FAQ - Polarity of a fusor supply

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Richard Hull
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FAQ - Polarity of a fusor supply

Post by Richard Hull »

A lot is contained in this FAQ forum on high voltage power supplies for a fusor. Contained in many FAQs are, how much power is needed, how much voltage is needed, how much sustained current is needed for both demo and real fusing systems, wiring diagrams, metering diagrams, etc. The polarity is also obvious and mentioned throughout. However, many newbies have no concept of the term "negative output high voltage power supply"! They often get tangled up in their underwear over the subject.

I am here in an attempt to make this clear for all.

High voltage power supplies are different from all other power supplies. They can easily kill a person. All DC supplies, be they in a common radio or a nuclear accelerator, have a plus, (positive) and a minus, (negative) output lead at the output of the internal supply electronics.

All typical high voltage supplies, being deadly, have the metal power supply case and ONE of the two electronic high voltage output leads connected to full electrical safety ground potential. As one of the high voltage polarity leads is, now, at ground potential and cannot kill, this output binding post is often called the "cold" or "ground" or the zero volt lead of the supply. Its polarity is whatever the lead that was chosen to be bolted or connected to the metal case ground from the internal high voltage electronics. By grounding one of the high voltage leads that is now safe and at ground potential, the other HV lead from the electronics is now called the "hot" or high voltage output lead. This is usually sent to a large high voltage insulator and is the real deadly, "business end" or "output" of the power supply.

As such, 99% of all manufactured high voltage supplies have their negative, internal high voltage electronics lead grounded to the case which is at full electrical safety ground. No insulation is needed for this negative output of the supply. The supply's high voltage and deadly output is positive in polarity. As ground is assumed to be negative, this type of supply is called a positive high voltage power supply!

Extremely rare are those supplies that have their internal, electronic, positive high voltage lead connected to the case ground. Thus, the positive high voltage is the safe, grounded connection while the negative lead is run to the high voltage output insulator and becomes the "hot" or deadly output of the supply. Such supplies are called negative output high voltage supplies.

The fusor demands and functions only with a negative output high voltage supply!!

There are no workarounds! You cannot take a positive output HV supply and reverse it to become a negative HV supply. Some very rare, large and hyper-expensive, multi-thousand dollar supplies are capable of being internally converted to either a positive or negative high voltage output. Powerful, reversible supplies, are much rarer, still. The issue with switchable polarity supplies is that, internally, their supplies must be made floating. This means insulation from the grounded chassis to what is normally a small crossover wired board that is reversible must be tremendous within the supply. In general, such supplies end at 20-30 kilovolts and often are limited to 5ma output. I cannot begin to tell of the great rarity of such a reversible high voltage, high power, 30 - 50KV supply that can handle high currents above 20ma.

All of the foregoing points out to the newbie that the fusor's entire assembly, the vacuum work, the fusor reactor vessel and the gas handling system are super easy to build and assemble, while the high voltage supply tends to end, forever, their dreams of doing fusion.

Most adroit and dedicated, would-be-fusioneers, are forced to build their own custom high voltage supply, or pay a premium price on e-bay for the rare, suitable negative power supply that might show up every year or two for sale or auction, online. The advantage in building your own supply is that you are intimate with its construction and can design, build, de-bug, and repair it at any moment. The drawback is, you have to know a lot about power HV electronics, and in the process, not kill yourself.

I hope, yet it is highly doubtful, that this FAQ will end questions so often asked about failure for the non-electronic types to understand the need for a suitable negative output power supply in the 30kv class capable of high current for fusor operation.

Points to remember

No one manufacturers high power, (>1kilowatt), high voltage (0->40kilovolt), adjustable power supplies that are fully floating with isolated output leads!
All manufactured supplies that can answer both the >1KW and 0->40kv+, fully adjustable requirements will have one side of the high voltage polarity grounded to the case or chassis for safety!
Thus, only one, manufacturer chosen polarity, will be the high voltage output of such a supply and 99% of all such supplies are Positive output supplies with the negative output at chassis ground


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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