8kV peak-to-peak at 20kHz

This device was designed to drive a TV cascade stable and at high power, which is mainly achieved by the high frequency. It makes use of a flyback-transformer and some circuitry to feed it with 15-20kHz rectangular pulses. You should have an oscilloscope and some knowlegde of this type of circuit, as a lot of things have to be adjusted depending on your flyback transformer, mains voltage etc.

This is the circuit diagram of my version together with an oscillosope screenshot (probed across the primary flyback winding, parallel to the VDR; 200V/div. vertical, 50us/div horizontal). (NOTE: VDR = Voltage Dependent Resistor = MOV = Metal Oxide Varistor!) The hairdrier protects the BU208A from overload. It makes it possible to virtually short-circuit the output without the transistor getting too hot. Instead, the heat is dissipated by the hairdrier. The diode SKE 4F2/08 can be replaced by any fast-switching diode capable of about 3A/800V. There's also a version BU208D of the transistor with a diode integrated, but it's more expensive, and I have't tested it. The best way to find out the correct terminals of the flyback is to have a circuit diagram of the TV it comes from; the second best is simply to try every combination (using a variable transformer to turn up "mains" voltage slowly, so nothing is damaged). The value of C depends on your flyback and which winding you use. Use one with about 1000V output and around 1nF for the cap. The VDR is essential, without it your BU208A will die with the first spark. But the whole regulation circuit (on the left) is optional, you have to adjust the frequency by hand anyway. A more detailed description how it works and how to adjust it will follow sometime, I hope. Email me if you want to know more.

As I said, this device is designed to feed TV cascades, and it does this much more efficiently than the above 5kV mains transformer (due to the higher operating frequency). You can draw a very hot arc from it as well. So if you have the necessary knowledge, it's really worth the labour (and price), especially when used with the 80kV super-cascade .

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Jochen Kronjaeger