EEVblog 1622 - The BIG BEGINNER MISTAKE with Multimeters

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.

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

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#ElectronicsCreators #Multimeter #Tutorial

[Breakstuffkhz]

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.

[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)

[SnabbKassa]

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

[clintondekock]

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

[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.

[darranrowe]

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.

[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.

[Joetechlincolns]

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

[----.__]

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.

[williamsquires]

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! 😊

[shaunclarke]

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!

[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.

[natecontarino]

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

[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).

[motivatedpeon]

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

[p_mouse]

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.

[RK-knud]

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.

[ТарасКорж-гт]

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)

[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)!

[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!