Circuit for providing electrical energy to a heatable means

Abstract

The present device includes a bridge circuit with a first intermediate terminal and a second intermediate terminal and with one leg of the bridge circuit being a heating element. Connected across the power lines of the bridge circuit is a main bidirectional switching circuit (having a gate element) which is turned on bidirectionally by control signals developed by an R-C circuit connected to the gate element. The resistors of the R-C circuit are for the most part integral components of two photoconductor isolator devices. These last mentioned resistors provide a low resistance after their associated LED's are turned on and the one of them whose associated LED turns off at the zero crossover remains at a low resistance value for a period of time after its associated LED's has turned off. In addition the present device includes a circuit to bypass the control signal which ordinarily would go to the main capacitor of the R-C circuit in the event that an overcurrent situation (e.g., a short circuit) occurs in the heating element circuit. The bridge circuit and the control signal circuitry (which is arranged across the bridge circuit) are designed so that the resistance of the heating element (which changes as the heating element heats up or cools off) acts as the sensing device to control or maintain the heat output at a predetermined temperature value.