RE: [TCML] Capacitor Questions

["Lau, Gary" <Gary.Lau@xxxxxx>]

Hi Ed,

The test you propose for the capacitor seems like a bad idea to me.  It doesn't subject the cap to the type of RF-duty that a TC would.  Instead it just repeatedly charges the cap, then discharges it into an effective dead-short (the spark gap).  This is a bad idea even for suitable caps.

Regards, Gary Lau
MA, USA

> -----Original Message-----
> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
> Behalf Of Ed Phillips
> Sent: Sunday, May 31, 2009 7:23 PM
> To: Tesla Coil Mailing List
> Subject: Re: [TCML] Capacitor Questions
> 
> Bob:
> 
>     If you have the patience I'd recommend the MMC route but the second
> capacitor MIGHT work.  Easy enough to get a quick check.  Rig a spark
> gap which just fires with the voltage across the unloaded NST secondary
> and then connect the capacitor across it and let it run for a couple of
> minutes.  If you get pretty hot sparks and the capacitor doesn't get
> very warm [notice both terms are subjective] it will probably due as a
> start until you get something better.  Alternatively I could send you
> three of the 0.004 ufd, 20 kV HV filter capacitors I'm using with my
> "big" coil [3" x 15" secondary, ~27" streamers].  That's the setup I use
> with a 12 kV, 60 ma transformer.  These things have lossy dielectric and
> a negative temperature coefficient but along with that have very low
> series resistance so they work fine until they get too warm and change
> the tuning [about 60 seconds at a crack with the 12 kV NST].  I've blown
> up a couple due to overheating but doubt if that would be a problem with
> your little NST.  I've been intending to dig out the bigger coil and try
> that transformer with it but haven't had the ambition to clear enough
> bench space.  I suspect it would give 12" + streamers at least.
> 
>     Alternatively I could send you three smaller 0.004 ufd 20 kV filter
> capacitors of the same type I use in the little TC you saw when you
> visited.  They have the same general attributes, lossy dielectric and
> high negative TC, but still have the low ESR and do indeed work OK for a
> while.
> 
> 73,
> 
> Ed
> W6IZJ
> 
> Bob Deuel wrote:
> 
> >Hello:
> >
> >I have questions regarding a couple capacitors that I have:
> >
> >1) The first capacitor is a Sprague type 45P5 "Vitamin Q" rated at 0.012 mfd. @
> 25,000 VDC. It physically measures approximately 2.5-inches in diameter and 6.5
> inches long with end caps threaded for 1/4-20 thread. It is an oil filled capacitor that
> has a glass cylinder with metal end caps. I understand that some capacitor
> dielectrics do not take kindly the rigors of Tesla Coil operation and overheat. I
> wondering if anyone has used a capacitor of this type? This one would use for a
> future coil if it is acceptable.
> >
> >2) I plan on building a table top coil using a 5kv @ 20 ma. NST soon. I keep
> looking at a capacitor that is on my bench that I believe was from an older IBM
> computer monitor. It is made by ERO and is 9000pF+80-20%/20KV. It actually
> measures 10,300 pF which is great. Physically, it is potted unit in a 1.5x1.75x2.75
> plastic case with a insulated HV lead and a "ground" lead. Are capacitors like these
> acceptable for a small coil or should I forget it and go with a MMC?
> >
> >I would appreciate your input on the feasibility of these two capacitors.
> >
> >Thanks,
> >Bob Deuel, K2GLO
> >
> 
> 
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