Re: AC vs DC charging circuit question

["Malcolm Watts" <M.J.Watts-at-massey.ac.nz> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Hi Gavin,

On 10 Aug 00, at 12:12, Tesla list wrote:

> Original poster: ghub005-at-xtra.co.nz 
> 
> Hello list
> 
> I've been wondering about something for the last few days. 
> 
> In an AC charging circuit it is common for the SG to fire near the 
> peak charging voltage (I'm disregarding LTR circuits). What 
> happens to the energy carried in the latter half of the charging half-
> cycle? I guess that this energy is just dissipated through the 
> 'shorted' spark gap - but wouldn't that make a fullwave DC charging 
> circuit twice as efficient? (i.e. since the charging circuit's energy is 
> stored right across the half-cycle)

When the gap shorts the transformer secondary/charging choke 
(by proxy if it's on the primary side of the transformer), 
energy continues to be stored in the leakage inductance. When 
the gap opens, that energy appears as an additional supply in 
series with the transformed mains and goes to usefully charge 
the cap. This is why it is theoretically possible to charge a 
cap to 2xVpk over a half cycle in a charging circuit that is 
resonant at Vsupply. 

> However, I understand that equivalently powered DC and AC coils 
> don't display a large amount of difference in their spark output(?).
> 
> Can anyone tell why this is? Of course I'm assuming that the DC 
> filter cap is isolated from the tank cap (otherwise it would also just 
> discharge through the SG). 
> 
> Of course that begs the question of how to actually isolate a DC filter 
> cap(s) from the resonance in the tank circuit. I'd be interested to 
> know what methods are in use.

You typically use a charging choke to do a similar boost job 
as outlined above. 

Regards,
malcolm