DMMs with coils

Mar 26, 2007 8 Replies
A

How do DMMs measure resistance? I was testing some relay coils that are specced at 50 ohms resistance, but they read 0.5ohms on both "Brand X" DMMs we have at work. If the DMM was in manual ranging in K mode, it would have read 0.05. The coils don't have built-in snubbers.



Naturally I will confiscate these meters Monday morning, but I want to know what's going on here.



At home, on a 10$ Mastercraft made in China analog meter, they read 50 ohms. On a Tektronix 249 DMM, they read 50 ohms.



The last time I put a scope across a DMM while it was measuring resistance I saw narrow pulses. This must cause the coil to react and cause false readings on naive DMMs?



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P

The inductance in relay coils can cause DMMs, especially with autoranging, to give erratic readings. Usually they will settle down to a correct value, and in manual ranging there should be no problem. Maybe there is a protection circuit kicking in and causing the current to cycle on and off?

Paul

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R

*All* DMMs that i know of use a simple battery and resistor in series with the unknown the metering circuit. Only the capacitive measuring circuit may use pulses. Methinks you may have done something incorrectly...
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P

** By using dual slope integration, a reference resistor and determining the ratio of voltages.
** The auto-ranging function is going haywire.

** But on lots of other, auto-ranging DMMs they will not.

Even Flukes.

** Yes - back emfs from the coil ( or a transformer primary) upset the meter.

Either lock in the low ohms range or add a suitable C across the coil.

...... Phil

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B

Watch out. That Mastercraft, although quite accurate has 1M input resistance for DC range of 20V I did not check other ranges. Part# 052-0060-2

Boris Mohar
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J

that's strange, i have no problems with the meters i use. i normally use FLuke Extech etc..

But i need to put them in Manual mode other wise, they just keep floppy all over.

"I\'m never wrong, once i thought i was, but was mistaken" Real Programmers Do things like this. http://webpages.charter.net/jamie_5
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J

Boris Mohar wrote in news: snipped-for-privacy@4ax.com:

Probably ALL the volts ranges are 1 MegR. The Harbor Freight $3 DMM I have is 1 MegR. I was thinking about changing it to a 10 MegR string,just for fun.

Jim Yanik jyanik at kua.net
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A

That ain't analog. I've got the cheap needle movement that moves when the meter is moved. Crap. Great way to test tunnel diodes though since it only uses a AA cell, no nasty 9V supply waiting to blow up delicate junctions.

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R

All decent DMMs use a 9V battery and *none* of them will stress, much less "blow up" any junction, delecate or otherwise.

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