Re: Fried Caps? (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Fri, 26 Oct 2007 10:32:58 -0700
From: huil888 <huil888@xxxxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: Fried Caps? (fwd)

OOPS!

There's a typo in my previous post on  GE 42L series cap failures.

The Cornell-Dubilier caps you want are the 942C series, not the 940C series. 
The 940's are metallized-film construction, just like the GE 42L series. The 
C-D 942 series utilize hybrid film-foil construction, with aluminum foil as 
both outer plates, and a metallized-film inner plate. The foil outer plates 
provide "real metal" for the sprayed-on end connections to attach to, which 
can handle high peak current. The metallized-film inner plate provides the 
"self-healing" feature that prevents catastrophic failure in case excessive 
voltage punctures the dielectric.

Details of this hybrid construction can be seen on C-D's website.

Regards,
Scott Hanson
----- Original Message ----- 
From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Friday, October 26, 2007 8:08 AM
Subject: Re: Fried Caps? (fwd)


>
> ---------- 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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>
>
>
>