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condenser testing

To: barneymg@ntsource.com
Subject: condenser testing
From: "g.cook" <litfuse@triton.net>
Date: Thu, 10 Dec 1998 15:32:53 -0500
I used to collect antique radios and I often had to test capacitors.  I
couldn't afford the fancy meters nor had enough training at the time to
know how to do it correctly, but I think I can help you with a quick and
sloppy method I've used.  It works with relatively high value capacitors
only, and I think ignition capacitors ought to.

Pretend the capacitor is a tiny nicad battery.  Put a charge on it with
a battery, say a 9 volt one.  Now check the voltage on it.  Granted, it
will quickly dissipate just by the drain of taking the measurement  (the
higher impedence of the voltmeter the better, for this reason), but if
the capacitor is good, and at least .05 uF, you will see a pulse.  It's
easiest to see on an analog (needle) meter.  The better the capacitor,
the longer the charge is retained.  Weeks, sometimes. 

Another related test: find a high resistance resistor, say 1 megohm
(green third band), and put it in series with the capacitor.  Now place
your meter to the highest resistance scale and measure the combined
resistance of the two, and make sure your sweaty fingers aren't part of
the measurement.  At first, it should read "low", like 1 megohm, because
the battery in the meter is charging up the capacitor and it is like a
dead short straight to your 1 meg resistor.  After a short while, the
capacitor will fully charge up, and will be high resistance to the DC
voltage of the meter; the resistance should go full scale.  Depending on
the capacitance, the time for this may be brief, so you might have to
look quick: 1 meg drifting to infinite ohms.

Bad capacitors will allow continuous dc current to pass through them if
they have  an internal short (and have a constant resistance), or will
show infinite resistance (without an initial low resistance blip) if
they fail open.  Actually, this was always the first test I did.

Go get your condensers and meter and give it a go.  I predict your bad
capacitor has a constant resistance (which can occur when high voltage
spikes cause internal pathways through the dielectric) and bleeds down
the stored charge needed by your coil.  A good condenser should last
indefinitely; I've often wondered why anybody replaces them.

I hope this helps.  It feels odd to be advising the Great Purple!



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