Re: Fried Caps? (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Thu, 25 Oct 2007 22:13:04 -0700
From: huil888 <huil888@xxxxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: Fried Caps? (fwd)

Victor -

Unfortunately, your capacitors are not capable of handling the current in 
your primary circuit. Of all the GE type 42L capacitors, the .01uF/3KV parts 
(42L4103) are probably the worst choice for use in a Tesla coil tank 
circuit.

These metallized-film caps have an RMS current rating of only 2 amps, and a 
peak current rating of only 28 amps (at 100KHz). These also have by far the 
highest ESR of all the 42L caps, which will aggravate internal heating. 
Finally, they have a very small body diameter, which means that the lead 
contact area with the edges of the metallized plates is relatively small. 
The usual failure mode with these caps is burning away of the edges of the 
plates at the end of the body. Run within their ratings, these caps are 
reliable, but they don't seem to have a lot of margin for handling peak 
current beyond their ratings.

You'd be much better off using the well-proven film-foil Cornell-Dubilier 
940C series caps, and selecting a value (like 0.1 or 0.15uF) that has a peak 
current rating of 100 amps or more. Use one of the Tesla coil design 
programs to determine the peak current in your tank circuit; this will tell 
you how many parallel strings you need to safely handle the current.

Also, in your photo there is a second vertical wire at the left-hand side of 
the photo, running under the caps. At least one cap is resting directly on 
this wire. This is bad, as the body of the capacitor has only a thin layer 
of insulation  over the outer foil. Each cap should have at least 3/8" of 
air gap all around it, particularly to any conductive surface.

Regards,
Scott Hanson
----- Original Message ----- 
From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Thursday, October 25, 2007 8:28 PM
Subject: Fried Caps? (fwd)


>
>
> ---------- Forwarded message ----------
> Date: Thu, 25 Oct 2007 23:11:59 -0400 (EDT)
> From: Victor Valencia <victor_valencia2@xxxxxxxxxx>
> To: tesla@xxxxxxxxxx
> Subject: Fried Caps?
>
> Hi,
>
>  I have a pretty basic TC that I built
> a couple of years ago.  It worked great
> in the beginning but I am having issues
> with the Tank Cap.  Here are the
> design parameters:
>
> NST 12Kv, 30mA
> Spark Gap: copper tube-based, 9 gaps with @20 mils/gap
> Tank Capacitor: 77x GE 42L 3kv, (7 strings of 11 caps) (.0066 uF total)
> Secondary Coil - 3.25" diameter, 801 turns 24 awg wire
> Top Load: 4.5" diameter aluminum ducting. total dia = 15"
>
> The problem I am having is that random capacitors just seem
> to heat up, melt, and/or catch on fire (lovely show, pic
> attached)
>
> Should I reduce my gap?  I thought that a 12kv
> input with 33kv for each cap string would be enough of
> a safety factor.  Is there some minimum spacing
> I should keep between the adjacent capacitors and/or
> the caps in the next string?
>
> Victor
>
>
>
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