High Voltage Supply?

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Rex Allers
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High Voltage Supply?

Post by Rex Allers » Tue May 15, 2018 9:40 pm

Miles Hawley posted a question about the suitability of two eBay supplies for a fusor. I acted as "enforcer" saying that post should have been made in the New User Chat Area. Two reasons for that: the question, "Is this supply good enough?", happens over and over from people starting to build a fusor, and also it gives two ebay links that will be gone sometime soon. (not useful for future readers)

I said he should re-post in the Chat section. Miles hasn't done that yet, so I am copying his post again here and will follow with my answer.

--- Original Question ---
High Voltage Supply?
Post by Miles Hawley » Tue May 15, 2018 6:14 am

Hi, I have been searching around the FAQ's for an adequate power supply to power a real fusion reactor. I found two high voltage systems, and wanted some other user input to help me determine if they could be used for detectable fusion. The first:
https://www.ebay.com/itm/SPELLMAN-High- ... 1438.l2649
The second:
https://www.ebay.com/itm/Spellman-X2189 ... 1438.l2649
Now of course the second is a lot more expensive, now just know I am a high school student, and am working on such a budget. If these two are not viable for fusion can someone supply some good ideas, as to some other systems. All suggestions are welcomed.

Rex Allers

Rex Allers
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Location: San Jose CA

Re: High Voltage Supply?

Post by Rex Allers » Tue May 15, 2018 9:41 pm

OK here are some reasons why neither of these supplies look good for a fusor.

In his introduction post Miles said he had been reading the FAQs and mentioned, "The ideal power supply should be 0-30kv 10-20mA DC, with negative polarity." This is true, but I would suggest a bit more voltage, 35-40 kv, might be even better. But this information seems to be ignored in the two ebay supplies mentioned.

-- Polarity
Both of these supplies give no indication that they are Negative HV. Both model numbers have a P between the voltage number and current number. That normally means a Positive HV supply. Also on a negative supply's label the high voltage number would have a minus sign. In the type of fusor usually discussed and built here, the shell is at ground potential and the grid has negative HV applied to it. Neither of these supplies look to be what you need.

-- Voltage
The first supply indicates max voltage of 18 KV. You are unlikely to do measurable fusion at such a low voltage. The second supply goes up to 125 KV. I'd suggest that above around 60 KV, managing and connecting the HV gets difficult and more dangerous, especially for someone new to this. You could set the supply to a lower voltage but there are other problems.

-- Current
The first supply says 6.7 mA. That's a good bit lower than the recommended 10-20 mA from the FAQs. Maybe 6.7 mA could be tried if the voltage was 40 KV or so, but 18 KV puts you low on both current and voltage. More later.

The second supply is rated at only .5 mA. Not surprising for such a high voltage supply but way too low for a normal fusor design.

-- Power
Another way I like to look at the HV needs for a fusor is the power consumed. Let's say a just barely good enough supply might be 30 KV at 10 mA. Looking at that as supply output power:

P = I x E ; (Power = Current x Voltage)

So 10 mA x 30 KV = 300 watts. So I'd say a useful fusor supply should have output power of a minimum 300W. From what people have been successful with, I'd say 400 W or more of output is desirable.

The first supply says 18 KV and 6.7 mA. That's 120 W. Very unlikely to be enough, but it gets worse. The label also says, 36 Watts Max. I haven't seen that on a Spellman supply before, but it seems to indicate the supply can't deliver rated voltage and current at the same time. (See the FAQs about neon sign transformers for more similar details.) 36 W at 18KV would be only 2 mA of output.

The second supply at 125 KV and .5 mA is only 62 watts. Very unlikely to be enough for fuson.

So I find output power to be a good way to look at supplies but you also need voltage around 30 KV or more.

-- Switching Supplies
These both appear to be smallish and light weight supplies. No doubt switchers or switching power supply design. Internally they operate at frequencies in the 10's of kHz as opposed to iron transformer supplies running at 50 or 60 Hz mains power.

If you read through the FAQs I think you will find that Richard doesn't think much of them for powering a fusor. The reason for this is that a fusor provides a load that isn't nice. The average consumption of a small fusor may be 300W for 30 KV and 10 mA, but as the plasma starts the current may spike up well over 10 mA. On many switching supplies this may be detected in the circuits and the supply will shut down.

It depends. I think a number of people have been successful with switching supplies powering a fusor but it would probably be good to have extra power margin from the minimum numbers.

-- Summary
Neither of these supplies look to have enough power to drive a standard fusor. They also have the wrong polarity (positive rather than negative).
Rex Allers

Miles Hawley
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Re: High Voltage Supply?

Post by Miles Hawley » Wed May 16, 2018 12:28 am

My bad on not switching this chat to new user. I think the information you supplied was very helpful.



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Richard Hull
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Re: High Voltage Supply?

Post by Richard Hull » Wed May 16, 2018 4:13 am

Rex is right on! Now, it might have been profitable to critically read, for its valuable content, the FAQ you should have read.


This speaks to fusor power and voltage, much as Rex spoke of. For some strange reason it was placed in the fusor power high voltage forum in the FAQs posted in that forum. It turns out this is just where it belongs and just where it should have been found and critically read.

The polarity "must have issues" are also completely covered in FAQ...


I can only fully explain all of this stuff in plain language. 100% of it all is in the FAQs, again, to be found in the ideal location for information on fusor high voltage requirements. You guessed it! Again, look in the High voltage fusor power forum in the FAQ section at the top.

Proper, power capable, high voltage supplies of a negative output polarity are one-in-a-thousand on e-bay.

Some few modern switcher supplies can internally reverse their polarity and even disable their current limiting circuitry. This is often outlined in the user's manuals that come with such supplies. Do not begin to think that the average seller on e-bay is even slightly informed about what he is trying to sell or that he has the user's manual.

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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