High-Voltage Supply First Draft

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Carl West
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High-Voltage Supply First Draft

Post by Carl West »

Hello everyone. I’m submitting my first draft high-voltage supply schematics for critical review. Attached to this post, you’ll find three images: a high-voltage supply with a neon sign transformer, a high-voltage supply with an x-ray transformer, and a high-voltage supply master switch and fuse box. All components have been sourced except for the x-ray transformer.

The primary circuit for the NST and XRT supplies are almost identical, omitting only the ballast resistor on the NST supply. I decided to design the circuit to ~85kVrms, ~120kVp specs, allowing me to use the same circuit for the demo fusor and the eventual full fusor, saving money (maybe) and time (certainly). These are electrical schematics only, and I haven’t fully decided how I intend to physically arrange everything.

In the master switch and fuse box, I intend to use standard male and female receptacles. Male to a power cord for line voltage, and female to the variac. On the hot wire, I employ a 250V 50A fuse (redundant with the mains 250V 50A breaker) and 120V 20A flip-switch. I found a little 120VAC red LED to use as an ‘on’ indicator. I like red lights for a HV application like this, since I suppose it’s been impressed into our collective subconscious as a ‘danger’ or ‘stop and think’ color. The grounds from the receptacles join the ground from the star point at a grounding bar. The neutral runs straight through from receptacle to receptacle. #12 awg will be used for all ground wires.

In the NST high-voltage supply circuit, a 0-130V output variac will control the primary voltage of the 0-15kV output NST. A string of three 50kV 1A diodes on each leg will provide full-wave rectification. These are some hefty diodes, but their combined 150kVp rating will comfortably accommodate the ~120kVp potential across them at the maximum 85kVrms I’ve designed for. If I must err, I would err on the side of making the circuit too robust rather than the opposite. Also, one isn’t exactly spoiled for choice when it comes to very high voltage diodes like this.

The center-tap will run to ground with a 10 Ohm, 5W shunt resistor, with a digital panel meter for measuring current.

A resistor string composed of ten 100M Ohm resistors in series, followed by a 10M Ohm,100k Ohm resistor, and 100k Ohm calibration potentiometer will comprise the voltage divider circuit for voltage reading. Both the voltage divider math and circuit layout are at their most parsimonious with this arrangement, and I noticed another user recently posted the same design, likely for the same reasons. Another digital panel meter will be used for measuring voltage here.

The XRT supply is almost identical, except a 60k Ohm, 100W ballast resistor is placed in series with the fusor. The high-voltage circuit will be under mineral oil when used with the XRT. As mentioned before, I don’t have a lead on an XRT, and in this schematic, the XRT is assumed to output up to the maximum voltage the circuit was designed for. I doubt I’d push it to 85kVACrms (~42kVDCrms output to the fusor) unless I was very comfortable doing so.

All grounds will be arranged in star point style, but are left distinct in the schematic to reduce visual clutter.

The FAQ has been a massive help. I’ve had to use parts of my brain that have been dormant for a long time, but it’s like riding a bike. I intend to make a demo fusor first. We live in an apartment right now, and my track record with spilling liquids (like milk or… mineral oil) is a bit troubled. My wife would be happy to tell you about it. Apartment living isn’t really conducive to making a full fusor anyway. I think something like that needs its own dedicated space in a garage or a basement. A demo fusor, however, is imminently doable with what I have now, and it’s probably for the best that it’s my only option at the moment. I’ve read wise counsel that one should become familiar with operating a demo before moving on to the real thing.

I’ve thoroughly enjoyed this process and welcome any input you’d care to give. I haven’t even begun to peruse the FAQs outside of the high-voltage category, so I’ve got some reading to do. I’ve used vacuum pumps to pump down the coolant system for refrigerators, but designing and building a vacuum system of this kind is new to me. I’m eager to learn.

I do have one question: I plan to run the voltage divider resistors under mineral oil, with only the potentiometer in air accessible to the operator (for adjustment only when the system is powered off). What is the conventional method of running the voltage divider setup?

Thanks everyone. I look forward to your input. I apologize if I missed something in the FAQ.

Fusor HV Supply XRT.png
Fusor HV Supply NST.png
Fusor HV Supply Master Switch & Fuse Box.png
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Richard Hull
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Re: High-Voltage Supply First Draft

Post by Richard Hull »

Make D1-D6 common microwave diodes.
You will never get more than about 10kv out of this supply, of course. Loading it into a fusor, you will be lucky to reach 8kv in real operation.

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
Carl West
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Re: High-Voltage Supply First Draft

Post by Carl West »

Thanks for the reply, Richard.

You’re referring to the 15kV NST circuit and not the 85kV XRT circuit, correct?
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Richard Hull
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Re: High-Voltage Supply First Draft

Post by Richard Hull »

Correct. Absolutely no need for 50kv diodes with any neon transformer ever made. I used a 6kv neon sign transformer for my first demo fusor back in 1997 with a single microwave oven diode and a 1 ufd 10kv capacitor filter. Microwave diodes are all that is ever needed in most any 60hz system.

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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Rich Feldman
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Re: High-Voltage Supply First Draft

Post by Rich Feldman »

Just remember than when using center tapped transformer, the reverse voltage stress on diodes is twice the output voltage.
If a "15 kV" NST is delivering -10 kV from one HV terminal, at some instant, the opposite terminal is at +10 KV so the "off" diode sees 20 kV.
All models are wrong; some models are useful. -- George Box
Carl West
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Re: High-Voltage Supply First Draft

Post by Carl West »

Thanks Richard.

I was aware that that the NST circuit output would likely be sub -10kVDC across the fusor, but I chose those hefty diodes for the circuit so that it could be reused for any voltage output up to 85kVrms / ~ 120kVp. Kind of a one size fits all circuit that can accommodate a range of transformers for any future fusor projects after the demo. Since I didn’t know what kind of XRT I’d be able to get my hands on in the future, a circuit that could accommodate up to that voltage seemed wise.

Now I’m starting to think it’s better to build the circuit to fit the transformer rather than the opposite.
Carl West
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Re: High-Voltage Supply First Draft

Post by Carl West »

Thanks Rich.

That was my impetus for selecting the three 50kV diodes. At a nominal 85kV output, the “off” diode string, as you said, sees ~120kV across it.
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Richard Hull
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Re: High-Voltage Supply First Draft

Post by Richard Hull »

Always the case, of course. So, only 2 12kv microwave diodes in each leg of the xfmr should do the trick, even with no load. With the collapse of the neon's output voltage under load the voltage is even lower. Three is a bit over kill in each leg. However microwave diodes are dirt cheap compared to higher voltage diodes. Volume makes for lower prices on these 12kv, .45amp wonders.

As of this date amazon has them at about a buck a pop!

https://www.amazon.com/Comimark-CL04-12 ... GBEALw_wcB

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
Carl West
Posts: 7
Joined: Sun Dec 18, 2022 5:56 pm
Real name: Carl West

Re: High-Voltage Supply First Draft

Post by Carl West »

Thanks Richard! That’s what I’ll do.
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