# Re: AC Resistance Measurements

```> 	I just ordered up one of those fancy HP 34401A multimeters because "I
need
> it" :-))  It will measure true AC RMS voltage up to 300kHz in the
> 100.0000mV AC range (6.5 digits).  If I use a resistor in series with a
> primary or secondary coil, I may be able to put say 10mA of variable
> frequency current through the coil.  The meter should be able to read of
> the voltage drop and give me the AC resistance.  With 10mA and 0.1uV of
> resolution, I should be able to read AC resistance down to 10 micro-ohms
at
> normal TC frequencies.  However, there will be radio interference and
> resonances and such in the coil that would mess around with such a fine
> measurement.  I was thinking of putting some value cap across the coil to
> swamp the RFI and lower the resonant frequencies down to where they would
> not mess up the measurement.  I was also thinking the same technique
could
> be used to measure the series resistance of poly caps.

I don't think you'll have any real problems with interference because the
impedances are low.  And, as for resonances, that's what you are trying to
measure... Measure the voltage and current at a series of frequencies, etc.

Of course, I think one of the new computerized antenna bridges, like the
one from AEA, would actually be better for measuring RF impedances, except
they don't go down low enough in frequency.

>
> 	I think the AC impedances of the coils or caps will make such
measurements
if
> it could work or not.  I would be interested in any hints anyone may have
> on this.

One standard technique is to combine your unknown device with a known
impedance of the opposite sign, so that the impedance magnitude of your
resistor and your unknown are comparable. Say you've got a coil with an
impedance of 100K and you want to use a 1K measurement resistor... If you
add a capacitor with a known impedance of 90K to the circuit, the net
reactive impedance will be only 10K, so the voltage across the coil and
resistor will be more similar.

>
> 	If anyone has any other ideas for useful TC measurements this meter
could
> do, let me know.  I don't think I'll be measuring the output current of a
> neon with it though :-))

Figure out some way to make a computer controlled frequency generator so
that you can make swept automated measurements (I assume the fancy HP has a
RS232 or 488 interface?).  Then, you'll have an impedance bridge.

>
> Cheers,
>
> 	Terry
> 	terrellf-at-uswest-dot-net
>

```