A 50kV supply with 33mA drive ability

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Finn Hammer
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Re: A 50kV supply with 33mA drive ability

Post by Finn Hammer »

All,

After a lot of beginners rant in the appropriate beginners forums, I am happy to be back here where I feel I am a bit more in controll of things, and then perhaps not. This delay in posting is the result of unforseeable trouble in obtaining a proper supply of high thermal conductive potting material.
But now I am in posession of 10Kg og Robnor PX700k-1/BK, and that ought to last my remaining lifetime.

I had previously potted one multiplier, a 30kV model and it functioned as desired, putting 50mA out without problems:

First prototype to look alright
First prototype to look alright

I also had a 60kV prototype on the floor, waiting to be potted:

Guts of the 60kV variant
Guts of the 60kV variant

This is the connector I use: a 4mm banana plug, some heatshrink and some pneumatic fitting. It works fine but I should probably put my lathe to some good use and produce something that looks more serious.

Plug detail.
Plug detail.

After the potting had cured It was time to hook it up to the trusty 1.6MOhm load:

60kV potted and ready to go
60kV potted and ready to go

And brace myself before starting to wind the variac towards the endstop. I admit to being scared this time, the ion wind from the hissing resistive tower load was noticable, but perhaps it was just the fear of a failiure that got me.


Scope readout
Scope readout

But then not so shabby! 63.8kV @ 37.5 mA. That ought to doo even for the most power hungry fusor. (for a while, at least ;-) )
Cyan and Magenta traces carry the real magnitude labels, although Magenta is in Ampere, not volts, for gate signal and primary current respectively, the yellow voltage trace magnitude labels are a result of a 20000:1 voltage probe and a probe setting of 20X so it reads 20kV/division.

So far so good, what now?. Obviously, a 100kV unit is within reach, just a matter of field controll here and there. I am willing to share, the question is how much more is needed before I have told enough for others to replicate, should they want to.




Cheers, Finn Hammer
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Mark Rowley
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Re: A 50kV supply with 33mA drive ability

Post by Mark Rowley »

Looking great Finn!
Potting is definitely the way to go for optimal performance. Tbh though, I’m not brave enough to do it. Just too worried about one component failing and not being able to replace it. It looks like you spared no expense by utilizing top quality caps and diodes so failure maybe be very unlikely.

It’s great to see more neutron capable power supplies using multiplier circuits. Undoubtedly the future PSU of the fusor hobby.

Mark Rowley
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Finn Hammer
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Re: A 50kV supply with 33mA drive ability

Post by Finn Hammer »

Thanks, Mark.

Spellman and all the other big ones have done it for decades, time to catch up.
One thing I'm pondering is the benefits and tradeoffs by having either a small or large stage capacitance.
A small capacitance will have a larger ripple, and I imagine this will lead to a neutron output determined not by the peak voltage, but rather peak minus half ripple. A problem in this scenario is, that the breakdown of the feedthrough is determined by the peak voltage, so this component will have to be overengineered related to neutron output.
A large stage capacitance otoh, has small ripple, so the neutron output will relate directly to the quality of the feedthrough. But the energy released in a short from grid to shell may shatter or even evaporate the grid.
The latter would be preferable with respect to the chances of schrapnell ending up in the turbo, already coasting down since the controller got fried by the EMF wave from the shorted supply.
Perhaps I worry too much.
These multipliers of mine have 22nF stage capacitance. I solemny promise to build a version with 3.3 nF stage capacitance, so some time in the not so far future, I will be able to answer that question myself.

Cheers, Finn Hammer
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Richard Hull
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Re: A 50kV supply with 33mA drive ability

Post by Richard Hull »

All of your work is appreciated my those who seek to make everything in the power circuit more bullet proof. Ripple vs. pure, or close to it, DC with its attendant stored energy capacity is always going to be a tradeoff. The critical breakover ionization potential is always an issue. That is why the ballast resistor comes into play to save the day and the supply and grid.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Finn Hammer
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Re: A 50kV supply with 33mA drive ability

Post by Finn Hammer »

All,

It is a big day, when a selfmade PSU goes into the cabinet of it's own, and as always, this cabinet shows up to be just a tad bit to small, but for now it is ok.


IMG_20210526_110900.jpg

Sitting nicely in the fusor's frame it has readout for Mosfet temperature, transformer core temperature and output current. The switch to the right is a combined Kill/reset switch.

IMG_20210528_091038.jpg

It powers the fusor with great authority, so great that I have plans for an upgrade consisting of a PFC frontend booster, and an intermediate buck voltage control.


The PCB below is from a TI reference design, for a 3kW PFC, and I will populate this board, but that is going to be a project for the next winter.

IMG_20210529_190137.jpg


Cheers, Finn Hammer
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Dennis P Brown
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Re: A 50kV supply with 33mA drive ability

Post by Dennis P Brown »

As always, your home made equipment is extremely professional and rather state-of-the-art. Great pic's and progress. Do be careful of x-rays - 60 kV at over 35 ma will produce a very dangerous flux.

I built a 120 - 150 kV voltage multiplier and while it was all under oil, its nearby static effects convinced me to shut it down and place it in mothballs the first, and only time I charged it up.
Voltages in the 100 kV+ range can get rather flaky and difficult to safely shield. I also realized the accelerator it was to be tied to would produce just too dangerous of an x-ray flux; I went with my nearly done fusor instead.
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Mark Rowley
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Re: A 50kV supply with 33mA drive ability

Post by Mark Rowley »

Very nice setup Finn. I especially like your temperature monitoring arrangement for the typical problem areas. I’ll probably follow your lead here when begin my next phase of psu upgrades this summer.

Mark Rowley
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Finn Hammer
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Re: A 50kV supply with 33mA drive ability

Post by Finn Hammer »

Dennis,
Thanks for your kind words, and yes, radiation is a subject I will have to pay very serious attention to from now on. At present I am relying on below 30kV and the lead glas in the Lesker viewport.

Mark,

If you want to try the LLC topology, and need a PCB, then you know who to contact.

Cheers, Finn Hammer
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