Electronics > Beginners

How would I test DMM leads to see if they are crap or not?

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DW1961:
I got a cheap meter lately that I tossed into the trash, but I kept the leads. I tested them for resistance and they had about .04 Ohms. I'm going to use them for 10 amps of current. I just want to now if I should toss them too? What else should I test them for? If they are decent, I don't want to waste a good pair of leads. They are insulated like they should be.

krzysssztof:
I would have done a simple test:
Connect one lead to the constant current source, as much Amperes as you can get out from the source (ideally 10A, because this is what  you want to measure later).
After a while measure voltage drop on the lead including connectors (this will allow you to calculate resistance quite precisely) and check if the lead did not heat too much - this will give you an idea if the copper wire diameter is suitable for such currents.

DW1961:

--- Quote from: krzysssztof on September 27, 2020, 07:36:26 am ---I would have done a simple test:
Connect one lead to the constant current source, as much Amperes as you can get out from the source (ideally 10A, because this is what  you want to measure later).
After a while measure voltage drop on the lead including connectors (this will allow you to calculate resistance quite precisely) and check if the lead did not heat too much - this will give you an idea if the copper wire diameter is suitable for such currents.

--- End quote ---

Yeah good idea. I don't have any 10A supplies. I really need to get a bench power supply. Is there on on Amazon you would recommend that isn't too expensive?

Doctorandus_P:
You can easily make a current source / sink from a LM317 and a 1.25Ohm resistor, (and some capacitors for stability)

With this you have a pretty easy / stable / reliable fixed 1A device that you can use for measuring very low resistances. Just put this 1A through your test probe, and every mV measured is a mOhm. With this simple device you also automatically have a 4-wire measurement, which is pretty crucial for accurate measurements. You can also probe the cable in multiple locations, measure contact resistances of intermediate connectors etc.

If you want more current, just put more of these devices parallel.

It's of course also not so difficult to make an adustable source / sink with an opamp, power MOSfet, sunt resistor potentiometer and voltage reference. But the fixed 1A is easier for measurements and does not get out of calibration easily.

fordem:
First - a cheap meter will have cheap leads - if you have any doubts about the leads just toss them.

Next - if the meter had a 10A scale, the leads are probably good for 10A - note - the meter's 10A scale may have a duty cycle, 10A for X seconds - assume the leads to have the same duty cycle.  What makes the leads crap for me is longevity, how long do they last before the copper starts to separate, before the insulation starts to crack, etc.

Are you using the leads around mains voltage?  Do they have a CAT rating moulded into them?  If you have any doubts just toss them - your life may depend on them.