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



Hi,

Those very long proven bleeder resistors don't hurt or effect anything. I gave almost 10,000 away free at a cost to me of like $200 in the old days :D If it just saved one person's hind end, it was well worth it!

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=10MH-ND

I no longer stock them. :p But I order $10K's of part from digikey now days and ship stuff all over the World daily, so twist my arm :D

If you do use a spark gap across "any" cap, you also definitely still need a current limiting resistor too! A dead short across these powerful Tesla coil caps can easily draw 1000s to 10,000s of amps which will damage/destroy MMCs and have been very well known to destroy those high dollar commercial caps like 'pop corn' too.

So I think you are just making things worse by trying to add multi spark gaps and now limiting resistors. The main gap is just like a voltage limiting spark gap anyway. If you think it might fail, just parallel in another cheap gap to function in case of main gap failure. The primary inductor limits the current then.

Of course, MMC caps are specifically designed to short internally too like 100,000 times! There breakdown rating is about 3X their DC voltage rating. They can internally take their "own" stored energy of a dead short. Thus again, I think you are just waisting your time there.

Best Regards,

   Terry



mddeming@xxxxxxx wrote:
Hi Connor,

Congratulations on not zapping yourself, YET.
With resistors running about 1-2 cents each in bulk, I can't see that it is a major expense for added safety. Many older coilers (30-yr experience+) believe they got to be that way by using both a J-stick AND discharge resistors.
Using 20 10-meg resistors would "waste" three MILLIWATTS per string.
I find it hard to believe that you could really notice a loss of 0.006 watts out of 2880 watts total. That's about 0.0002%! Seems like a very small price to pay for the added safety.

Matt D.


-----Original Message-----
From: CONOR WHYTE <cjwhyte@xxxxxxx>
To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
Sent: Sun, 17 Aug 2008 11:37 pm
Subject: Re: [TCML] mmc cap protection by spark-gaps-any ideas?



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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