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80kV 2-stage CW
120kV 6-Stage CW Tower

A reliable high power, high voltage source is something useful to have kicking around for physics experiments, and after seeing a creative design by Mates on the 4HV forum I decided to make my own voltage multiplier. I started by constucting a 4-stage model, but after a year I upgraded it with another two stages for even greater voltages.

Schematic

The circuit is just a standard voltage multiplier, or 2-stage Cockcroft-Walton multiplier, or Villard cascade depending on your favorite historical inventor. It could also be called a Greinacher, but that's a marginally different circuit so we'll keep it at CW for now. The circuit works by alternating between charging up the left and right side capacitors up the ladder until they have all reached full potential. The capacitors and diodes will have the twice the AC potential across them, as in the voltage from negative to positive peak. Add some overhead for safety, and you're left with 60kV ratings when using a 20kV source. Such high voltage ratings are easy to achieve with homemade materials however.

Capacitor construction

I used four strings times 60 RGP10M diodes, and four HV capacitors made by Mate's recipe. Construction was pretty straight forward. First I made the capacitors with tin foil and overhead transparencies. I made sure to give at least 4cm of clearance from the sides so it wouldn't arc over. Basically the capacitors are made by paralleling 5-6 overhead transparency sheets, then putting one sheet of tin foil on top, another 5-6 overhead sheets, one more tin foil, then finally rolling together and taping. The voltage standoff is surprisingly good given the quick and cheap construction.

240 diodes  small diode strings
Then I moved on to the diode stack. 240 diodes, and they all had to be chained together. It took a few hours from I started until they were all linked together in strings. I chose RGP10M diodes because my electronics source had them in large quantity for a good price. Besides convenience, they are avalanche rated which means if the voltage exceeds their rating they can safely breakdown and dissipate a certain amount of energy. This is necessary when putting diodes in series as it prevents a diode from exploding if it is slightly mismatched and too much voltage is dropped across it. The use of 60 diodes per string also give a decent safely margin. RGP10Ms also have about 500ns of recovery time, which is acceptable when using an input source up to 100kHz.

diode tower  diode tower in towerprojectfiles/cwtower_80kv/diodes_intower.JPG

Here the diodes are spiraled around a thin PVC pipe to give them some structure and voltage stand-off in a smaller package. Beside them is the pipe they will soon be fitted into. The diode tower was centered and suspended with twine within the main PVC pipe. Wires from the diode tower were pulled through small holes in the large PVC pipe. Some duct-tape really eases construction. Be sure to see the video as the pictures don't do justice, capturing a still photo of an arc at it's greatest is nearly impossible. Remember when drawing arcs from a high voltage CW like this, that care must be taken not to simply short the output leads together. The capacitors are charged to about 40kV, and will dump all of their energy into the spark and diodes at once, resulting in large peak currents flowing through the diodes. For one thing it's damaging to the diodes and will lead to failure. If the driver is too weak it can also prevent an arc from forming, as the capacitors will not have sufficient energy left to sustain an arc, resulting in sparks instead.

For driving the CW Tower I use my home-made HV transformers, driven by a multi-purpose inverter.

Finished tower  Quadrupler setup  Arc  40kV Sparks
The finished multiplier and setup. The arc picture was taken just as a strike occurred, it can be drawn out much further. the sparks were taken when the CW was driven from my smaller 10kV homemade transformer. It was ballasted which prevented arc formation unless the leads were brought within close proximity to eachother.

Youtube Video!



Update! Now 120kV CW Tower

In order to provide the required voltage for my Coolidge X-ray tube I had to add another two stages to the Cockcroft–Walton Tower. With some handy plumbing parts and some more duct-tape it was a breeze to upgrade. The beauty of multipliers is their ease of scaling up with more stages. In conjunction with my recent X-ray experiments, I've measured the voltage from the CW multiplier without load, powered from the Big-Mofo transformer. It turns out the voltage from the transformer is only about 13kV. Meaning in the above video the voltage was really 50kV, not 80kV as thought previously. With the two extra stages it's now 75kV no load. The pictures speak for themselves, the upgrade only took an afternoon of work. When drawing arcs the power consumed is over 1,5kW!

Internal structure  Pipe  Done.
Arc 1  Arc 2

And of course a video:




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This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License.

Disclaimer: I do not take responsibility for any injury, death, hurt ego, or other forms of personal damage which may result from recreating these experiments. Projects are merely presented as a source of inspiration, and should only be conducted by responsible individuals, or under the supervision of responsible individuals. It is your own life, so proceed at your own risk! All projects are for noncommercial use only.