Study of a Variable Feedback Oscillator and its Use in Automatic Temperature Control

Many methods and devices have been utilized in the automatic control of temperature. However, the accuracy requirement of a temperature controlling device depends upon its use. The purpose of this work was to design a device to obtain accuracy of control to the extent of better than one tenth of a degree centigrade of deviation of the specified temperature. This can be accomplished by using a vacuum tube device. The theoretical analysis of the nonlinear characteristic of the electronic circuit will constitute the main part of the work. The complete control system consisted of a feedback type oscillator, a power amplifier, a switching circuit, and heating elements. The principle idea here was to put a thermistor into the feedback network so that the oscillator would have output whenever the ambient temperature of the thermistor is lower than the specified value. This output is being amplified and fed into a relay circuit to switch on the heating elements so that the temperature can be raised. Because of the high sensitivity of the bridge circuit and the critical response of oscillation, a high accuracy of control can be successfully achieved