NST and Bridge rectifier to stop res rise
Hi, Malcolm and all,
Your suggestion of adding a bridge rectifier to the charging circuit
to prevent resonant rise is very interesting. The addition of the
rectifier does "de-Q" the resonant charging circuit. However, there are a
few ways other implications which came to mind.
If the cap is connected across the rectifier output there will be a
problem. The problem arises when the gap fires and the cap voltage
tries to swing negative, forward biasing the diode bridge. This
will drive the diodes into heavy conduction and spoil the primary
ringdown. You could put a small RF choke in series with the
rectifier output but I guess that is defeating the object.
If the spark gap is placed across the bridge, the situation seems
perculiar. When the gap fires, the spark gap should conduct the
positive half cycles of the RF tank current, and the diode bridge
should carry the negative RF half cycles which would normally also
be conducted by the spark gap. At the start of each negative going
RF cycle, the diode stacks should start to conduct before the gap
reignites ? The positive volt drop will be the normal volt drop of
the spark gap, whereas the negative volt drop should be the sum of
the diode forward drops. (ie. A lot less.)
At first sight, this seems like it would almost eliminate the high gap
voltage drop on negative RF half cycles ! Therefore almost halving
losses in the primary circuit. I wonder if it happens this way in
practice. Maybe if the gap only conducts on positive RF half cycles
it will run cooler, and exhibit a higher voltage drop, negating any
benefit gained ? I would like to know other peoples thoughts on this.
The diodes stacks would have to be suitably rated to hold off the
peak HV voltage, and also to withstand the peak tank current I think.
(I hope I'm not stating the obvious here, but the diode chains
should have equalising resistors and equalising caps accross them.)
Has anyone tried this out with good or bad results ? I would really
like to know whether spark gap losses are reduced significantly.
- In Sunny Newcastle.