A thyristor is a semiconductor that is used for switching purpose. In many cases a thyristor may replace a solenoid or relay for controlling the load current to the motor . electrical load switching through thyristor is advantageous because there are no moving parts thus problem with wear corrosion and arcing are eliminated.theyristor
There are two common types of thyristors, the silicon controller rectifier (SCR) and the triode ac semiconductor (triac).either the SCR or triac will allow current to flow once a certain level of gate signal is achieved .if the gate signal is then removed,the current will continue flow through the anode-cathode circuit until that signal is interrupted.this characteristic of a thyristor makes it ideal for switching warning circuit on aircraft.for example ,if an excessive temperature is reached in a turbine engine for only a split second and then decrease, an engine warning light will illuminate. the light will continue to glow until the pilot interrupts the light circuit and turns off the thyristor,thus turning off the light.the thyristor has made it possible for the pilot to receive continuous indication of a warning condition that existed for only a very short time period. Thyristors are also used for controlling large amounts of current flow to motor, heater, or lighting circuit.