Re: Re[2]: Capacitor Size (long)

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Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>



Original poster: FIFTYGUY@xxxxxxx

Original poster: "Lau, Gary" <Gary.Lau@xxxxxx>

> I disagree on two points.

>1) The width of a static gap should NOT be set to
>achieve 120BPS.  While I agree that adjusting the
>static gap width does permit one to adjust the
>BPS to almost any desired value, it ignores the
>most important consideration in setting gap
>width.  The width must be such that the breakdown
>voltage does not exceed what's safe for the NST.


"     I think we would all agree that this would
be the most prudent setting without a safety gap.
However, I think it can't hurt to open up the
main gap to where it works best, so long as the
safety gap is adjusted properly."

If the safety gap is properly set, yes it wont hurt to open up the 
main gap. However, once the main gap becomes too wide, it stops 
firing and the safety gap then determines the BPS (it probably 
doesn't quench as well).  If you are trying to lower the BPS by 
opening up the main gap, there will be a minimum BPS you wont get 
below because the safety will be firing.  The only way to lower the 
BPS from this value will be to increase the Cp or lower the input 
voltage to the NST.


> 2) A static gap firing more than 120 times per
> second is not uncommon, it's the norm.  I base
> this opinion on scope measurements of my coils.


    So assuming folks "properly" set their main
gap by using just the NST output alone (no prim
cap), how come the break rate is *higher* than
120? How is resonant charging occurring to drive
the cap charge voltage up, if the bps is higher
than 120 and the cap value is properly LTR?

120 BPS is an approximate estimate for Cp = LTR for static gaps 
(1.6*Cres) with the main gap set to fire at the rated output of the 
NST (for 15KVrms NST with 120V input, firing set to occur at 
21.2KVpeak).  If you have the safety gap set to just not fire with an 
unloaded NST using a variac at 140V, the main gap set to fire at 
21.2KV and the variac at 140 volts, you will have BPS much higher 
than 120.  Also many NSTs will put out more than their name plate 
specifications.  For example, it is not uncommon for a 15KV nst to 
put out 15.5KV unloaded or more than there 30ma when shorted.  The 
Cres of a particular NST should be based on actual measured Vs_oc and 
Is_ss for that NST with whatever input voltage you are applying.

Gerry R