DMMs with coils

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

*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...

** 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

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

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

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

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.

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