be followed with "oh dear" and a sustained chuckle.

[@mrcampbell7674]

I'd like to know how you address this from a business perspective. Did you send the customer a new meter? Send the old one back? Etc ?

[@EEVblog]

I authorised the return on ebay even though the meter isn't faulty, he hasn't sent it back yet. Also informed him about all this.

[@modrobert]

I assume the customer agreed to be featured in the video, if not then it would surprise me if he comes back for more. The huge differences between meters in the same setup was surprising to me even if the bad reading itself wasn't.

[@johnpublic6582]

@@modrobert Yeah, that part where he divulged the customers name, address, phone number, taxID number, graduation date, and mother's maiden name was just wrong. He should have just said, "a customer" and not given all those PII details if he wanted to include him in a video.

[@modrobert]

@johnpublic6582 The customer know he was featured when seeing his photo of the measurement, the rest of the video pointing out his rookie mistake to a broad audience will certainly not help customer relations unless he agreed to the video. This has nothing to do with leaking PII (Personal Identifiable Information), it's about the customer knowing with 100% certainty his mistakes are being featured.

[@johnpublic6582]

@@modrobert The customer needs to know he did a dumb. No one knows who he is so consent is an excercise in stupidity.

[@TrickyNekro]

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.

[@johnpublic6582]

-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!

[@Pootycat8359]

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!

[@EEVblog]

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."

[@bob1505]

@@EEVblog Okay Dave, now you know how customer service is supposed to work. No winner winner, no chicken dinner for you!.

[@kriswillems5661]

@@EEVblog Nothing feels more frustrating than fighting stupidity and then losing....

[@vincei4252]

@@EEVblog Stupid people don't know when to stop. They know everything that needs to be known and will beat you with their knowledge.

[@radman999]

Damn I hate people like that. He messed up and should own it

[@Keith_Ward]

Those Parrot test leads caught my eye. Dave or anyone have anything to say about them and if they work as well as claimed? Which sizes work best, etc.?

[@lokiva8540]

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.

[@Electronics-Rocks]

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!

[@kriswillems5661]

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).

[@cdh76]

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.

[@tgirard123]

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.

[@ItsTristan1st]

It is worth noting that capacitors retaining charge can mess with quick measurements too. If the reading is changing then discharge the caps first.

[@HL65536]

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.

[@dougn7bfs]

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!

[@311superfly]

Yes you have schooled me. My cheap probes have given me headaches. Now i test their resistance frequently before testing anything.

[@ctrlaltdel02]

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.

[@Pampali]

Wonder does Keysight U1217A in "smart Ohm" mode should do the trick... it may be confused too due variable voltage levels on CAN BUS.

[@EEVblog]

I tested this and it doesn't work on the CAN bus or on the PSU. It should work on the PSU based on the claim of DC offset up to 1V or something on the 300ohm range, so I'm not so why it doesn't. CAN is outside the spec.

[@Pampali]

@@EEVblog Yeah, that is what I expected. It think U1217A in smart ohm mod constantly switching between voltage and Ohm mode, because voltage on CAN BUS is variable it could not include the appropriate offset amendment during resistance measurement.

[@Ayleen_Hazar]

Отрицательное сопротивление открывает прекрасные перспективы для энергетики! Зачем нужны установки термоядерного синтеза, когда каждая машина с can-шиной может уже сегодня генерировать энергию без затрат! Прекрасное открытие😂

[@phantom21629]

This was literally one of the first things they taught us in electronics class. Not even kidding, day one.

[@mopar3502001]

Likely the tech taking the resistance reading isn't aware of burden voltage/current either. It's just another trap for young players.

[@photohounds]

I've never even considered measuring 'ohms' on a live circuit. It simply does not make sense to do so...

[@keresztesbotond740]

To my knowledge DMM-s apply some constant current to the circuit then measure the resulting voltage and thus they can calculate ohms. Would have been interesting to also measure what currents the meter applies in different ranges. For instance at work I was measuring with a milliohm meter (40 ohm, 4 ohm and 400 milliohm ranges) and was curious what currents it was working at. Turns out in the 400 mohm range it is applying 200 mA of current. Was surpried to see that high of a number, but I guess you need that kind of current for those low resistances.

[@tl1024]

It's easy to avoid this, always "park" your volt meter on a "volts" setting. That way it'll be safe.

[@bryanwheeler1608]

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.

[@eDoc2020]

@@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.

[@kenchilton]

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.

[@marccrane4830]

When the CAM operates, it has a Digital signal above 0 volts with pulses Above or Below the no-signal line.

[@deezelfairy]

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.

[@johnpublic6582]

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.

[@deezelfairy]

@@johnpublic6582 It's magic, definitely all magic. I'm amazed more people don't know 😂

[@calholli]

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.

[@johnpublic6582]

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.

[@calholli]

@@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

[@johnpublic6582]

@@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.

