Why doesn't LCR meter measure expected reactance?

Question

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.

Answer 1

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.

Answer 2

Just a hunch: it may be interpreting its resistance reading as parasitic resistance (parallel leakage resistance?), rather than reactance.

The x100 error is suspicious, though.

Answer 3

"Then I tried measuring with the resistance function, and it gave me:"

When you switch to the resistance mode in an LCR meter, it expects to find a resistance connected across it's terminals.If I'm right, one cannot measure Inductive reactance or capacitive reactance using an LCR meter.Like the manual says, you can measure L,C,R,Q and the dissipation values only.

Update: An LCR meter cannot be used to measure (as in display) inductive or capacitive reactance.The reactance value depends on a lot of factors,including the parasitic influences mentiond by others.

It internally computes the reactance using an AC voltage and freqency and a suitable circuitry and from it the C or L values are approximated and displayed.Various meters use various circutry to achive this approximation.The series or parallel mode also changes the circutry and approximation model.

In practice , use the series mode for large capacitive and small inductive values and the parallel mode for vice-verca,assuming you know aprori the range in which the value of the component falls.

I still believe that an LCR meter is just what it is say it is: An L meter, a C meter and an R meter.When it is in the R,mode, it expects a resistor to be connected across it's terminals.Connecting a cap/inductor in the R mode and trying to make sense of the reading might be a waste of time. Feel free to correct me if I might be wrong :)

I found a very good link that explains impedance measurement in detail.