Testing picofarad

[dosyl]

I have some capacitor to test; they have 27J .3 kv. I suppose that they have 27 picofarad.
I have a capacimeter, but the lower range is 200 pico.
Do you know a way to test them ?
Thank's

[jts1957]

Sencore LC-102 or LC-103 Capacitor/Inductor testers will handle testing that value.

[dosyl]

Thanks for replying so fast.

I forgot to say: "Without buy another tester" if possible;
And the LC-102 should be expensive.

[jts1957]

Not once you've paid it off. Got mine maybe 12 years ago.

[torbjorn]

Another method of testing small capacitors in the range of some tens of picofarads is to connect a potentiometer of a few Mohm in parallell to the capacitor, and then supply a square wave from a tone generator to an oscilloscope via that R-C network. Usually, 1 kHz or 10 kHz is a suitable frequency. First, connect a known good capacitor of roughly the same capacitance and adjust the pot to get a clean square wave response, measure and note the resistance value, then connect the capacitor to be tested and again adjust the pot for a clean square wave. The ratio of the capacitances is the inverse value of the ratio between the pot's resistance in the two cases.

This is exactly the same method as is used to match a 10:1 scope probe to the scope's input capacitance, although a trimmer capacitor in the probe is adjusted in that case.


If you have an old grid-dip meter available, then another possible method is to connect the capacitor in parallel to a known coil and then check that circuit's resonance frequency with the dip meter.