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

["DC Cox" <resonance@xxxxxxxxxxxx>]

Your best protection is to not push them too hard.  Use 16 to 18 caps per
string and they will last for 10-15 years.
We have some out in museums now for over 3 years actively in use 3-4 times
per day x 7 days/week.  So far,
with 18/series, not a single cap failure.

Also, if one gets too greedy, say drops it down to 12/string, then, when one
or two caps fail it puts excessive
stress on the remaining units and they fall like dominos.

As the cap value changes, your coil goes slightly out of tune which puts
more stress on both the
pri caps and the NST.

I use 18 on all my designs (.0083 uF per string) and have no cap failures
running typical 200-400 KHZ frequencies.

Terri Fritz's design is rock solid and works great with all NST powered
coils.

Dr. Resonance




On Thu, Aug 21, 2008 at 8:47 PM, Mark Olson <kc5gym@xxxxxxx> wrote:

> Snips and comments:
>
> > .  Despite anything
> you can do, no two condensers are exactly the same. They each have a
> different internal resistance which means that when you connect a
> string of them across a voltage source, they will not have the same
> voltage across each condenser in the series
>
> $$$ The above statement is true without question.
>  _______________________________________________
> .  IT IS THERE TO PROTECT THE CAPACITOR AND NOT THE TECHNICIAN.
> The fact that resistor across the capacitor will discharge it is just
> a bonus but the fact that resistors fail is a fact of life.
>
> The high voltage was only 6000 volts d.c
>
> Ray
> _______________________________________________
>
>
>
> Yes, for DC applications, like power supply filter caps in series, the
> resistors certainly do help equalize the voltages across each cap.  Thus
> they do add some protection to the caps.
>
>
> $$$ The above statement is true without question.
>
>
> But in TC usage, the voltage across the series strings of caps is RF,
> changing polarity on the order of 100,000 times per second.  In this
> situation, the RC time constant for a bleeder resistor across an MMC cap is
> so large (seconds)in comparison to the roughly ten microsecond cycle time
> that the resistors will do practically nothing to equalize the voltage
> across the caps.
>
> $$$ Have you ever known of a condenser discharging a-c of any frequency?
> .
> I highly recommend adding resistors across caps in an MMC as one of several
> human safety precautions.  Cap protection is achieved by using caps of all
> the same type and having an ample voltage margin, e.g. run the caps at 1/2
> or 1/3 of their DC ratings.
> --Steve Y.
>
>
>  $$$ Not a bad idea, but I dont trust them...
> .
> Marko
> KC5GYM
>
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