Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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Old July 16th 17, 03:29 PM posted to sci.electronics.repair
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Default Resistance measurements

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?

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!

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Old July 16th 17, 03:39 PM posted to sci.electronics.repair
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Default Resistance measurements

On Sunday, 16 July 2017 14:33:04 UTC+1, 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?

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!


Are they calibrated?


NT
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Old July 16th 17, 04:01 PM posted to sci.electronics.repair
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Default Resistance measurements

On Sunday, July 16, 2017 at 9:33:04 AM UTC-4, 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?

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!




Do your DMMs and analogues agree with each other when using fixed resistors out of circuit?
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Old July 16th 17, 04:08 PM posted to sci.electronics.repair
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Default Resistance measurements

On Sun, 16 Jul 2017 07:01:34 -0700, John-Del wrote:

Do your DMMs and analogues agree with each other when using fixed
resistors out of circuit?


Yes they do. Sorry, should have mentioned that in the OP.

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Old July 16th 17, 05:05 PM posted to sci.electronics.repair
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Default Resistance measurements

On Sun, 16 Jul 2017 10:41:02 -0400, Ralph Mowery wrote:

The analog meter has enough voltage/current to turn on the junctions of
diodes and transistors. The DMM will not have enough to turn them on.

If you have a diode out of the circut and use an analog meter you will
often see a small resistance in one direction and if you reverse the
leads a high resistance. The DMM will usually show a high resistance in
both directions unless you use the diode setting if the dmm has one.


Damnit, Ralph! You beat me to it. I was going to say that. ;-)
I have a couple of analogue meters too. They test for resistance at 15V
which is more than enough to turn on those semiconductors, but also more
than enough to destroy a lot of chips that can't tolerate much more than
5V. Horses for courses/different strokes and all that.
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Old July 16th 17, 05:36 PM posted to sci.electronics.repair
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Default Resistance measurements

On 16/07/2017 14:29, 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?

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!


Try with different probe leads and resistors with different termination
metals, maybe small dissimilar metals producing voltages that are
interpreted differently by the different metering systems
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Old July 16th 17, 06:26 PM posted to sci.electronics.repair
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Default Resistance measurements

On Sun, 16 Jul 2017 16:36:56 +0100, N_Cook wrote:

Try with different probe leads and resistors with different termination
metals, maybe small dissimilar metals producing voltages that are
interpreted differently by the different metering systems


It's okay now I believe Ralph has nailed the problem entirely in his
post. (thanks, Ralph).

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Old July 16th 17, 07:50 PM posted to sci.electronics.repair
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Default Resistance measurements

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


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