Thermistors for the Gradual Application of Heater Voltage to Thermionic Tubes

Thermistors which are thermally-sensitive resistors having large negative temperature coefficients of resistance can be aptly used for the gradual application of heater voltage to thermionic tubes, thereby diminishing thermal transients and reducing mechanical failures. Full voltage is first applied to the thermistor and load in series and after temperature equilibrium is reached, the thermistors are shorted out. From a cold start thermistor resistance decreases by a factor of approximately 100 and heater resistance increases by a factor of 5 to 10. By selecting a set of series-connected thermistors to limit the current to a maximum of 120 per cent of the operating value during the voltage application, the initial voltage on the heaters will be less than 5 per cent of rated, and the voltage before shorting out, about 75 per cent of rated. Peak currents occur twice, once while the thermistors are in the circuit and again when the thermistors are shorted out at 90 seconds. One set of thermistors can be used to limit the current satisfactorily over a wide range of loads. The most favorable number of thermistors to connect in series is determined experimentally. Commercially available units in washer form can accommodate heater loads up to 1100 watts.