Demo Fusor troubleshooting

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Anthony_Saffioti
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Demo Fusor troubleshooting

Post by Anthony_Saffioti »

My fusor setup looks good, but I can't increase my voltage passed a certain point without tripping the safety on the transformer. I checked my rectifier, changed the size of the inner grid, changed the outer grid, checked voltage, resistance, and current, especially at the ground lug. When I use the bypass button on the transformer it works as it should. I'm using high voltage feedthrough's from Max-Gain systems, an Allanson 15kV NST, and a variac from Amazon.

fusor1.jpg
fusor2.jpg
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Dennis P Brown
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Re: Demo Fusor troubleshooting

Post by Dennis P Brown »

You are exceeding the transformer current limit, so the breaker is doing what it is supposed to do. This allows an arc to form creating a high conductive plasma path to ground. Your system is likely in the torr range. Check your vacuum.
John Futter
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Re: Demo Fusor troubleshooting

Post by John Futter »

As Dennis has said
ypour vacuum needs to be a 100 times better

You need to get a Pirani or thermocouple gauge to see what is going on
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Rich Feldman
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Re: Demo Fusor troubleshooting

Post by Rich Feldman »

Mixture of right and wrong advice in previous responses.

Having too high a pressure (which you do) means the arc or glow discharge takes all the current your NST can give, without the secondary voltage getting very high, even with full primary voltage. But that has nothing to do with "tripping the safety". You could short the outputs of NST to each other with a metal wire, and it would be happy for decades. Normal neon sign operation is much closer to short circuit than open circuit conditions. In fact many cheap high frequency "NST's" turn off, on purpose, if they sense no secondary current after a few tries.

Since Anthony mentions a bypass button, I bet he's talking about the Secondary Ground Fault Protection feature. That's found in most newer NST's. What does the label have to say? Hobbyists often colloquially, but wrongly, refer to it as GFCI. Ground Fault Circuit Interrupters, which is the right name for the devices protecting electric outlets in bathrooms and kitchens, work differently.

The "SGFP" in NST's is intended to trip if you get an arc to ground from either HV terminal, since that could start a fire.
Unfortunately it will also trip if you connect a rectifier to one or both HV terminals and try to drive a demo fusor (or charge a capacitor, as I can attest). Except maybe a 4-diode bridge rectifier, with both ends of load "hot", but that's not for beginners.

Demo fusor experts can tell the pressure close enough by the plasma behavior.
Your pressure will get much lower when you fix the vacuum leaks. As others have said, a proper gauge helps. The range of interest (from too high to too low) is all indistinguishable from perfect vacuum on common Bourdon-tube vacuum gauges.
All models are wrong; some models are useful. -- George Box
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