Re: MMC Problems!

["Malcolm Watts" <malcolm.watts-at-wnp.ac.nz> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Hi Arthur,

> Original Poster: "Arthur Mavros" <mpiece-at-one-dot-net.au> 
> 
> Hi all,
>         I have just completed building a MMC and I am having major problems.
> The caps are WIMA MKP-10's 8.2nf 2000vdc/700vac. I am running these as
follows.
> 15 in series=0.00054uf (30,000vdc)
> 12 strings=0.0065uf   (matched to NST)
> 15 strings=0.0081uf   (LTR to NST)
> Power supply is one 15kv/30ma NST.
> Spark gap is variable ,8-12 gaps 25thou each ,vacuum motor
quenched(variable).
> I have safety gaps+NST protection caps on each HV leg.
> All MMC caps have 10 meg 1/2 watt resistors across each cap.Each cap is
> soldered to the next with solid copper wire 1mm thick.
> The caps are mounted on Perspex with each string bolted to a thick 15mm wide
> brass strap. This is mounted in a wooden box with a fan for extra cooling. I
> thought that this set-up should be very durable.(WRONG)
>  
> As soon as I applied the power 1 or 2 of the caps went Bang.
> I checked for poor soldering or cross arcing but nothing, so I replaced the
> burnt caps only to have the same problem continue! The funny thing is
that some
> of the caps have gone bang as much as 1 minute after shutdown, I cannot
> understand why if they have bleed resistors across each of them?

What type of resistors did you use? Have you checked their values 
after the failure? If they are carbon film types some may well have 
gone high or open circuit. If such was the case, the stress placed 
on the cap across which such a resistor would be plaecd would be 
considerable. If this is the case and no other resistors are to hand, 
you may be better off leaving the resistors out altogether.

Regards,
Malcolm