#9 FAQ - Ballasting the fusor power supply

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Richard Hull
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#9 FAQ - Ballasting the fusor power supply

Post by Richard Hull »

First, for any power supply using a 60hz neon sign transformer or an oil burner transformer of the Iron core type,(older types used and sold into the early 2000's), no ballasting is needed ! They are self current limited. These are usually found in Demo fusor type power supplies. In these cases, the builder almost always builds their own, negative output, high voltage, DC power supply around this type of magnetically shunted transformer.

A formal fusing fusor, designed to do significant fusion, works in the regime of a controlled glow mode, right on the edge of arc discharge. Normal operational power input is on the order of 500-1000 watts from the high voltage supply.

All 60 hz transformers of the X-ray, Plate, or Microwave oven type will need secondary ballasting. Usually, this ballasting occurs at the negative high voltage output point. Such dangerous supplies of this type ranging from 15,000 to over 50,000 volts will have the transformer and all the high voltage rectification under oil. All such transformers have a maximum rated output current. Beyond this current, they can be easily destroyed! These rare and highly valued transformers must be protected against such destruction. The ballast should also reside under oil as well. Such power transformers of this type, can and will try to supply many amps in an outright arc situation! Most will burn out their secondaries in a few seconds trying to do this.

The fusor is like a Geiger tube, a gas fulled device that detects via break down. All GM counters has a ballast or current limiting resistor in series with the high voltage. When the tube breaks down to a gas arc, the ballast resistor has all of the voltage placed across it reducing the voltage across the tube. The tube no longer conducts until the next count. The fusor is no different. At startup, the fusor must enter glow mode. The voltage to enter glow mode is much higher than the desired for running and fusing. Like the GM tube, at start up, the fusor might experience a 1 amp draw!! This is not to be desired, needless to say! By placing a ballast resistor of suitable size in the high voltage output of the supply, it will have a high voltage develop across it, dropping the fusor voltage, allowing the operator time to quickly dial back the power supply voltage to a desired run current. This ballast resistor must have some high wattage capability. Typically a 50 watt wire-wound resistor will suffice. These are hard to find in the very high ohm range today, other than at hamfests.

Figuring out your ballast resistor value

The average working fusor might run at 25-35 kV. Typical run currents are from 7-20ma. Let us say that our supply max is 30kv and we want to limit the current to 30ma. The impedance of such a supply scenario is 30,000/.03 or 1 megohm. Let us say we want at a near shorted situation of 100ma to drop 6,000 volts across the ballast this would demand a 60,000 ohm ballast resistor in series with the high voltage output. For a continuous 100ma output, which we DO NOT WANT, the resistor would need to be a 600 watt resistor. Now we will know that this 100ma draw is transitory only as this will not last any longer than it would take to have the operator reduce the voltage to a stable running level. We could be quite safe with 1/10 this wattage for a second or two. Thus, a 60K 60 watt resistor would be ideal!

Now let us say we during a real fusor run get good results at 30kv and 12ma. How much voltage will we lose at the output due to the ballast resistor....Or more to the point, at the fusor HV terminal??
.012 X60,000 = 720 volts At 30kv this is not a bad loss in trade off for protecting the precious transformer and rectification diodes.

I have a 45kv supply and ballast it with a wire wound 63k ohm resistor of 75 watt capability under oil.

I general, Ballast resistors are in the 50k ohm to 100kohm range Much smaller and you will pull a lot of needless over current and may damage your supply. Much higher and the plasma won't stay lit in many instances. The object of the ballast is to give you time to reduce the voltage, and thus, the current once the plasma strikes. Your job is to work fast to reduce the voltage while keeping the plasma lit and dropping the over current on the supply to an acceptable running level. This effort is usually done in under 2 seconds. Get good at doing this.

A typical ballast resistor is usually 5-inches to 12-inches long in the value and wattage we would typically need. They are rather rare now as they were much more common to the vacuum tube era.
They are rather large and ugly and are green or brown in color. I attach an image of the type thing you will need if your supply demands a ballast resistor. pay no attention to the value in the image as it is too low, but the wattage dictates the size.

TIP: Ballast only with a wire-wound resistor!!!
Attachments
power resistor.jpg
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