|Jochen's High Voltage Page|
|Rectifiers and diodes. (1) diodes from microwave oven 6kV>300mA, (2) rectifier rod (selenium) from b&w TV, (3) silicon diode 20kV, (4) rectifier tube (valve) from old b&w TV, (5) silicon rectifier 155kV (about 20cm long). Large version|
Nowadays, high voltage rectifiers are usually a string of solid state rectifier diodes (silicon or sometimes selenium). However, there are still rectifier tubes (valves) around, e.g. in old TV sets. Many rectifer tubes can still be bought from specialized dealers. For both types the following parameters determine their usefulness for a given purpose.
Silicon high voltage rectifiers can be bought from electronics dealers, suitable types are e.g. BY509 (11kV, 4mA) and BY713 (20kV, 3mA) which cost around 1DM. High power diodes can be found in microwave ovens (about 6kV/300mA). Selenium rectifier rods (20kV, 1mA or so) can be found in older b&w TV sets (near the flyback transformer). For higher currents, it is possible to make a string of lower-voltage diodes such as 1N4007 (1000V/1A). Parallel to every diode should be a resistor (around 10MOhm) and maybe a small capacitance (around 1nF) to equally distribute the reverse voltage. This method is only good for low frequencies such as 50Hz (mains).
Rectifier tubes need an additional heating (or filament) voltage, which must be connected to the cathode. As a result, the cathode must be either grounded or the filament transformer providing the heating voltage must be insulated for the maximum reverse voltage of the tube - this makes tubes a little complicated to use.
|Keep in mind that every high voltage rectifier tube is a potential xray emitter. This is not usually a concern as long as the tube is used within its specifications. It may become a problem when the reverse voltage is larger than allowed, causing significant reverse current and xrays as a result.|
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