[@georgedavall9449]

Dave’s knowledge is only surpassed, by his vast collection of meters! 😊

[@martyyoung598]

Or ability to talk fast! (TIC!)

[@georgedavall9449]

@@martyyoung598 This is funny! 👍

[@teresashinkansen9402]

I heard he owns about 4% of Australia's GDP in measuring equipment.

[@georgedavall9449]

@@teresashinkansen9402 LUV this!

[@tlhIngan]

Is that board doing a LIN device or a LIN host? (LIN is a low cost automotive network as well, except lighter weight than CAN. It requires a host to query devices. It's basically a serial protocol.

[@BensSightSoundandAuto]

Auto sparky here, and yeah its always important to ensure all devices on the network were shut down before checking the network integrity

[@PhilReynoldsLondonGeek]

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.

[@thomasmaughan4798]

Yes. I leave my Simpson 260 on the highest voltage range. It does not have an "off". Some models do, and they short circuit the meter movement to damp its movement when being carried.

[@ka7znm]

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.

[@erikdenhouter]

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.

[@lucdesvaux8230]

I have a similar issue but when messing an AC voltage.I just came across this error with my multimeters. The first, a BM869S reads 0v when messing 230v AC and the other one (also a BM869S) reads 156v AC. On DC they both ready incorrectly (3v instead of 5v on a lab psu). Could not understand why. I have to ask in the EEVblog if think.

[@LeoH3L1]

CAN bus being a differential signal would explain the negative resistance readings, as different mulitmeters likely send their resistance measurement current out in the other direction, and it's likely that what ever bit of code was processing the returned value was likely not an unsigned datatype, or if it was hardware it was a design oversight.

[@MrSupro]

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.

[@spehropefhany]

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.

[@johnc1575]

You can also get a negative resistance measurement by reversing the 4 wire current with the voltage sense.

[@catalinalb1722]

I noticed too that CAN BUS must be off in order to measure correctly. I have PeakTech multimeter.

[@pivotmastex]

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.

[@gordonwelcher9598]

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

[@RZ-zv3jr]

Why have battery testers like RC3563 have no problem measuring the inner resistance of a battery? Would it work here correct? AFAIK they use AC current or do two measurements. ?

[@HansS-vh8de]

The most simple ideal/theoretical model of a battery is an ideal voltage source (intrinsic resistance 0Ohm) in series with an ideal resistor. You do a voltage measurement without loading the "circuit" thus you get the "inner voltage" or voltage of the ideal source. Then you perform a second voltage measurement with a known loading e.g. 1A. You calculate the difference and thats your voltage drop at the resistor. If you divide it by your loading current of 1A which is the current flowing throug the resistor, you get your inner resistance. You can also do those measurements at different currents or with an AC current but at least you need two current/voltage pairs for your calculation. I think for batteries you can also play with different frequencies of the current to obtain other informations because the chemistry is slow and the ions can not keep up with the speed. The CAN bus usually has a 120Ohm parallel termination on each end which is also parallel to the voltage source, line drivers and protection circuits and you don't know what the driver stage and protection circuit is doing when you draw a curent or apply an external current therfore those methods are at least questionable.

[@MatthijsvanDuin]

A solution for doing a live measurement on a CAN bus (or similar transmission lines) would be to inject a very-low-frequency AC current instead of DC current, and measure the correlation between the voltage and the injected current (in other words, measure the real part of the impedance at some low frequency). Any DC bias or high-frequency signals present on the thing you're measuring will get ignored, and in the event there _is_ an interfering signal present at (approximately) the same frequency as what you're using for the measurement, it won't be perfectly synchronized so you'll see the measured resistance oscillate thus revealing that the measurement is unreliable. This usually won't help with measuring things in-circuit of course, which is possible in some cases but it'll very heavily depend on the specific circuit.

[@bobblum5973]

Back in the mid '70s my tech school electronics instructor showed us an open cathode resistor in a vacuum tube circuit. While troubleshooting it he hooked a Simpson 260 multimeter across that resistor to measure the cathode voltage. The circuit started working! The Simpson had a low input impedance (20k ohms per volt, maybe 50k?) so it became the cathode resistor. As my instructor said: "You leave the meter in-circuit and charge the customer for it!" 😄

[@thomasmaughan4798]

20k ohms per volt for a Simpson 260. I bought one a couple of years ago, mostly as a sanity check on my Fluke. How do you KNOW the DVM is even within a magnitude of being correct? Not much can go wrong with a Simpson 260 provided it actually works. Breaking it is fairly easy. Plus if I intend to just leave it hooked up as a monitor, the Simpson is ideal. Old schematic diagrams assuming you are using a Simpson or other 20k ohm per volt meter and already adjusted for meter loading. Using a high impedance DVM on old equipment can sometimes give misleading voltage measurements even though it is more accurate.

[@bobblum5973]

@@thomasmaughan4798 Thanks for clarifying the ohms/volt on a 260, it's been years since I laid hands on one, and was too lazy to look it up at that moment. Still, 50k ohms/volt seemed high in my mind. VTVMs and FET-VOMs... the good old days!

