|Jochen's High Voltage Page|
|Several types of cascades salvaged from old TVs.|
Older TV sets that do not have a diode split flyback usually contain a flyback that gives about 8kV peak, and a subsequent cascade which triples this to about 24kV. These cascades are moulded modules with a number of wires or solder points for input and output, usually Ground, AC input, 8kV (focus) output and 24kV (acceleration) output. The acceleration output is easily recognized by the thickly insulated cable directly plugged into the CRT at the other end. The rest of the inputs/outputs are often labelled.
|Circuit diagram of a TV cascade (type (2)).|
The schematic above shows the internal circuit of the most common type of cascade (number (2) in the photo above) and the usual input/output labels. It´s basically a Villard circuit but lacks one cap in the first stage (compare schematic above with the Villard circuit). The AC input (labeled "~") goes directly to D1/D2, and the Ground connection is split into seperate branches for D1 (labeled "D") and C1 (labeled "A").
To make this type of cascade work, there are two possibilities. Either, you connect the input AC voltage directly to "~". In this case, "A" must be grounded, while "D" remains open - otherwise, D1 would be a short-circuit for the negative half-wave of the input AC. Or, you connect "A" and "D" together to ground and externally add the missing cap between the input AC voltage and "~". The additional cap must be rated about 10kV, 1nF. In principle, the latter method gives a slightly higher output voltage.
Below is an X-ray picture of the module corresponding to the schematic above. The components are also labeled as in the schematic. The circle refers to the Super-Cascade section.
|X-ray picture of a cascade module (type (2)).|
TV cascades can be salvaged from old TV sets or bought from surplus dealers. Best thing to feed them is a flyback transformer, but Obits are possible as well, though less suitable due to their low frequency. Only one-half of a symmetric Obit can be used. Output voltages of up to 30kV are possible, also higher if you´re willing to take some risk of early failure.
Cascade modules are not suitable for rapid discharge, which occurs e.g. when arcing to ground. I lost several units this way. Even worse, the input AC source may be damaged through the extremely high voltage spikes occuring during rapid discharge. A damping resistor in the 100kOhm range at the output is therefore highly recommended. Of course, this must be a high voltage type. Many cascades have an internal resistor, but the high voltage spikes it has to withstand usually lead to failure after a few discharges. Suitable external resistors can be obtained by putting several 10kOhm in series, or by filling a short piece of plastic hose with tap water.
|Cascades keep their charge for quite some time after disconnecting. Always discharge them (through a resistor!) before handling them.|
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