Measuring Capacitors

[Martin Czech]

A simple method is to charge a RC network (C is DUT) and R is known.
Time between any two levels is then proportional to C.

You need: stable reference supply, a few stable R,
two comparators (one for the lower, one for the upper level),
some logic to control charging, to discharge etc.
The levels should be such that they are far away from the
flattening of the curve ,  < 60% of full reference value will do.
Now you have to measure the time between both comparator
switching, a pulse wave should therefore be generated.

There was once a schematic in a German electronics magazine.
The charging was slow, so a TL084 was used.
The additional wiring capacitance  can be simply subtracted from the pulse
via an adjustable monoflop (adjust to 0 if no cap is connected).


Of course: this method assumes ideal capacitance, the more
non idealities, the more random the reading.
You won't get any information about the cap inductance
and resistance, which govern the behaviour -say- at 1
MHz or higher...
A bridge can do that but is very uncomfortable.

Cheap multimeters do it that way. They are so cheap that diy
is not very usefull apart from the educational value.


m.c.

:::From: "Jon Darby" <jdarby at lplizard.com>
:::To: <synth-diy at node12b53.a2000.nl>
:::Subject: Measuring Capacitors
:::Date: Tue, 19 Dec 2000 13:57:16 -0500
:::Content-Transfer-Encoding: 7bit
:::
::: 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