250 kV High Voltage DC Power Supply with Neat Trick for Switching Polarity

Posted on Feb 7, 2014

The Cockroft-Walton multiplier uses a cascaded series of diodes and capacitors to generate a high voltageDC potential from anAC input through acircuit topology thatuses diodes to charge capacitors in parallel and discharge them in series. The output polarity of the Cockroft-Walton multiplier depends on the way in which its diodes are oriented, so

250 kV High Voltage DC Power Supply with Neat Trick for Switching Polarity
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the output polarity(referenced to ground)of a high-voltage DC power supply is usually set during the design. However, since some of our physicsexperiments requireone or the other polarity, we build our Cockroft-Walton multipliers with an extra capacitor so that we can make our HV power supplies output either positive or negative high voltage referenced to ground. The schematic for our reversible  Cockroft-Walton is shown in the following picture (click to enlarge): If the high-voltage AC output of the flyback is connected to point A  of the voltage multiplier, and point B  is connected to ground, then the output at point D  will be positive. If however point C  receives the high-voltage AC, and point D  is connected to ground, then point B  will be negative. Banana connectors are theninstalled onthe plastic container and wired directly to points A, B, C, and D. The connectors must be sealed very well using silicone RTV: The plastic container should then be filled with pure mineral oil (may be purchased at a pharmacy) to completely submerge the multiplier circuit assembly, which prevents high voltage breakdown between components: In this AC power supply, a push-pull oscillator drives a TV flyback transformer from an old color TV (a flyback without embedded tripler). The well-known hack is that the original primary of the flyback is not used. Instead, new primaries are made by winding two sets of four turns each of...

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