RE: [TCML] LTR/STR

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

Hi Neal,

First, let me try to make it clear that operating with a cap that is mains-resonant with the NST secondary is NOT a guarantee that bad things will happen.  It's merely an "enabler" of bad things.  With a mains-resonant cap, the circuit will "want" to ring up to a voltage far in excess of the NST faceplate voltage, but simply by having your static gap or safety gap at a setting that will fire at the peak NST secondary voltage will ensure that hazardous levels are not reached.  Trouble is, particularly with beginners, folks find that the wider they open their gaps, the better performance becomes, so setting the gap width PROPERLY is often not done.

That's where using LTR caps comes in.  If the cap size is sufficiently larger (or smaller) than what would resonate with the NST secondary at 60 Hz, there will be minimal mains resonant rise, and the hazard of opening the gap too wide is reduced.

Now to your situation.  With a 15/60 NST, the mains-resonant cap would be .01uF, or 10 nF.  So your 4 nF cap is significantly STR.  That's OK as far as avoiding mains-resonance, but it's limiting the performance of your coil.  I have found that the optimum cap size for a 15/60 NST is .02uF.  Spark performance correlates with "bang size", or the amount of energy that the cap discharges each time the gap fires.  This energy is calculated as 0.5 * C * Vgap * Vgap.  The higher the capacitance, the bigger the bang.

If you are planning to downsize to a MOT (not what I would call table-top sized!), you will need an even larger nF cap size, since the voltage is much less.  For my table-top coil using a 4/20 NST, I use a 25nF cap.  Are you locked into the 4 nF cap?

Regards, Gary Lau
MA, USA



> -----Original Message-----
> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
> Behalf Of Neal Namowicz
> Sent: Sunday, February 10, 2008 6:14 PM
> To: Tesla list
> Subject: [TCML] LTR/STR
>
> Hi all,
> How does one go about determining whether or not their cap bank is LTR, STR,
> or? From what I understand about it, if it's at line resonance, the smoke and/or
> resulting fire would let you know. :) I'm fairly sure that my current coil is safe, it's 4"
> secondary, single static gap, 4nf capacitance, and powered by 15kv/60ma nst. I'm
> looking to downsize though to a small, small tabletop coil, probably driven by a mot.
> I've got basic meters but no scope, and if it involves any type of math formulas, I
> ask that you utilize the k.i.s.s. principle with your explanation!
>
> Thanks for your help,
>
> Neal N.
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