Re: NST PFC cap question

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

Original poster: "Terry Fritz" <twftesla-at-uswest-dot-net>

Hi Kevin,

At 06:49 PM 12/14/2000 +0000, you wrote:
>I just hooked up a pair of 100uF 100V non-polar electrolytic caps in series 
>(closest to the needed 41uF I could get) to the primary of my 15kV, 30mA NST 
>for phase correction. (I don’t fully understand the PFC idea, but it sounded 
>good and I thought Id give it a try) After running for 10 seconds they were 
>getting warm and after another 10 seconds they were significantly warmer. Im 
>sure if I continued to run them they would overheat. 

Yeah, and they blowup pretty quickly past this point with a very load bang!
 50uF across 120VAC will draw 2.26 reactive amps.  Those little caps simply
are not designed for that current.  You need real motor run or PFC caps for
this that are rated and have internal safety stuff so they can be put
directly across the AC line.  Motor shops and junk places that have motors
will have them.  Some mail order places have them as surplus for very good
prices too.  DigiKey's #P9404-ND   20uf 180VAC caps are what I use (fast
and easy but not cheap).  Two will run you about $17 with shipping.

>What can I do to fix 
>this? Can I put a resistor in series to cut down the caps current? The 
>resistor shouldn’t effect the phase shift of the caps right? 

No, you need to draw the heavy out of phase current and a resistor will
just reduce that current.  You need the right caps...  Others may know
where to get the cap or have some around that they can get to you.

Cheers,

	Terry


>100V was the 
>highest rating in the non-polar caps at the local electronics shop, so 
>putting caps in parallel isn’t an option. Any ideas or links to PFC info 
>would be greatly appreciated. Thanks
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