Measuring an unknown capacitor with a Tenma 72-960 LCR meter, I got 89 nF at both 1 kHz and 120 Hz, which I believe because I measured other known capacitors, too. Then I tried measuring with the resistance function, and it gave me:
- 180 kΩ at 1 kHz
- 1.5 MΩ at 120 Hz
But the reactance of an 89 nF capacitor is:
- 1.8 kΩ at 1 kHz
- 15 kΩ at 120 Hz
Also confirmed that in resistance mode, it measures 1 kΩ for a 1 kΩ resistor at both frequencies.
Why are the measured values off by exactly ×100? Am I misunderstanding what the LCR meter measures? (Is it magnitude of total impedance \$|Z| = \sqrt{R^2 + X^2}\$ or just the resistive component R in \$Z = R + jX\$?)
Update with some more measurements:
10 µF:
- 9.393 µF @ 1 kHz
- 9.71 µF @ 120 Hz
- 185 ohm @ 1 kHz (reactance is 16 ohm)
- 5.6 kΩ @ 120 Hz (reactance is 133 ohm)
680 nF:
- 683.5 nF @ 1 kHz
- 686 nF @ 120 Hz
- 63.22 kΩ @ 1 kHz (reactance is 234 ohm)
- cannot measure at 120 Hz (reactance is 1.9 kΩ)
So the exact ×100 numbers may just be a fluke.
Best Answer
Well it's plausible but the numbers seem suspicious (I think). Your meter's manual refers to measuring capacitance/resistance in "parallel mode" by default, but that this can be changed to "series mode". This suggests that it is trying to compute the equivalent parallel resistance of the network. The real part of the impedance of a parallel RC network is
\$\frac{R}{1+\omega^2R^2C^2}\$
... and this is frequency dependant. The equivalent parallel resistance in this case is constant with frequency (by definition it is R) and this is what I would expect the meter to display. However in a real capacitor, the equivalent shunt resistance is not formed by a real resistor.
It would be interesting to switch to "series mode" if possible and see what the numbers are then.