[R-390] DMM bounce

[Bob Camp]

Hi

If you are using a trusty VACCUM TUBE instrument then you will have no 
problem with vacuum tube impedances.

A modern super duper high speed auto ranging DVM will to just a little 
nuts trying to measure the resistance of your 360 Henry inductor. Some 
of them at least drive the device under test with a constant current 
source. This is fine for a capacitor or resistor. On an inductor the 
current ramp gives you a voltage ramp. Up goes the voltage and it auto 
ranges. Even if it runs a constant voltage it will still auto range if 
it's fast enough. The coil will look like an open circuit at first so 
the things little brain decides to go to a higher resistance scale. In 
either case you can get a situation where it just bounces back and 
forth with no tendency to settle at all.

Simple solution - hook a low voltage power supply (like a D cell) in 
series with the DVM and the secondary. That should give you about a 
milli amp though the secondary. Unless this is a very low level 
transformer the current shouldn't bother it to much.

I use a similar setup for checking caps on the R390. A 60 volt  power 
supply, a 5 micro amp DC meter and a 22 meg  resistor all in series 
with the capacitor makes for a pretty sensitive ohm meter / cap leakage 
tester.

You can use the same resistor in series trick to keep the current in 
the secondary low. In that case I probably would measure the voltage on 
the secondary and figure the resistance out by formula. Figure that 
kind of stuff out for a living and they make you a Lord ....

Don't know if that helps or not.

	Take Care

		Bob Camp
		KB8TQ

On Jul 12, 2004, at 7:03 PM, Dan Merz wrote:

> Hi,  here's a general question related to making resistance 
> measurements
> when an inductance is involved,  e.g. the winding on an audio
> transformer.  Generally,  I make such measurements either with my BK
> DMM on a fixed scale setting or with a Simpson 260 old-style
> multimeter,  and I usually have no problem.  But recently I repaired an
> audio transformer and sent it to the lucky guy and he emailed that the
> primary was ok but his digital meter just bounced on the secondary and
> never settled on a reading.  I thought maybe I made a bad solder
> connection.   The approximate inductance of this secondary is about 360
> henries.  He sent it back to me and I measure it as ok,  about 1500
> ohms,  using my usual methods.  I do notice that the reading shows a
> couple of high reading before settling down.  This doesn't happen on 
> the
> primary which is much lower inductance.  My question - are there some
> DMMs,  maybe the autoranging types,  that have problems with such
> measurements.  I could imagine this could happen depending on the rate
> at which it tries to "range" the reading but it would seem that it
> should eventually settle down.  He tried it with two different digital
> meters and couldn't get a stable reading.  I sometimes use this
> measurement method on 390a parts,  hi.   Hopefully waiting for
> illumination,  Dan.
>
>
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