Electronic fuse (safety device) for a transistor output stage

摘要

An electronic short-circuit fuse for a transistor output stage of an electronic control, testing or supply device, the output stage of which contains an npn transistor whose collector is connected to the control output via a series resistor and whose emitter is connected directly to the circuit earth, contains, in parallel circuit thereto, an auxiliary transistor whose collector is likewise connected to the control output and whose emitter is connected via a resistive potential divider having at least two resistors to the circuit earth. The base of a safety transistor is connected to the centre tap of this resistive voltage divider, the collector of which safety transistor is connected to the base of the power transistor and the emitter of which safety transistor is connected to the circuit earth. Control signals produced by an operational amplifier are fed, in each case via a series resistor, to the base of the power transistor or the base of the auxiliary transistor, respectively. In normal operation, the power transistor and the auxiliary transistor are on (conducting), whereas the safety transistor is off (blocked). In the event of a short circuit in the load or of an overload of the output stage, the current in the sensor circuit passing via the auxiliary transistor increases and thereby the control voltage at the base of the safety transistor also increases, the safety transistor therefore becoming switched on, which turns off (blocks) the power transistor. After operation of the fuse, the device must be switched off, before ... Original abstract incomplete.