Thermistors for the Gradual Application of Heater Voltage to Thermionic Tubes

Thermistors which are thermally-sensitive resistors separately. The separate application and removal of having large negative temperature coefficients of resistance can be power to the individual sectioijs facilitates trouble aptly used for the gradual application of heater voltage to thermionic tubes, thereby diminishing thermal transients and reducing mechanistin andmnizes thener of time theent cal failures. Full voltage is first applied to the thermistor and load in system is energized and de-energized. In such large inseries and after temperature equilibrium is reached, the thermistors stallations, power is applied and removed more often are shorted out. From a cold start thermistor resistance decreases by from any given section by local control than by the conia factor of approximately 100 and heater resistance increases by a trol which operates the entire power system. Consefactor 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 quently, famernv tageistusu alied at the secduring the voltage application, the initial voltage on the heaters will tion level rather than at the systems level. For this purbe less than 5 per cent of rated, and the voltage before shorting out, pose, thermistors, which are resistors having negative about 75 per cent of rated. Peak currents occur twice, once while the temperature coefficients of resistance, may be applied to thermistors are in the circuit and again when the thermistors are give gradual filament heating. shorted out at 90 seconds. One set of thermistors can be used to giermg limit the current satisfactorily over a wide range of loads. The most temistor e be enuse intmeldel applicfavorable number of thermistors to connect in series is determined tions, temperature measurements, and amplifier stabilizexperimentally. Commercially available units in washer form can acing circuits. At least one manufacturer has used them in commodate heater loads up to 1100 watts. series heater strings for television receivers. All of these are low current applications. This paper will discuss the I NTRODUCTION use of thermistors to control more than one kilowatt of I N LARGE electronic systems, such as those empower. A physical description of typical commercially ploying large scale digital computers where the available thermistors and recommended mountinig tube-count can run to tens of thousands, long tube methods are presented in the Appendix. life is a necessity. To extend tube life, one precaution that should be taken is the prevention of thermal shocks PERFORMANCE OF THERMISTORS