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Re: Another MMC cap candidate (UK)

    Michael wrote:
Michael wrote

>I am about to make a MMC for my 4" coil with 3 x 15/30 NSTs. Here in
>Australia these same caps listed below (from farnell) cost $1.45 each (up
to
>25), and being a 16 year old i am hardly "in the money". Would i be
>presenting too much stress on these caps to run them in a string of 10
>(1500v ea) for total of 15Kv DC rating from 15Kv (AC) NSTs?

The caps are cheaper if you buy 25+ or 100+ (anyone else interested in OZ)
Voltage wise you may get away with it but its iffy. I was trying out
shorter strings
on the grounds that if 6 works then 9 will be fairly bullet proof at 8KV
AC.

IkV AC is equivalent to 1414V peak to peak DC.  The current thinking on
this
list is that we should match the total DC rating to the peak to peak votage
input.
So 1500V DC is equivalent to 1000V AC so for safetly you need 15 caps in
each
string. 

Money wise its better to build something that lasts that something thats
likely
to fail. If you do decide to run close to the edge then be aware that a
small coil
because of its high frequency (say 200kHz plus) will heat the caps more
than
a large coil with its lower frequency (say under 100kHz).

Also, If something went wrong, would i be likely only to blow one cap, one
string, or
the whole lot?? One string, the caps fail short circuit increaseing the
voltage
strain on the rest.

>I'll be using 4 strings of 10.
0.018uF, 10kVAC  ZAP I'm afraid.
try:
0.009uF,  3 strings of 15
or 
0.012uF, 4 strings of 15.

You can compensate with more primary turns. 
(You don't need to use all of you transformer current)

>Also do i still require a safety gap?
Yes

>One other question (pardon my lack of knowlege) what does MOV stand for?
Metal Oxide Voltage? widget - its like having 2 zener diodes back to back
at a given voltage it conducts - useful for taking transients out of the
mains
but not really practical as an alternative to safety gaps.

My thanks also to Mike for the suppliers, RS and Farnell tend to have a 50%
or 100%,
mark up, on the one off prices but they have to be much closer to wholesale
on the 
1000 off prices or no one would use them, plus delivery if free and they
take credit cards.

>I plan to buy another 130 of these 47nF/1500V units from RS.
>I can't find a cheaper supply.  However there is a significant
>reduction in unit cost for a bulk order, if any UK people are
>interested ?

>100 off -at- 0.348   500 off -at- 0.32   1000 off -at- 0.254,

I intend to buy 200 of these caps soon.

Anyone else - it would be nice to get to 1000+
The differnce between 100+ and 500+ wouldn't pay the postage costs.

>When I built my MMC I simply stacked the capacitors in an
>8 by 8 block shape, (kind of like a hexagon pattern.)

If it works I can't argue but I'ld worry about heat build up and voltage
insulation.
The total length of the leads will be no longer on a pcb - more induction
though.

I was planning to use separate strings and a resistor across every
capacitor.

What coil are you running off this - transformer voltage, frequency,
spark length?

Alan



Alan