100 kV DC gives 5 inch sparks from a voltage multiplier driven from a dental x-ray transformer.
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It is from an old dental x-ray machine and is rated at 60 kV. It looks tiny for a mains transformer to give this even under oil. It doesn’t have any shunts and can draw 15 A from 240 V. I connected it up and with about half power (125 V) it puts out about 30 kV at 20 mA before internal shorting occurs and the oil starts to bubble. Occasionally it will run to a 2 1/2 inch spark on the full 240 V. It has a lot more power than an NST.

The 60 kV transformer with a bit of suppression circuitry (ex microwave). Installed here as under oil in a PVC pipe running as a Jacob’s ladder with a drink can next to it for scale. A Jacob’s ladder is a spark gap in which a spark forms initially between the lowest and closest points then rises as the plasma heated air rises. It eventually extinguishes near the top then restarts at the bottom. A favourite backdrop for old Frankenstein movies.

To obtain around 100 kV DC from this 60 kV x-ray transformer I input 150 V AC which gives 36 kV AC out. With a diode and a 0.015 uF 80 kV mica capacitor connected as a voltage doubler, around 100 kV DC is achieved giving a best spark of 5 inches (13 cm). Note the spark is hazy (and fairly quiet) due to current limiting with a resistor and an inductor to reduce strain on the capacitor. The whole setup crackles with corona when in action.

The spark above is attenuated with a resistor (below) placed in a short poly tube which has 67 x 1.8 k ohm resistors and is almost exactly 100 k ohm. The voltage rating is unknown but for these 2 or 3 watt types is probably around 1 kV each. It was placed in a PVC pipe and filled with paraffin wax. I have had these in my junk box for around 20 years so I hope they aren’t past their use-by date.

The capacitor (the upright tube in the photo) is a 2.2 nF rolled polyethylene in oil cap composed of 2 sections with 8 layers of sheet in each side. I have a 1 3/4 inch (4 cm) safety gap which only starts to fire at output sparks with the voltage doubler of 4 inches (10 cm). This was replaced with the mica caps for the highest voltages. Ignore the wiring above as it does not include the mica caps and also is not configured for longest sparks.

The diode is composed of 290 x 1N4007’s in a PVC pipe about 15 inches (40 cm) long. Each one is rated at 1000 PIV at 1 A. This was designed to accommodate the full 60 kV AC = 169 kV peak to peak. Allowing for 10% increased voltage from the variac gives a total 186 kV which allows 50% headroom only. Note no dropping resistors were used. So far no problems (but many lesser ones have come to grief). I now have two of these to allow another stage to my voltage multiplier. They seem to work well for 100 kV DC sparks.
I also have an inductor made from 21 g wire close wound 15 inches on a 1 inch (2.5 cm) former. The inductor and resistor are needed to limit the current for devices such as the lifter.

Related pages

  100 kV x-ray unit

Try something else

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External links

Photo Date: 2004