Measuring capacitors

[Lincoln Fong]

>>Howdy,
    Is there an accurate way to measure capacitance? My Forrest Mims pocket
>book is the only place I have read how to test a capacitor, but his method
>doesn't measure it. I have a drawer full of those assorted ceramic disc
>capacitors that I have no idea what their values are and would love to put
>them to good use. Thanks a million.

>Jon Darby


I built a little potential divider using a variable resistor as one side and
the unknown cap as the other side. I then feed in a sine wave of known
amplitude across the ‘outer lugs’ and find a frequency and resistance value
where the output drops to a half of the input. IE the reactance equals the
resistance of the pot. It’s easiest with a dual trace scope. Then you can
use a frequency/reactance(in ohms)/capacitance table to determine the value
or better yet switch in a test cap of known value to compare readings. This
may seem a bit longwinded and it’s true it won’t measure the smaller values
because of stray capacitance error but if you build a jig and have a whole
box to do (and are very cheap like me) it will work fine down to a few
nanofarads.

Lincoln
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