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Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Ok, I've been doing a bit of reading up on TC's in general, and SGTC's in particular, and my first question is which of these basic primary tank circuits is better and why?
Most people, I think, say the second has advantages, although on Ritchie's site he tends to depict the first, which is also the layout I used when I built an HF TIG welder some years ago.
I'm after in-depth answers, please.
Also, are there any differences when driven from AC or DC supplies, or when using static or rotary gaps, etc?
(I've left out details of power supplies/diodes/chokes/resistors, etc. for now, although would either of the circuits above have advantages/disadvantages as far as the various alternatives regarding supply circuitry is concerned?)
Registered Member #42796
Joined: Mon Jan 13 2014, 06:34PM
Location:
Posts: 195
Ash Small wrote ...
Also, are there any differences when driven from AC or DC supplies, or when using static or rotary gaps, etc?
from what i read and from my attempts to build a SGTC: DC has the advantage of allowing voltage doublers but it will not work with a static gap or you can use a static gap and blow alot air through it (unreliable) or better use a rotary gap. Another advantage of rotary gap in DC operation is that it doesn't need any synchronization (DC motors can be used) and you can adjust the speed of the motor aka the BPS to whatever value you want
Registered Member #6038
Joined: Mon Aug 06 2012, 11:31AM
Location: Salado, TX
Posts: 248
Dr. Dark Current wrote ...
The first one stresses the transformer with RF AC voltage much more.
Agreed. I have fried several NSTs with this configuration too. Would use option 2 as a preference. Even still I prefer to have some RF chokes and safety gap across the transformer.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
The first topology sends sustained RF oscillations from the tank circuit back into the power supply. The second one sends a single abrupt voltage step as the gap fires and the voltage across it falls to zero.
You could argue for a while over which of these is more likely to damage the power supply. I think in practice the RF oscillations work out worse. In particular, the oscillations will quickly destroy a DC supply by reverse biasing the diodes and sending part of the tank current through them.
A good filter like the "Terry Filter" would probably protect the power supply in either case, but I think the RF oscillations would waste more energy in the filter than the spark gap firing transient.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Steve Conner wrote ...
The first topology sends sustained RF oscillations from the tank circuit back into the power supply. The second one sends a single abrupt voltage step as the gap fires and the voltage across it falls to zero.
You could argue for a while over which of these is more likely to damage the power supply. I think in practice the RF oscillations work out worse. In particular, the oscillations will quickly destroy a DC supply by reverse biasing the diodes and sending part of the tank current through them.
A good filter like the "Terry Filter" would probably protect the power supply in either case, but I think the RF oscillations would waste more energy in the filter than the spark gap firing transient.
Thanks for the detailed reply, Steve.
Of the various power supply circuits I've read about, DC resonant charging seems the most appealing (link to Ritchie's page on DC resonant charging here: ).
I assume the inductor in the DC resonant charging circuit will choke the RF (and spark gap spike, for that matter) and give adequate protection for diodes, etc?
(In my HF TIG welder I just used a high(ish) value resistor to reduce any RF that tried to get back to the DC LOPT (flyback) that powered the primary circuit)
I'm currently considering using an MOT with doubler lifted straight from a microwave oven, charging a series string of electrolytics, as the supply for the DC resonant charging circuit, maybe even with more multiplier stages, if practicable, as this seems a cheap, efficient and simple solution (I have several old MOT's, etc. in the shed).
Does this sound like a practical and viable solution?
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Steve Conner wrote ...
DC resonant charging worked for me!
That's pretty much what I had in mind, Steve.
I assume the reason you used four chokes was to reduce the voltage across each one to ~2.5kV, but is an inductance of that order really necessary?
(By the way, Maplin no longer list that part number, but I'm considering winding some myself, maybe with a lower Ohmic resistance. I'll do a bit more research first, though. I've wound a number of inductors over the past year or so although I've been a bit too preoccupied to do much for the past few months)
Registered Member #30656
Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208
If you need more voltage, you can put two MOTs in series without any extra insulation stress, just connect both transfomer cores to ground, and with the parallel primaries phased correctly you'll get ~4kV between the output QC tabs, balanced around ground.
This arrangement (with a DC resonant charging circuit) was the basic plan for the coil I never got around to building about a decade ago, so it sounds good to me.
Should be reasonably easy to get a quick simulation of the TC (or at least the power supply/tank circuit) going in LT Spice to sanity check everything and get a handle on how much inductance is required. Modelling the spark gap and streamer load will involve a large amount of guessing but you'll at least be able to get a good idea of how everything should work.
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Basic SGTC questions
