Re: [TCML] Cap Discharge Questions

[Gary Lau <glau1024@xxxxxxxxx>]

Comments interspersed:

On Tue, Jan 18, 2011 at 9:15 AM, Derek Esq. <a_key_move@xxxxxxxxx> wrote:

> Hi all,
>
> So I'm considering using a Maxwell 37667 for the capictor in an NST spark
> gap coil. After reading the old posts, I have a couple questions on
> discharging the capicitor. And this could apply to any suitable larger pulse
> capacitor I suppose:
>
> 1) I read and agree that after running the coil and turning it off, the
> capictor can and should discharge through the tank primary and NST secondary
> windings. In one of the older posts, it was suggested that using a hot stick
> to the cap would short circuit the discharge mechanism through the primary
> and NST windings. While hot sticking the capacitor would short circuit it to
> ground, wouldnt' that be equally acceptable as it would allow the
> capacitor to discharge to the RF ground? And that would also assume you were
> able to turn off the coil and get over to it in time to short circuit it to
> ground prior to the capacitor discharging through the tank and NST windings.
>

I think that hot sticks are typically used to short the two ends of a
capacitor together, regardless of whether they are tied to mains ground, RF
ground, or no ground at all (which is the case in most NST-powered coils).
Shorting the two ends guarantees that the cap is discharged, and that's all
that matters.  It might be that one of the cap terminals is tied to a
chassis ground, in which case the other terminal could be hot-sticked to the
chassis, if its extremely obvious what's tied to what.

>
> 2) It would seem to me a safe process to discharging the capictor would be
> to allow an appropriate amount of time for the cap to discharge through the
> NST windings. Check the cap with a multimeter with a suitable high voltage
> probe. And then if it's not discharged, discharge it with a capcitor
> discharge stick or a hot stick. Recheck the cap at that point and then do
> any adjustments to the coil, etc. Or am i missing something?
>

The discharge time through the NST primary is extremely quick - surely less
than a millisecond.  Using a meter is unnecessary, as its easier to simply
assume that there's a charge present and always use the stick to discharge
it, rather than first measure, then conditionally discharge (two operations
rather than one).  It also gets into the question of, are you sure that the
meter is properly connected and functional?  Would you bet your life on
that?

>
> Lastly, assuming your capacitor maintained a voltage on it somehow, is it
> better to use a capaictor discharge stick or a hot stick to discharge the
> cap or does it matter for a pulse capacitor since puluse capacitors are
> designed to take rapid voltage reversals? Could you damage the capacitor by
> hot sticking it?
>

Excellent point/question.  The hazard of discharging a cap with a direct
short is not voltage reversal, but of excessive discharge current, which
might damage internal capacitor connections.  Normally the cap discharges
into the primary inductor, and the current is limited by that inductance.
If you instead apply a direct short across the cap, the short circuit
current is now limited only by the cap's internal resistance and inductance
- both extremely low - and the cap's maximum current spec will be violated.

My personal philosophy is to keep a well-constructed and tested bleeder
resistor permanently attached across the cap, to ensure that it will be
discharged, even if the NST path is lost (and this happens!), without having
to resort to manual procedures.  Some would argue that the bleeder resistor
could fail or become detached without being evident.  I would counter that
the odds of that happening are less than the likelihood of becoming lazy and
not using the stick every time.

Regards, Gary Lau
MA, USA

>
> Thanks,
>
> Derek
>
>
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