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Old 3rd March 2011, 03:12 PM   #1
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Default High Capacitance Reading

I have some vintage paper in oil capacitors in the small uF range (2 to 4uF) and when I check them with my LCR meter they give a rather high reading. What does this mean? Is it the capacitors or the meter? Thanks a lot for your help.
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Old 3rd March 2011, 03:30 PM   #2
Boofers is offline Boofers  Canada
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You could verify that your meter is okay by testing some other new capacitors.
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Old 3rd March 2011, 03:43 PM   #3
AndrewT is offline AndrewT  Scotland
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Do you have any 1% or better capacitors?
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Old 3rd March 2011, 03:52 PM   #4
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Oh Yes, thought about it after I sent the message. Checking some .22 and .1uF they measure correctly on the meter. So it seems the capacitors are kaput? In what sense are they faulty? Does it mean they are leaky?
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Old 3rd March 2011, 03:54 PM   #5
AndrewT is offline AndrewT  Scotland
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Measure the leakage current.
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Old 3rd March 2011, 04:21 PM   #6
DF96 is offline DF96  England
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They probably have some moisture in them. As water has a high permittivity (around 80) this increases capacitance. As water is conductive it also increases leakage. This is a standard failure mechanism for paper-based capacitors.
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Old 3rd March 2011, 04:37 PM   #7
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Quote:
Originally Posted by AndrewT View Post
Measure the leakage current.
Thanks Andrew,

How easy it is to measure the leakage current? Does anyone know a simple setup? Thanks to all for your answers and help.
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Old 3rd March 2011, 05:43 PM   #8
Boofers is offline Boofers  Canada
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Quote:
Originally Posted by sonata149 View Post
Does anyone know a simple setup?
You could put a resistor in series with the cap, say 100 ohms or so. Then connect a DC power supply with ~20V on the cap and ground on the resistor. Put your DVM across the resistor. An ideal cap with no leakage you would measure 0V. If the cap is leaking you would measure some voltage. Practically speaking, a few mV would probably be ok. I can't say at exactly what point the cap would be considered bad, depends on your situation i guess.

Last edited by Boofers; 3rd March 2011 at 05:46 PM.
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Old 3rd March 2011, 06:35 PM   #9
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Thanks a lot Boofers. I'll try it.
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Old 3rd March 2011, 08:25 PM   #10
Boofers is offline Boofers  Canada
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Hey sonata149 in my guitar tube amp repair book I found a different procedure for checking caps for leakage.

It says to connect one side of the cap to +V and the other side straight to your voltmeter, no resistors, using common ground of course. Your meter should initially read close to +V and it will slowly decay down to zero. If you have any residual reading on your meter, the cap is bad. It says anything over 1V means the cap is bad, but they assume +V is a tube amp voltage (120VDC and up).
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