RE: Helping someone with broken coil (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Wed, 22 Aug 2007 08:40:37 -0400
From: "Lau, Gary" <Gary.Lau@xxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: RE: Helping someone with broken coil (fwd)

Hi Chip:

While it's possible to suggest what capacitor value would be the best
match for the NST, it's also likely that the coil's owner would not wish
to alter the secondary, topload, and primary.  If I were to suggest a
capacitor value based solely on the NST, it's entirely likely that you
may also need to adjust these other components in order to maintain
resonance.  Or does the owner want to replace whatever's necessary if
bigger sparks are likely?

Assuming no changes to the other components, I would suggest that you
measure the secondary/topload resonant frequency, using either Terry's
Tesla Coil Tuner, or the usual function generator & oscilloscope.  If
you don't have that gear, then we'd need much greater detail on the
secondary (wire gauge, estimated total turns, and exact winding
dimensions) and topload configuration (exact dimensions) to calculate
that.  Then calculate or measure the primary inductance (WinTesla says
it's 47.4uH), and work backwards to the required capacitance value.

FWIW, with my 15/60 NST and a static gap, I got best results with a
.02uF cap, so that would translate to .04uF for a 15/120 NST.  I assume
the guy's coil also has a static gap?

Regards, Gary

> From: Chip Atkinson <chip@xxxxxxxxxx>
> To: tesla@xxxxxxxxxx
> Subject: Helping someone with broken coil
> 
> Greetings,
> 
> I was contacted recently by a guy who has a display coil  The
capacitor
> blew up and he would like to get the thing running again.
Unfortunately
> he was unable to get any specs from the cap so I'm kind of winging it.
> 
> His secondary looks to be 7" in diameter and 22" long.  The coil is
> powered by a 15kV/120ma transformer (nst?) and the primary has 10
turns
> with 4.5" inner radius and 13.5" outer radius.
> 
> My question is what is a good capacitor value replacement?  I figure
one
> rated for 45kV, but what capacity?  What have others used on coils of
> approximately this size?  My guess is 0.020-0.025uF.  Worth noting is
that
> 0.021 uF is a resonant capacitor so it should probably be avoided.
> 
> Thanks in advance.
> 
> Chip
> 
>