@John Thanks for the photos! Where do you get those nice corona rings? I was thinking I would make one out of copper tubing, but if I wanted to get a commercial one I have no idea where I would look.
Speaking of, with all the HV experience here someone will be able to tell me - am I likely to need more than one corona ring if I'm going up to 40 to 50 kV? My plan is to connect my resistors in a helix like in the Glassman example Preston posted early in this thread. Probably six resistors per turn, approx. 15 cm diam., with 2.5 cm turn-to-turn separation. That makes about 5 turns for a stack of 30 or so resistors about 13 cm tall. Then a corona ring on top, and a terminator where I can connect my HV cable. Is there a rule of thumb about how often to include corona rings? Seems like I might need one mid-way, at the 25 kV point? Or would it be sufficient to try to keep my connections between resistors as smooth as possible? I can just try it and see and add one as needed, but thought there might be some engineering principle I should learn.
@Rich Thanks for the tip! I knew resistor voltage was limited by flash-over in addition to dissipated power, but wasn't aware that these power resistors could also break down internally even if the power/current rating was not exceeded. A quick study online seems to indicate that exceeding the voltage rating causes a gradual decline of resistance value over prolonged use rather than catastrophic failure. I couldn't find any ratings for resistors like these, so I'll just give it a go and keep an eye on it. Good to be aware of.
-- pwf
Dummy load for HV supply
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Paul W Fontana
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Bob Reite
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Re: Dummy load for HV supply
My eight stage CW multiplier has a corona ring at each stage, but that is probably overkill, unless I decide to push it to 100KV. So maybe have a ring for every 12.5 KV?
The more reactive the materials, the more spectacular the failures.
The testing isn't over until the prototype is destroyed.
The testing isn't over until the prototype is destroyed.
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John Futter
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Re: Dummy load for HV supply
The corona rings are 12mm aluminium rod rolled into a circle and welded
Bob
a corona ring every 12 kV is conservative but you will not regret it
I seem to have more trouble after 20kV with the odd sharp edge
Bob
a corona ring every 12 kV is conservative but you will not regret it
I seem to have more trouble after 20kV with the odd sharp edge
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prestonbarrows
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Re: Dummy load for HV supply
On a normal PCB, around 300 V per mil is a conservative design point with uncoated traces. More than that and you need to start looking into adding slots and so on.
In air, breakdown starts at electric field strengths of around a megavolt per meter, or about 10kV per cm. The best geometry for between a given voltage drop is two spheres. Planes, or worse points, will concentrate the field and lead to breakdown at lower absolute voltages.
Past a few 10's of kV, design rules are hard to pin down. Everything goes non-linear and becomes much more empirical. If you have the capability on your setup, adding one stage at a time while appropriately scaling the load voltage and recording the drain current will show you exactly where any corona losses are happening. This will show up as an anomalous current not accounted for by Ohm's law and the basic resistor circuit to ground.
A classic designer's trick is to imagine stretching a rubber sheet over the HV section towards ground. Where the sheet would break first is where you should expect the most arcs.
Always avoid sharp points and keep the voltage gradient headed in one direction without folding it back on itself.
In air, breakdown starts at electric field strengths of around a megavolt per meter, or about 10kV per cm. The best geometry for between a given voltage drop is two spheres. Planes, or worse points, will concentrate the field and lead to breakdown at lower absolute voltages.
Past a few 10's of kV, design rules are hard to pin down. Everything goes non-linear and becomes much more empirical. If you have the capability on your setup, adding one stage at a time while appropriately scaling the load voltage and recording the drain current will show you exactly where any corona losses are happening. This will show up as an anomalous current not accounted for by Ohm's law and the basic resistor circuit to ground.
A classic designer's trick is to imagine stretching a rubber sheet over the HV section towards ground. Where the sheet would break first is where you should expect the most arcs.
Always avoid sharp points and keep the voltage gradient headed in one direction without folding it back on itself.