Heating circuit

Abstract

An energizing circuit for applying maximum electrical power to a heatable element having, at least, a high initial thermal lag. By sensing the resistance of the element, which varies as a function of its temperature, a control voltage is obtained which is used to vary the effectiveness of the heating current so as to provide rapid initial heating without the danger of burn-out. More specifically, the circuit is useful in energizing the glowplug of a model airplane engine. Current pulses are obtained from a storage battery which is connected in series with the glowplug and a semiconductor switch. The switch is turned on and off by a multivibrator to produce a series of current pulses. The duration of each "on" pulse is controlled by a bridge circuit which senses the resistance of the glowplug, which is a function of its temperature, and produces a control voltage which varies the timing of the multivibrator. The energizing circuit is manually disconnected as soon as the engine starts and may be accidentally short-circuited when the connecting leads are dropped. A control circuit is therefore provided for sensing such a short circuit and shutting off the current. A further control circuit may also be included for minimizing overheating resulting from contact transients set up when initial or erratic contact is made with the glowplug. In addition, the circuit is designed so that when the contact is removed from the glowplug the control circuits are deactivated automatically and the circuit draws no stand-by current from the battery. Undesired effects that may occur as the battery becomes discharged are also taken into account.

Dependencies are inferred from the claim text.