Re: Testing Metal Film Capacitors

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

Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>

Hi,

At 04:41 PM 5/18/2004, you wrote:
>   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.

Chances are that these older caps are not going to work well.  MMC type 
caps are pretty modern things and not common.  I think you would be best 
off selling them on e-bay and using the money to by caps that work for 
sure.  There is really no way to easily use the older caps other than just 
hooking them up and seeing what happens.  But they probably have like 50X 
the dissipation of good caps so it is unlikely they are of any use.

Cheers,

         Terry