DMM giving different readings between mv and ma

[Bionic-Badger]

That does you absolutely no good when you use a 10% power resistor which drifts rather drastically with heat. 10% power resistor + 0.0000001% voltmeter will always be vastly inferior to a 1% ammeter. If you can get a <1% 0.1 ohm resistor for cheap (can you? Maybe, I dunno), my qualms pretty much go away. But like I said, that's what an ammeter is anyway. No one uses those, though. It's always a junk 1 ohm.

Yeah, I'll admit a shit-ass 10%-tolerant power resistor is going to be pretty bad. Though if they're using one of those, it's probably more reason to keep them on the dummy-load road than burning out their equipment.

 

[norbyx]

I never bothered with using a test load like Cyparagon said, I just short the driver through the ammeter. If it's a true constant current regulator, it shouldn't hurt it.

Hi,
I know this is an old thread (not too old though) but since I am going to start my first build from scratch soon I would like to understand this statement.

So I have to picture the situation.
I have my variable voltage/amperage power supply, my DMM (a good 50.000 count data logging meter proffesional grade with 0.7% amp meter accuracy and 0.05 Volt meter accuracy), and my driver.
I hook the driver to the power supply and turn the power on one side and my alligator clips on the driver were the diode positive and negative go. I set up my DMM to 10A, and power everything up.
I should get a reading of the mA, out of the DMM just by shorting the driver on the diode side?

In that case there would be no sense in having a dummy load correct?

Also what happens if I powerd up the driver with no load? As for measuring the voltage that the driver will deliver to the diode would I need just to power everything down, take the test probes out of the DMM 10A and put them on the DMM V socket and power everything up again? BUT in this case since there is no load than there would be no voltage drop ether, so a dummy would be needed....

I am confused.

:thinking:

 

[Cyparagon]

^That really, REALLY depends on the driver topology. Is it linear? Buck? Boost? Buck/boost? Charge pump? Shorting the output is fine for linear, but switching regulators will complain.

 

[norbyx]

And the dummy works for all of them indiscriminately?