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
> 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
> 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
> 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