Re: AC Resistance Measurements
> I just ordered up one of those fancy HP 34401A multimeters because "I
> 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
> 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
> 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
> impossible. However, someone may know all about this and could tell me
> 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
> 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.