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Re: MMC partial failiure
Hi Nick,
> Original Poster: NickandSim-at-aol-dot-com
>
> Hi All,
> my little coil's mmc seems to be undergoing some sort of weird
> failiure mode.
> The coil functions normally, throwing nice 18" arcs and drawing 80ma -at- 5kV,
> then it starts to pulse in and out of tune.
> The secondary discharge stops almost totally and the spark gap makes the
sort
> of hiss that is does when the coil is very badly tuned. After a few seconds
> it pulses back into tune only to drop out again and repeat what I have
> already described.
> My theory is that in one of the 3 strings of caps is a cap close to death
and
> that it is breaking down reducing the capacitance of the string and taking
> the coil out of tune, then self healing a bit then breaking down again.
> Unfortunately not having an accurate C meter I can't measure the strings to
> check this.
> The caps are 1600V 47nF philips 378 series in strings of 10 with 3
strings in
> parallel and the neon is a 6/60 (I said 80ma 5kV because it sags quite a
lot
> under load)
> It is not a reso-cap so I don't see how a large enough voltage to damage the
> caps is being generated off a 6/60 neon?
>
> Semi-baffled in GB
> Nick Field
What is the surge impedance of your primary? It is important to
realize that the number of parallel strings must be proportioned to
primary current. I am currently running 15 strings in parallel for a
peak primary current around 700A (that's about 50A peak per string)
and I can detect some mild warming of the capacitor leads. The caps
are being run about 40% below their DC rating.
I suppose this problem doesn't occur with other types of
capacitor? Might not the gap itself be getting too hot and failing to
quench?
Malcolm