Re: Cap charging

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

Original poster: Bart Anderson <classi6-at-classictesla-dot-com> 

Hi Luke,

Tesla list wrote:

>Original poster: "Luke" <Bluu-at-cox-dot-net>
>Can someone tell me if I have this right?
>
>Let's assume a static gap and an NST of 15KV 60mA for this question.
>
>If the gap is set to fire at 15KV the gap will fire at each peak of the 
>cycle (120bps).

If the gap is set to fire at 15kV, the gap will fire when it charges to 
15kV. If the charge is slow, it may take many cycles to reach the potential 
to arc the gap.

>If the gap is set to fire at 20kv the cap will continue to charge until it 
>reaches 20kv and then poof the gap fires.

>Do I have a correct analogy for how this happens?

Yes.

>I think of it like the NST being a battery and the cap being another 
>battery that will charge the cap.
>So the NST will charge the cap to peak, then on the next cycle the NST's 
>polarity will be opposite so the cap and NST can act like two batteries in 
>series (like         + bat -  + bat - ) so now the NST will combine with 
>the cap will produce more and the cap will store that charge.  This will 
>continue with each cycle until enough voltage is built up to fire the gap. 
>Is this correct so far?

Ok. That will work. The cap charges. Should it not be charged sufficiently 
to fire the gap, the cap will continue to charge through the peak. As the 
voltage passes through zero, the cap polarity reverses and continues to 
charge. This system continues until the gap fires (or something fails).

>Now if the gap is set to fire again at 20KV and the cap is of a small 
>value so it charges up very quickly can the cap charge to the 20KV in one 
>half of the ac cycle?

Yes.

>A post I found in the archives made it sound like it could.  This confuses 
>me.  Because in only one half of the ac cycle we essentially have a dc 
>voltage that is simply rising in value to a max.  and if dc is applied to 
>a cap no matter how high the dc voltage is the cap will never reach a 
>voltage over that.  So if in the case of a small cap that charges rapidly 
>can reach say the 20kv in one half cycle of the 60hz cycle how does that 
>happen?

The voltage increases from 0 to 20kVp (peak transformer Vout) in one half 
cycle. It is mainly the "current" and "cap size" that determines how fast 
the cap will charge to this voltage. If the cap is small in size and the 
current large, the cap can easily reach 20kV peak in one half cycle.

>   If it cannot and it indeed does happen over successive cycles then I 
> would like someone to at least let me know I am on the right track.

If you can find a copy of Microsim 8 or similar analysis program, that is 
very helpful to get a feel for cap charging. You can view say cap volts 
with input volts (or whatever). You can even model your own transformer and 
run through various charging cycles. Change only cap size (start decrease 
it's value in half) and watch how the charging cycle reacts.  It all gets 
pretty wild.

Take care,
Bart