SOLUTION

The silicon-controlled switch (SCS) is a tetrode thyristor. That is, it has four electrodes It has an anode gate (AG) like a PUT and a cathode gate (KG) like an SCR. A current of Efficient size on either gate will fire the SCS. A large reverse current through the anode gate may be used to turn the SCS off.  Its use is largely in voltage or current sensing circuits as signals on either gate fire it. It operates like an OR gate. its power capabilities are limited to timing, logic, and triggering applications. 

An advantage of the SCS over a corresponding SCR is the reduced turn-off time, typically within the range 1µs to 10µs  for the SCS and 5µs to 30µs  for the SCR. Some of the remaining advantages of the SCS over an SCR include increased control and triggering sensitivity and a more predictable firing situation. At present. however, the SCS is limited to low power, current, and voltage ratings. Typical maximum anode currents range from 100 mA to 300 mA with dissipation (power) ratings of 100 mW to 500 mW.

Applications

The SCS is a low-power device compared to SCR. The power rating of SCS is very low compared to SCR. The SCS handles current in the mA range. But it has the advantage of faster turn OFF. The application areas of SCR and SCS are the same. The SCS is sometimes used in digital applications such as counters, registers, and various timing circuits.