Pushing a CW Multiplier

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Matt_Gibson
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Re: Pushing a CW Multiplier

Post by Matt_Gibson »

I also have a homemade multiplier that I have put away…

It’s a 4 stage using 0.068uF 6kV capacitors in series (so 0.034uF and 12kV). Under oil. I need a safer setup as it is very hard to control and has gunshot level discharges when it bypasses whatever resistor I try to throw at it :-) A lot of spraying and then BANG!!! It’s also for a future path that I may take along this journey:-) For now, my 40kV glassman is the man.
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Dennis P Brown
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Re: Pushing a CW Multiplier

Post by Dennis P Brown »

Here is my old voltage multiplier without its vertical oil tank. If I recall, the diodes were 20 kV. I intended the unit for a corona field generator - not for current. Used the wrong caps but was rather ignorant of design of those devices. The driver was 3 kV, high current x-former (close to an amp) that also had a single stage multiplier of its own but no need for that section to be under oil.
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Eight stage using 15 kV door knob caps
Eight stage using 15 kV door knob caps
Matt_Gibson
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Re: Pushing a CW Multiplier

Post by Matt_Gibson »

I have some 40kV 2nF doorknobs that I was going to use but decided to save them for a Marx generator.

How did this multiplier perform? Did you get some current/voltage measurements? I abandoned the idea of using the doorknobs when I saw how bad the voltage ripple was going to be at just a few mA.
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Dennis P Brown
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Re: Pushing a CW Multiplier

Post by Dennis P Brown »

I tried to measure it but the field generated was too intense - even with the multiplier's top electrode under oil (about a cm), my 45 kV probe pegged when it got to within 5 cm while still in air. Worse, the oil surface was moving towards the probe! I calculated the stack could hit (with a better HV supply) 120 kV - 140 kV safely (for the components, not necessarily me, apparently) and assumed it would max to 100 kV with the reduced supply I was using. However, the variac was only at half scale so I thought I was within the limits of the HV probe - apparently not. That was just too dangerous so I abandoned the linear accelerator and built a normal fusor.
Matt_Gibson
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Re: Pushing a CW Multiplier

Post by Matt_Gibson »

Sounds like you need a good “dummy” load for the multiplier?

I’ve been meaning to put one together, but will save that for fusor 2 once I outgrow my 40kV Glassman.
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Dennis P Brown
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Re: Pushing a CW Multiplier

Post by Dennis P Brown »

I considered it - oil filled plastic tubes with a center HV wire from the voltage multiplier tank all connected to a small sphere for corona discharge; then sense the field using my 45 kV probe. The issue was that I felt the voltage multiplier had far too much power to safely get near. Not sure how to create a dummy load that would work in a safe manner - a really long Giga-ohm and long mega-ohm resisters under oil to act as a voltage divider connected to the VM output? We are talking a 100 kV or more with a large surge current (low frequency). I haven't abandoned the approach or project completely (that is why its still assembled in its plexiglass tube/tank. ) Any ideas would be appreciated.
Matt_Gibson
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Re: Pushing a CW Multiplier

Post by Matt_Gibson »

I decided to revive this…

Instead of leaving this as a 10 stage designed for 100kV at 1mA, would it be worthwhile to convert to a 5 stage and double up/parallel on the capacitors?

I’d go from 10 stages, 0.015uF capacitors to 5 stages and 0.03uF capacitors. Running through https://www.extremeelectronics.co.uk/ca ... alculator/

I see less than 1600v dropped at 15mA (24w wasted). Seems reasonable, no? Capacitors are rated for 15kV and diodes are rated for 30kV. I should have no problem pushing 60-70kV?

100kV at 1mA doesn’t seem very useful and I’ll probably never tackle 100kV insulation.
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Rich Feldman
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Re: Pushing a CW Multiplier

Post by Rich Feldman »

Sounds like you are planning to reconfigure a kit of high voltage C's and D's.

When you figure the stack's voltage drop under load, beware of calling it "lost power"
or, as mentioned by someone early in this thread, saying that the I * dV product heats the components.
It's reactive voltage drop, without power dissipation if the D's and C's are ideal.

Real D's and C's are not ideal.
The C's have ESR and dielectric loss which figures into their RF current ratings, as Tesla coilers know well.
The D's have forward dV when on, and reverse I while turning off.
But those are not, AFAIK, major contributors to the output resistance of practical voltage multipliers.
All models are wrong; some models are useful. -- George Box
Matt_Gibson
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Re: Pushing a CW Multiplier

Post by Matt_Gibson »

Rich,

That’s what I’m up to…Going from 10 stages to 5 stages frees up enough capacitors to double up. I’d simply remove the extra diodes and keep them as spares.

My thoughts are that 5 stages should be a lot less “lossy” than 10 stages and this multiplier wouldn’t need oil since it was designed for 100kV in air use.

Any reason this won’t work or be an exercise in futility? First glance, this looks like a nicely packaged up option to get a 60kV supply that can source some decent current, all without needing any oil. I might need to blow some air across the boards for cooling…
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