[@johnbeer4963]

It's funny, at one point I had to measure very low resistances of around 1 ohm. Eventually I got a 4 wire setup but at first i tried just the multimeters. My Brymen BM257 would not fix a reading on those resistances. A "Mastech" 830 style meter from the late 1990s however, would. The cheapest DMM you can buy, that has spent 20 years in the boot of a car. I also tested it on DCV vs the brand new Brymen and found it to be ONE COUNT out from the Brymen's reading.

[@DD4DA]

Well, if you try to messure a resistance in a circuit, you get any values you can image, but not the real value of the resistor. Depend on the multimeter, it could be dangerous for the meter, if you select OHM and put power into it.

[@KeritechElectronics]

Not getting caught. Whenever something seems off, I just test it on a known good resistor. Never using an ohmmeter live. Why is it off? It's all written in Ohm's and Kirchhoff's laws...

[@paulmuff9883]

Many thanks for explaining this, us novices that just do things like playing around with old cars etc in my case because I’ve learned a lot from this video, manny thanks 🙏🏻 😊

[@wtmayhew]

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.

[@rasungod0]

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.

[@handbackurmbe]

I wonder if negative Ohms means the electrons are falling out?

[@TheTrueCBaer]

There is a way around this. One time i wanted to measure the impedance of the analog telephone port of an Fritz!Box while powered on. If I remember correctly I used an audio transformer and the Network Analyzer function of the Analog Discovery 2. Came out to be 600 Ohms. Only the readings where the frequencies of signaling tones are, where wrong.

[@ralfneumann8619]

Protek 608 low Voltage Ohm range .25V is very handy for use on un powered circuit.

[@JarzanX]

@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...

[@rdormer]

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.

[@edwardhuster8466]

Never measure ohms while circuits are live. On voltage your battery will change bias the chips and other components. Possibly damaged parts on circuit board.

[@whiskyguzzler982]

7:42 Is that a 10k between CAN-L and CAN-H? Just to drain voltage? Or why?

[@mckryall]

What if the customer's meter is just putting out a ton of current? That might even damage the can bus if you connected it while devices were communicating, or at least cause confusing faults by interrupting the flow of data

[@shawnmccori]

I am thinking it might be circuit jumping cause the EEVV meter is more sensitive it found it. While the old one has less conductivity it wasn't reading the short or arc.

[@2oqp577]

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.

[@pa4tim]

My Keysight has a function for measuring resistance in situ (1273A) but not in a life circuit (Although I never tried it, it's a bit pointless)

[@EEVblog]

I tried it, doesn't work in thsi case of th CAN bus or PSU.

[@MrMaxeemum]

How did his old fluke get the reading correct? Surely the customer used the 2x meters under the same circumstances. He is unlikely to have measured with the Bryman with the bus powered up and then measure with the Fluke with the bus powered down. I can't imagine any meter getting the right result with the bus powered up.

[@sw6188]

Yup, if a Fluke can't give an accurate reading of resistance with power applied then there's no way a janky old Kamakooza meter is going to be able to (which is what his meter was, it wasn't a Fluke).

[@InspectorGadget2014]

I'm from the era where multimeters were still analogue. Making such "mistakes" would often make the needle of the meter go to the extremes. (and could sometimes cause damage, if not careful) We should not forget that modern-day digital meters are using quite different methods to measure things, where the result indeed may be affected by voltages, impedances and other components that may (also) existing in-circuit. BTW, there are specific-purpose can-bus tools (often quite expensive) that do display (measure?) the resistance and/or flag when there is a problem with the termination. Using a multimeter in automotive situations can be helpful but in can-bus I like to use a scope (indeed) or the aforementioned (digital) can-bus tools. Indeed, multimeter 1-0-1, I guess we all have to make such silly mistakes to learn from it. Measure twice, think three times! ;-)

[@saksaelectronicsconsulting2023]

A multimeter could perhaps be made to sense any voltage in the circuit when in ohms range, showing "1d107" at the user.

[@Dave-bp6ju]

I wonder how did the customer got that 60 ohm with the other meter... Different conditions? So much different measurement method in the meter? Faulty CAN?

[@ValuedTeamMember]

I never dreamed i.e. it never crossed my mind to have any kind of a "voltage" around or near a resistance measurement. That's just CrAzY

[@michaelwilkes0]

what if it wasn't the can bus? maybe having 2 dmm connected in parallel, the stronger one reads correctly, the weaker one is confused by the stronger one.

[@hausaffe100]

How do loop and earth impedance meters work on powered circuits ?

[@dahlia695]

Negative ohms? Maybe that can be used for faster than light travel?

[@sw6188]

It's how the Tardis works. 😁

[@ralphj4012]

Some semi's will power up if you apply miniscule voltages only to their GPIO pins, TI micros spring to mind.

[@splitprissm9339]

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 :) ).