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DMMs with coils

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?
 
P

Paul E. Schoen

Jan 1, 1970
0
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?
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
 
R

Robert Baer

Jan 1, 1970
0
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?
*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...
 
P

Phil Allison

Jan 1, 1970
0
How do DMMs measure resistance?


** By using dual slope integration, a reference resistor and determining the
ratio of voltages.

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.


** The auto-ranging function is going haywire.

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


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

Even Flukes.

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?


** 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
 
E

Eeyore

Jan 1, 1970
0
Robert said:
*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...

I've know the reactance of some transformers to defeat a DC resistance
measurement with a Fluke 77.

Graham
 
B

Boris Mohar

Jan 1, 1970
0
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.
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
 
R

Robert Baer

Jan 1, 1970
0
Eeyore said:
Robert Baer wrote:




I've know the reactance of some transformers to defeat a DC resistance
measurement with a Fluke 77.

Graham
THe "reactance" of the meter to the "resistance" *must* have been a
"fluke".
 
J

Jamie

Jan 1, 1970
0
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?
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.
 
J

Jim Yanik

Jan 1, 1970
0
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

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.
 
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

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.
 
R

Robert Baer

Jan 1, 1970
0
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.
 
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