On Sun, 16 Jul 2017 13:29:07 -0000 (UTC), Chris
wrote:
I'm finding I get different results (vastly different in some cases) when
measuring the total resistance of a circuit with a) a DMM and b) an old
analog meter with a physical needle. And this doesn't only happen at high
impedance points, either. What could account for this?
If you're doing in circuit resistance measurements, you're probably
measuring the resistance of a non-linear device such as a transistor
or diode. These will show different resistances at different applied
voltages. Umm... this assumes that you've unplugged the circuit that
you're testing and have discharged any BFC's (big fat caps).
Disconnect whatever you're measuring. Take one of the DVM's that has
the highest input resistance, set it to VOLTS, and measure the VOLTAGE
across the leads of the other meters. You'll find quite a bit of
variation. My guess(tm) is that the meter with the highest voltage,
will read the lowest resistance.
If you have an ESR (equivalent series resistance) meter, you can do in
circuit low resistance measurements without worrying much about the
effects of semiconductors. That's because the voltages involved are
so small, that the semiconductor doesn't even being to conduct, and is
therefore essentially out of the circuit.
There's really no way to "fix" the problem of measuring in circuit
resistances. If I want to accurately measure a resistor that's in a
circuit, I usually have lift one lead, and measure only that resistor.
I've got four DMMs and two analogs. The DMMs agree with the other
DMMs
and the analogs agree with each other. But the different types don't
agree with each other!
The analog meters (VOM) require more current in order to obtain a
resistance reading. More current means more applied voltage across
the leads, which means that the semiconductors in your test circuit
are well into conduction. Try measuring a resistor and diode in
parallel and you'll see the problem in action.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558