TV cascades

Description: Older television sets that lack a diode-split flyback typically feature a flyback transformer that produces approximately 8kV peak voltage. This voltage is subsequently increased through a cascade circuit, which triples it to around 24kV. These cascades are often encapsulated modules with multiple connections for input and output, typically including Ground, AC input, 8kV (focus) output, and 24kV (acceleration) output. The acceleration output can be easily identified by the thickly insulated cable that connects directly to the cathode ray tube (CRT). The remaining input and output connections are usually labeled. The internal circuit of a common type of cascade is depicted in the accompanying schematic, which corresponds to the second module shown in the referenced photo. This configuration resembles a Villard circuit, albeit with one capacitor omitted in the first stage, which can be compared with the standard Villard circuit schematic. The AC input, denoted by the symbol "~", is directly connected to diodes D1 and D2, while the Ground connection is divided into separate branches for D1 (labeled "D") and capacitor C1 (labeled "A"). To operate this type of cascade, there are two options: either connect the input AC voltage directly to "~", ensuring that "A" is grounded while "D" remains open to prevent D1 from short-circuiting during the negative half-cycle of the AC input, or connect "A" and "D" to ground and externally add the missing capacitor between the input AC voltage and "~". This additional capacitor should be rated for about 10kV and 1nF. The latter configuration generally yields a slightly higher output voltage. An X-ray image of the module, which corresponds to the schematic, is available, with components labeled as in the schematic. The circle in the image indicates the Super-Cascade section. Cascade modules can be salvaged from old television sets or purchased from surplus dealers. The optimal power source for these modules is a flyback transformer, although Obits can also be used, albeit less effectively due to their lower frequency. Only one-half of a symmetric Obit should be utilized. Output voltages can reach up to 30kV, or even higher, though this comes with the risk of premature failure. Cascade modules are not designed for rapid discharge events, such as arcing to ground, which can lead to unit failures. Furthermore, the input AC source may sustain damage from the high voltage spikes generated during rapid discharge. To mitigate this risk, it is highly advisable to incorporate a damping resistor in the range of 100kOhm at the output, ensuring it is rated for high voltage. While many cascades include an internal resistor, the high voltage spikes they must endure can lead to failure after a limited number of discharges. Suitable external resistors can be created by connecting several 10kOhm resistors in series or by using a short length of plastic tubing filled with tap water.

The described circuit utilizes a flyback transformer to generate high voltages suitable for applications such as CRT operation in older television sets. The flyback transformer produces an initial peak voltage of approximately 8kV, which is then increased to around 24kV through a cascade circuit. This cascade consists of diodes and capacitors arranged in a configuration that allows for voltage multiplication. The presence of two diodes (D1 and D2) facilitates the rectification of the AC input, while the capacitors store and release energy to achieve the desired voltage levels.

The design of the cascade requires careful attention to the connections and grounding of components. The grounding of capacitor C1 (labeled "A") is critical to prevent short circuits during operation. The option to externally add a capacitor rated for high voltage introduces flexibility in design, allowing for adjustments in output voltage based on circuit requirements. The use of high-voltage-rated components throughout the circuit is essential to ensure reliability and safety, particularly given the potential for high voltage spikes during discharge.

To enhance the circuit's robustness, the inclusion of a damping resistor is recommended. This resistor serves to limit the current during rapid discharge events, protecting both the cascade module and the input AC source from damage. The choice of a high voltage resistor is imperative, as standard resistors may fail under the conditions present in this application. By implementing these design considerations, the circuit can achieve efficient operation while minimizing the risk of component failure.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. 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. 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.