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Values of small capacitors

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Values of small capacitors

Postby Folderol » Sun Aug 09, 2020 3:22 pm

That's both small in size and in value. Not only is the print hard to read, sometimes it gets smudged, and other times it printed in a confusing way. As an example I found a bag with a few marked:

100
220

So, what is what? is that 100 something at 220V or 220 something at 100V
Most capacitor testers are for electolytics, and won't give any useful information on caps less that 1u, and proper capacitor testers are expensive. However if you have a reasonably good digital test meter and a signal generator, or indeed any stable audio source you can find out. You also need a selection of known capacitors.

So here's the plan.
First you get two known and equal caps and wire them in series, with the extremes across your audio source. Then measure AC volts from the junction to ground. If it's not sensibly close to half the source voltage, increase frequency till it is (cap impedance drops with increasing frequency so masking the effect of the meter).

Now replace the bottom cap with your unknown one. If it's half voltage, bingo! you got it in one :) If the voltage is higher the cap is smaller, and conversely a lower voltage means it's value is higher.

If the difference is fairly small, and the printing is readable, you can easily work out what it's value is supposed to be. So, with the example above, if my reference caps were 100nF and the voltage was about a third instead of a half, it would be 220nF. If it's above half by a fair amount it could be something in the 10-22nF range or even smaller.

Just keep 'range changing' with pairs of caps until you can pin it down.

P.S.
The ones I had turned out to be 220pF, therefore @ 100V
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Re: Values of small capacitors

Postby James Perrett » Sun Aug 09, 2020 3:37 pm

That's a handy hint Will.

However the cheap component testers from Ebay will measure low values fairly well (just tried a 100pF Suflex which measured 103pF). So far I'm very impressed with the one that I have which is based on an open source design and cost less than a tenner from China.
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