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Re: QUEUE



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William,
         You write:

> 1. how do you decide what diameter tubing to use for primaries - I am building 
> a small coil, and I bought a box of 3/8 tubing, but several persons 
> recommended 1/4.  I won't be messing with the tubing for a couple of weeks so 
> there is plenty of time to return it and get the smaller (cheaper) stuff.

If you have 3/8" tubing, use it. It is better than 1/4" (lower loss).
 
> 2. A more technical question.  I don't understand the need for the HV primary 
> voltage.  As best I can figure out, what drives the output is primary current, 
> not voltage - after all, there is no way you will have 15KV across a copper 
> buss bar - what you are doing, it seems to me, is creating a big pulse that 
> then resonates back and forth through a tank circuit made up of the primary 
> and the capacitor (whilst the spark gap conducts), and the primary only 
> (higher freq osc) when the spark gap is not conducting.

But you are not dumping the cap into a short circuit. You are dumping 
it into an inductance. Iprimary = Vc/Xprimary
where Xprimary = 1/2*PI*F*Lp
 
> So, why can't I just charge up a farad or so of "computer type" electrolytics 
> to 100VDC and then dump the charge through a 1,000 amp SCR into the primary 
> directly????

The peak current will most likely kill you SCR and the inductance you 
need to resonate the tank with your secondary will be vanishingly 
small. Also, the electrolytic will not stand the voltage reversal it 
will experience when the tank swings through zero and charges it up 
with the opposite polarity - if it could with a one-way conducting 
device in the circuit :)


>  Has anyone tried this??  I would imagine having a LC circuit of 
> the tubing for the L, and a suitable AC capacitor, as a tank 
> circuit.  Then, 
> the SCR and the capacitor bank would share a common ground with the tank 
> circuit.  The SCR would fire, dumping charge, and then the reverse current 
> would shut off the SCR.  The pulse would resonate in the tank for a while, 
> then you would fire the SCR again.   A crude schematic is represented below:
>               Gate Drive
>              /
>             /
>    L-----|---------------SCR----------||||||||||||||||||||||||||  ------ +++   
>  
>    L      C                                  CCCCCCCCCCCC     charging circuit
>    L-----|____________________|||||||||||||||||||||||||||_____- - -
> 
> This would seem to have several benefits over a spark gap interrupter - lower 
> loss, faster switching, lower (hence safer) primary voltages, and lower cost 
> primary power supply.  Comments???

Watch the dI/dt rating of the SCR. The circuit cannot ring unless the 
"switch" can conduct in both directions.

Malcolm






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