Control circuit for a source of heat

Abstract

The present system includes a bridge circuit (with four legs) with one leg including a heating element. The mid-points of the bridge circuit are connected to a silicon controlled rectifier (SCR) which is further connected to the control element of a monostable multivibrator. Further connected to the power input elements of the monostable multivibrator is a D.C. power source. The monostable multivibrator is connected to a transistor which has a light emitting diode (LED) in its cathode circuit. The LED is part of a circuit which further includes a light sensitive triac. The light sensitive triac is further connected through a pulse generating circuit to the control element to a second triac which in turn is connected to supply heavy electrical current to the heating element. Accordingly, when the monostable multivibrator causes the transistor to conduct, the light sensitive triac conducts and fires the second triac and hence causes electrical current to pass to the heating element to generate heat at the heating element. Normally, the triacs would turn off at the zero crossover but the circuitry is arranged to have them in a conducting state into the next half cycle after the zero crossover. The present system includes a soft start feature, a short circuit proteciton feature and an electronic temperature selection control.