CAPACITORS
Mr l Kaya
We have got a capacitor here. Would it be possible to discover whether it is
faulty using a DVM?.
Thanks in advance.
Lami & Simon
Eric Freudenthal
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From: es...@warwick.ac.uk (Mr l Kaya)
Newsgroups: sci.electronics
Date: 19 Aug 91 10:09:56 GMT
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Hello everbody,
Lami & Simon
You can test for gross failures:
If, after the cap has had enough time to charge, it should not
conduct. So the ohmmeter should indicate infinite resistance.
If the cap is small, a dmm is not sufficient.
If the cap is big (greater than 100uF):
1) discharge it by shorting
2) measure its resistance with an ohmmeter. It should rise
over time.
3) most caps are rather cheap. Buy a new one anyways.
Michael A. Covington
Yes, partly.
Measure it with the ohmmeter. It should show infinite resistance
(no conduction). That will tell you it's not shorted.
If it's a relatively large value (0.1 uF or more), then the ohmmeter
should show a non-infinite resistance for a moment before reading infinity.
And if it's a really large value (10 uF or more) it will take seconds or
minutes to go up to infinite resistance.
--
-------------------------------------------------------
Michael A. Covington | Artificial Intelligence Programs
The University of Georgia | Athens, GA 30602 U.S.A.
mcov...@aisun1.ai.uga.edu mcov...@uga.cc.uga.edu
Mr l Kaya
Lami & Simon
Gary Coffman
Well yes you can determine go no go with a DVM, but it's easier with an
analog meter like the old faithful Simpson 260 or one of the newer DMMs
with capacitance checking built in.
Measure the capacitor for a short on your highest ohms scale. If the capacitor
is a small value, it should show infinite ohms. If the capacitor is bigger
than about .1 uf, it should show a charging ramp to a very high resistance
value over a period of seconds. An analog meter is useful here because it's
easy to see how rapidly the capacitor ramps up by the rate of meter movement.
This gives you a feel for the value of the capacitor. Now reverse your meter
leads. A fairly high value capacitor will kick your meter hard while a small
capacitor may show no reading at all. Experience will tell you if the readings
you get indicate a good or bad capacitor.
Another method, and the one often used in digital capacitance meters, is to
construct the following circuit.
________
|A---||-C-/\/\/\--D
AF Gen | |___^ |
|B________________|
_______|
Set the audio generator to produce a convienent voltage across A and B, say
2 volts. Now measure the voltage across C and D. Adjust the pot until the
voltage across C and D is half the voltage across A and B. Disconnect the
audio generator and measure the resistance of the pot. This is the reactance
of the capacitor at the frequency of the audio generator. Now use the formula
C=1/(2*pi*f*X) where X is the value you just measured in ohms and f is the
audio frequency in Hertz and C is the capacitance in farads.
Gary KE4ZV
Jerome Grimmer
From a practical point of view, I don't think so, at least not with any
degree of accuracy. From a physics point of view, you could theoretically
do it with a Galvanometer though, but this won't be real accurate either unless
you are really good at reading transient peaks on a galvanometer. I would
suggest acquiring or building a capacitance meter. I've seen projects
in various magazines about how to build one.
--
Be excellent to each other, and compute on!
Jerome Grimmer
ap...@Cleveland.Freenet.Edu, ST6...@SIUCVMB.BITNET, ST6...@vmb.cdale.siu.edu
Eric Yoxtheimer - EECS
>
>>Hello everbody,
>>
>>We have got a capacitor here. Would it be possible to discover whether it is
>>faulty using a DVM?.
>>Thanks in advance.
>>
>>Lami & Simon
>>
>
New Fluke Series II Multimeter it does have a continuity check, not to
mention a capacitance reading. This might do the trick.
Or this, do you know what's size it's supposed to be? Could you get a
o-scope and look at the output (I know...this evades the DVM limitation,
but it might work...maybe ;-). Just a couple of ideas...
-Eric Yoxtheimer
Computer Engineering Grad Student (who doesn't always know what
he's talking about :-)
eyox...@eecs.wsu.edu
Harold Wong
I would like to know how to test a capacitor for leaks as well. How
would I do this with a scope?
Harold
Gary Coffman
> I would like to know how to test a capacitor for leaks as well. How
>would I do this with a scope?
Well if your scope has a built in DMM it's pretty easy. You just series
the DMM with a power supply and the capacitor and read off the microamps
of leakage current. If your scope doesn't have a built in DMM, then it's
somewhat harder to get accurate results. What you can do is charge the
capacitor with a power supply and time the decay as you watch the trace
return to zero volts. This will give you a rough idea of the value of
the capacitor and it's leakage. Figure that your scope will present
about a 1 megohm load on the capacitor and calculate the time constant.
If the measured time constant is less than the calculated constant,
the difference is a function of the amount of leakage in the capacitor.
Gary