The present invention relates in general to a circuit coupled between a semiconductor switch and a load for interrupting power to the load upon failure of the semiconductor switch. More particularly, the failure control circuit is particularly adapted for use with a bilateral switch such as a Triac.
Various types of circuits exist for detecting a fault condition associated with the load. However, most of these circuits are quite complex and involve substantial additional circuitry for the detection of one or more conditions associated with the load.
Accordingly, one object of the present invention is to provide an extremely simple means for detecting a fault condition at a load, and in particular when the load is driven from a switchable semiconductor device, especially a bidirectional device such as a Triac.
U.S. Pat. No. 4,252,252 shows a control circuit for controlling a dispensing machine which includes as one component thereof, a solenoid driven from a Triac. It has been found that the Triac failed usually only in one direction at a time and typically failed by shorting thus in effect applying continuous voltage to the load. Where the solenoid controls water flow in a dispensing machine, it is extremely critical that such failure not occur as substantial water damage will be the result. The Triac shorting can occur in many different ways. For example, the Triac that is subjected to a destructively high di/dt generally shorts when MT2 is positive. Also, an excessive off-state voltage, backed by large current, may cause a short in either direction, but seldom in both directions simultaneously. This shorting failure essentially causes a substantially d.c. voltage across the load. When the load is an electromechanical device such as a solenoid, there may even be a fire hazard with such d.c. saturation.
Accordingly, one object of the present invention is to provide a simple circuit for use between a Triac or the like and a load for interrupting power to the load upon failure of the Triac, and in particular failure in the form of a permanent short in one direction.