Re: Testing Metal Film Capacitors

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux" <jimlux-at-earthlink-dot-net> 

One would have to carefully consider how much your time is worth if you want
to test as opposed to buy new.. However, assuming that time is cheap and new
capacitors dear...

Two aspects of the capacitors are of interest: the current carrying ability;
and the loss (they're related, though).  The tricky part is doing it at the
working frequency of the coil (or somewhere near there... say 100-300 kHz).

Loss you could measure in a sort of bridge circuit. Maybe a 555 timer
generating 200 kHz, feeding the capacitor in question and a resistor in
series and measuring the voltages across resistor and cap.  Maybe even just
hooking the cap in an RC  timing circuit and measuring the frequency
(accurately).  Or maybe you have an impedance bridge.

Current handling is probably best tested in a proof testing fixture.  That
is, figure out what sort of RF current you are going to have flowing and set
up a test to put that current through, and see if the cap lasts.  A handy
test fixture might be a tesla coil....


----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Wednesday, May 19, 2004 7:31 AM
Subject: Testing Metal Film Capacitors


 > Original poster: "Chris Fanjoy" <zappyman-at-hotmail-dot-com>
 >
 >    I want to put together an MMC, to use as the tank cap on the TC I'm
 > building. Because of the tight budget I'm on, I really can't afford to
 > spend money on a good pulse cap or anything like that. Instead, I want to
 > build an MMC with the old caps I have laying around.
 >    I have literally hundreds, if not thousands, of metal-film capacitors
 > pulled from various electronics. Most are probably about 15 - 25 years
old,
 > and nearly all of them are the "tear-drop" shape. There are a wide range
of
 > voltages and values, such as 0.068 / 1600V, up to 4.7 uF / 250V. They come
 > in many different colors, by many different manufacturers. Most are
 > generic, some bear the Matsushita logo and there are even a few Sprague
 > "orange drop" types. The big questions is - how do I distinguish the
 > polypropylene types from the ones with other dielectric materials?
 > (polyester, mylar, etc). Is there any kind of test critera one can use to
 > determine if a cap is suitable for a TC? Perhaps some kind of test jig,
 > that would apply an AC voltage of 3 - 4 times the capacitors rated value?
 > (and if the cap doesn't short/explode within a certain amount of time,
 > consider it a pass?)
 >    There must be some way to make use of all these nice old caps. Or maybe
 > I'll just have to build an MMC anyway and see how it fares.
 >    Thanks for any advice.
 >
 >