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Re: SGTC

Original poster: "resonance" <resonance@xxxxxxxxxxxx>
Case# 2 (xmfr output in parallel w/spark gap) is usually the best option to prevent undesireable kickback into the xmfr sec windings. Also, always use a Terry filter or equivalent between your NST and the parallel spark gap.
Even PT and pole xmfrs need a protection circuit if you want to keep the high freq currents out of the secondary windings --- you only want 60 Hz current there. The HF currents form "spiders" tracking across the HV windings and can lead to break down of the sec winding. These are caused RF which likes to "track" across insulator surfaces. Not good inside a power xmfr. Pole xmfrs will tolerate some abuse but good design says to eliminate the problem before it occurs --- prevention.
If your design risks shorting the xmfr I would suggest your spark gap has a serious quenching problem that needs to be addressed. 

Dr. Resonance



An interesting question for the group:

Which circuit makes a better SGTC and why?
1) The cap in parallel with the HV transformer and the spark gap in series with the TC primary 

OR
2) The spark gap in parallel with the HV transformer and the cap in series with the TC primary 

OR
3) The SG in parallel with the HV transformer and caps in series with EACH side of the TC primary
I have plans dating back 100 yrs to the present and all configurations are used for small and large coils. There does not seem to be any preference and I have circuits using 20 KV transformers with all options above.
The gap in parallel configuration seems to be slightly more present in designs but not by a large margin.
The SG in parallel adds some over volt protection to the HV xfmr but also the risk shorting the transformer is present if the gap is left closed for any amount of time.
The third option completely isolated the TC primary from the HV transformer making it by far the safest for shock hazards. 

Thanks, Frank