[R-390] DMM bounce

[Dan Merz]

Hi Bob,  ok,  that helps.  I wondered if the autoranging process did what you
mentioned - and didn't allow the current thru the inductance to settle enough
before the next range check.  I never ran into this before with my meter
which isn't autoranging.  And the other guy evidently hasn't run into it with
his autoranging meter either but this particular transformer has a high
permeability nickel core that contributes to the high inductance compared to
most audio transformers that I've run across.  Most of them have about 5, 10
or maybe 20 H primary with corresponding 45, 90 and 180 H secondaries based
on typical 3:1 winding ratio.  I'm kicking myself for not suggesting that he
try an old style ohmmeter before sending the thing back to me.   thanks for
the response,  best regards,   Dan.

Bob Camp wrote:

> 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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