Re: DC Tesla Coil

From: 	Gomez[SMTP:gomez-at-netherworld-dot-com]
Reply To: 	gomez-at-netherworld-dot-com
Sent: 	Saturday, January 03, 1998 3:48 PM
To: 	Tesla List
Subject: 	Re: DC Tesla Coil 

Tesla List wrote:

>     If my tank capacitor is charged with DC only, it seems to me that it
> would be much easier to get a solid resonance occuring in the tank circuit
> with a fixed spark gap (well quenched).
>     For instance, lets say my TC is resonant at 100 khz, it seems to me that
> all that is necessary to get a solid resonance is a fixed spark gap that
> will quench and fire 100,000 times per second, and capacitor power supply
> combination that exactly replenishs at this same rate.

That just isn't going to happen.  In fact, when your spark gap fires, it does 
_not_ quench and stop conducting with each cycle of the RF- in fact, you 
_want_ the spark gap to continue conducting for at least long enough to 
establish a "ringing" at the resonant frequency of your tank circuit.

If all one had to do was bang the primary periodically and let the secondary 
ring, we wouldn't need to make the tank resonant.  But in fact, RF 
resonance in the tank is the only efficient way to transfer energy into the 

>  DC avoids having to
> have your spark synchronous with a (AC fed) capacitor that 60 times (USA
> 60hz) every second has *no* charge on it.

Have another look at the sine wave that you're feeding that cap.  The period 
of time your cap has "no" charge on it, is almost infinitessimally small.

>     Can a fixed spark gap be quenched at that rate? 

No.  In fact, spark gaps are so eager to stay conducting because of the 
ionized arc channel that they'll happily stay conducting through several 
60Hz cycles, which we _don't_ want.  And _that's_ why we use 
mechanisms like quenched gaps or RSG's, to overcome their poor turn-off 

(side note- spark gaps are a good "closing switch", and they've been around 
for a long time.  The design of a fast "opening switch" is one of the design 
challenges that frustrates high power switching people to this day)

>  To get DC into the tank
> capacitor, is it as simple as feeding rectified AC into your transformer?

Um, last I checked, transformers steadfastly refused to operate on DC.  If 
you've got one that will, I will make you very, very rich!

Gomez: card-carrying mad scientist, extreme fetishist, fiction dabbler, 
pyrophiliac, technomage, goth, SF fan, lighting designer, dominant 
pervert, and juggler of labels... http://www-dot-netherworld-dot-com/~gomez