Re: LTR MMC Question - "Geek Group" caps

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Mark Broker by way of Terry Fritz <twftesla-at-uswest-dot-net>" <broker-at-uwplatt.edu>


>
>
> OK, so I think I'll go with MMC for my under-construction first coil,
> and am considering the Cornell-Dubilier 942C (aka  "Geek Group") caps
> (0.1uF or 0.15uF, 2000Vdc) for their high capacitance to cost ratio, but
> I'm having trouble figuring out the string length (caps in series) and
> how many strings I need.  Ideally, I'd like to be able to run with
> either two or three 15/60 NST's, and want to use an LTR cap size, so I
> think I need to be able to get 0.03uF and 0.045uF from the MMC. 

Looks good for a static gap.


>
> Question 1: How critical is the exact cap value when using an LTR cap? 
> For instance, can I use 0.05 with three 15/60's? 

not terribly critical.  "LTR" is a pretty broad description encompassing a
large range over resonant size.  The generally accepted value for an "LTR" cap
for a static gap is 1.5 to 2 times the resonant value (which is 10.6nF for a
15/60)


>
> Anyway, here are some of the options I was considering:
> 
> A) Using 0.15uF caps, 3 strings of 10.  
>    2 strings = 0.03uF, 3 strings = 0.045uF.
>    Quite cheap, but seems pushing it voltage (20KVdc) and current (?)
> wise. 

Looks really good.


>
> B) Using 0.15uF caps, 5 strings of 15.
>    3 strings = 0.03uF, 5 strings = 0.05uF.
>    30KVdc seems quite adequate.  Will 5 strings handle the current with
> three 15/60 NST's?  
>    And will 0.05uF be a good LTR size for three 15/60's? 

Going way overboard with the voltage rating.


>
> C) Using 0.01 caps, 6 strings of 13.
>    4 strings =  0.030uF, 6 strings = 0.46uF
>    26KVdc - adequate?, less than $150 - not too bad, good match to the
> capacitances I think I need. 

A good medium between A and B.


>
> Any thoughts?  This (the cap) is the only item I have left to purchase,
> and having made the mistake of buying a batch of 940C's in the past
> (before I found this list), I want to be careful this time. 

These CD caps are pretty tough little guys, and will take overvoltage in
stride.  Keep your safety gap set well, and you shouldn't have a problem with
solution A.


Mark Broker
G-3 Geek #1019