Circuit with a Thermal Starter

Circuit with a Thermal Starter Points : circuit with a thermal starter Fig represents the tube circuit which consists of a choke L, condenser C and a thermal starter. The thermal starter consists of two bimetallic strips and a heater coil H. When no current passes through the heater the bimetallic strips make contact with each other. When the tube is switched on to the mains, the current passes through the choke L, heater H, left electrode of the tube and back to the mains through the right electrode as shown in Fig.

As the current passes through the electrode of the tube, they are heated and produce electrons into the tube path, at the same time the heater H heats the bimetallic strips and this heating causes them to spring apart, thus the current into the, circuit is interrupted. The sudden interruption of the current in an inductive circuit causes a high voltage surge across the electrodes of the tube, which is sufficient to start the tube in operation. Note that the function of the choke is to produce a blast (high voltage at starting) for starting the tube. As the choke is in series with the circuit, it makes the tube to work at a very low power factor, a condenser C is connected across the main supply. The value of the capacitor should be 7.5 F for an 80-watt tube, and 3.25 for a 40 watt-tube.