RE: [TCML] mmc cap protection by spark-gaps-any ideas?

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

I would agree with Scott's (Bunnykiller) reply, in that the power used by bleeder resistors isn't terribly significant.  Did you actually compare the spark length with the same cap and everything else, with and without the resistors?

I also didn't follow your point regarding the 10% tolerance and ability to handle voltage spikes.  Do you mean able to handle a higher voltage in general, and is somehow correlated with the tolerance?  Tank caps see 60 Hz activity and tank frequency activity, both to the same peak voltage, but I'm not aware of "spikes" outside of these realms.

Regarding safety gaps across caps, yes, this is a bad idea because it subjects the cap to unhealthy currents should it fire.  The same level of protection can be had by putting the safety gap in parallel with the main gap, instead of across the cap.  There is no way possible that the voltage across the main gap can exceed the voltage seen by the cap, so the protection is identical, except that the discharge current is then safely limited by the primary inductance, rather than directly shorting the cap.  Exactly how bad shorting a charged cap with a safety gap is isn't clear beyond the fact that it violates the current spec of the caps.  What is clear is that it's better not to do that since an equivalent degree of protection is available by putting the safety gap in parallel with the main gap.

Discharging an MMC with a safety wand is not the same as having bleeders across each individual cap.  Discharging an MMC end-to-end without resistors will ensure that the NET voltage is zero, but the charge on the individual caps may or may not each be zero.  You'd have to short each individual cap to guarantee that.

Regards, Gary Lau
MA, USA

> -----Original Message-----
> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
> Behalf Of CONOR WHYTE
> Sent: Sunday, August 17, 2008 11:38 PM
> To: Tesla Coil Mailing List
> Subject: Re: [TCML] mmc cap protection by spark-gaps-any ideas?
> Importance: High
>
> I don't use resistors on my MMC. I find these waste energy and cut down on my
> spark length. I  use CDE 942C series 1.6Kvdc 680nF capacitors, I use 20 per string
> 34nF @ 32Kvdc x2 . I run 12Kvac @ 240mA. I have been running my MMC now for
> 2 years and have had 0 problems.
> Quite often many people are scared about residual charge in the cap bank this is
> why people add the resistors. It bleeds charge away after the unit is turned off. You
> can do this with a discharge wand MUCH cheaper.
> Like I said I have never used them. It is a waste of energy.
> If you feel like you want to protect your cap bank, I'd might make an extra gap that
> one could adjust.
> maybe say to be safe a 3.5cm gap 35Kv.  I would also make sure the MMC total
> DC rating is around 3x that of the input voltage.  12Kvac -- 32-36Kvdc.  Another
> factor I would also check is the tolerance rating of the capacitors most are 5%...
> which is ok... CDEs on the other hand are 10% and can handle much more voltage
> spikes.
>
>
> ----- Original Message -----
> From: vatercox@xxxxxxx
> Date: Monday, August 18, 2008 10:47 am
> Subject: [TCML] mmc cap protection by spark-gaps-any ideas?
> To: tesla@xxxxxxxxxx
>
> > Dear List
> > I have 30 x 47nF polypropylene metal foil caps, each rated at 2
> > kVDC
> > which I intend to wire as three strings of 10 capacitors for
> > approximately 14nF at 20kV DC. While normally each capacitor in
> > an MMC
> > would be protected by a 10- or more megohm high voltage resistor
> > these
> > have proved difficult to obtain (due to lack of demand?). In liu
> > of
> > this would it be possible to protect each cap with a miniature
> > spark-gap -perhaps a pair of closely-positioned stiff wires
> > connected
> > to the capacitor leads notwithstanding the fact that 2 kV is
> > usually
> > regarded as a bit on the low side for satisfactory spark-gap operation
> >
> > Jolyon Francis Vater-Cox
> >
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