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Registered Member #59110
Joined: Mon Apr 11 2016, 04:35PM
Location: Camarillo, California
Posts: 74
I am sorry if this is an obvious or previously answered question.
I am working with various HV circuits to generate HV AC to drive electrode-less plasma tubes. One of the circuits is push/pull MOSFET driven flyback. (the circuit is from Build the Poor Man's Plasma Globe, by R. Iannini and Marc Spiwak).
The circuit works well and is able to drive the small plasma tubes I have filled to date. I am currently attaching one side of the secondary winding to the tube and the other side to the other end of the tube.
However, in the final design I want to have only one electrode attached to the base of the tube. Like a commercial plasma globe. This leads me to my problem. When I have the two leads of the secondary floating, I can draw a decent arc of either side. When I ground one lead, the output voltage goes down significantly.
My only theory is that since the primary winding is symmetrical (to first order) and is grounded in the center, that the secondary winding (if it is symmetrical) has a virtual ground in the center and by grounding one side I am effectively shorting out half of my AC voltage.
Does anyone have a more feasible idea and information on how to get a single ended ie ground referenced HV AC signal?
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
Do you have any more info on your transformer? A picture of the transformer and the setup would also be useful to diagnose the problem.
The problem you describe makes me think that something funny is going on, maybe the secondary winding is configured in an unusual way. It is also possible that grounding one end alters the resonant properties of your transformer, causing the driver to operate farther from the optimal point.
Registered Member #59110
Joined: Mon Apr 11 2016, 04:35PM
Location: Camarillo, California
Posts: 74
Wolfram,
Thanks for the reply. I'll have to try the resonance theory. It seems plausible.
I've tried two transformers. One is a custom one designed by a user called kiat. It's an iteration on something called a fiddy flyback (another user?). The link to the thread on this transformer.
The second is a transformer sold by CHIRK. It's available from Images Scientific. They both have this issue/feature.
I've got a few antique flyback units on order. I'm curious if they do the same.
Registered Member #59110
Joined: Mon Apr 11 2016, 04:35PM
Location: Camarillo, California
Posts: 74
Wolfram,
At first look your resonance shifting idea has merit. I don't have any AC voltage measuring equipment, at these frequencies (10's of kHz) such a thing seems like it would be difficult or expensive. So I'm using arc length in a long higher pressure (450 torr) neon tube as a rough guide.
When I get the "bipolar" (floating?) arc to strike I tried grounding one side of the flyback and going uni-polar. Initially the arc does not happen but after some frequency tuning the arc starts up again. It doesn't look quite the same but that may be do to the change in frequency.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714
Nice tube there, Alan. Is your iodine at a pressure in equilibrium with a bit of the condensed element? Are you familiar with electrodeless sulphur lamps?
Registered Member #59110
Joined: Mon Apr 11 2016, 04:35PM
Location: Camarillo, California
Posts: 74
klugesmith,
The tube is just practice. I'm trying to get some real globes and larger tubes built.
The iodine is at equilibrium. Without the neon the vapor pressure is a fraction of a torr.
I seem remember that sulfur lamps require pretty intense microwave bombardment to operate, put out good quality white light and are hard to make small. That was back in the 90's.
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Flyback Output Question
