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Registered Member #160
Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938
I know that Richard Hull made a Tesla coil using a thyratron and some guy in germany did as well, but has anyone here made one? I was wondering if it is possible to audio modulate a thyratron coil. Seems to me it is just a matter of pulsing the grid with audio frequencies. Ideally there would be two thyratrons in reverse parallel to allow each RF cycle clear run. Otherwise it would only be a half wave unit.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Your reasoning is sane, dunno if ive seen it done though.
Perhaps even easier would be to use a DC power supply and just bypass the reverse thyratron current with a big HV diode (needs to handle big peak currents still). But i only say this because im not sure how thyratrons are triggered, so if the triggering is easy then just use 2 of them.
Registered Member #160
Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938
Yeh, the HV diode was the other option, significantly cheaper too than two thyratrons. Huge current capability though. The unit I was looking at has a peak current of 90A @ 8kV. Grid needs to be between 175-200 V. Don't know how easy that is to control. AC power supply would simplify switching topology I believe. It would need to be shielded from the coil too as not to ionize the gas in the tube. They come in different sizes but perhaps it is possible to parallel them for greater current capability?? The larger ones are not so easy to find.
Registered Member #1381
Joined: Fri Mar 07 2008, 05:24PM
Location: Hungary
Posts: 74
Dont think it's possible to audio modulate a Thyratron , cause the way it functions. The ones (25kv , 1kA/s) we use at the lab (work). Won't conduct till the grid gets a "big" + Voltage (around +120-150V) , and you can't "turn off" the device via controlling the grid voltage , it will keep conducting untill anode voltage or current drops to zero.
Registered Member #1025
Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566
I think Mathias is pretty right...The thyratron as switch works simillar like a xenon tube - once triggered it stays open until the anode voltage drops to minimum.
Registered Member #160
Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938
Ohhh...my bad. I was thinking that the grid was negatively biased but it is actually creating the conduction rather than repelling it. That explains the difference between the thyratron and the triode. There goes that idea then. It is a wonder though that more people haven't used them in TC use though, I suppose the half wave problem isn't attractive.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Oh, i was figuring you would audio modulate it like we do the DRSSTCs, with pulse frequency modulation rather than some sort of amplitude modulation of a CW output.
Registered Member #160
Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938
Either way, it wouldn't work with a thyratron I realize now. PWM crossed my mind as well, but the grid voltage is used to turn it on, once on the current won't allow it to go off even with zero grid voltage, therefore unless a negative voltage can prematurely arrest the current flow, I can't see either PWM or AM working.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Nope, you mis-understood me again.
Fire the coil like a spark gap coil, except you control when, and how often, it fires. This would work fine with a DC power source and say a resonant charging inductor and diode set up to charge the tank cap up to some fairly high break rate.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
In tetrode thyratrons, the auxiliary grid is used to modify the anode delay time.
An amplitude modulated signal applied to the auxiliary grid would have the effect of advancing and retarding commutation (within rather narrow limits) and so would produce a type of pulse position modulation.
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Thyratron Coil
