Excess current protection circuit for electronic switches

摘要

An excess current protection circuit for operation in AC circuits for an electronic switch which is to be connected in series with the electronic switch to be protected consists of a series circuit of a resistor (7) for detecting the current intensity of the load current and of a power switching transistor (8), which is switched on in normal operation and is switched off via a switch arrangement (9 to 16), which interacts with the switching transistor as a multivibrator, if a limiting voltage drop on the resistor (7) which is a function of a predetermined threshold value of the load current intensity is exceeded, and interrupts the load circuit for the remaining half cycle. As a consequence of the invention, the protection circuit is intended to be constructed in such a manner that it reliably protects the electronic switch without unnecessary disconnections occurring. To this end, the invention provides that the circuit arrangement (9 to 16), which acts as a multivibrator, contains a capacitor (17) which charges additively whenever the threshold value is exceeded and whose potential also determines the tripping threshold of the multivibrator in such a manner that the threshold value of the load current intensity is controlled to an ever lower level irrespective of the magnitude of the overload and of the duration of the overload, until the circuit arrangement transfers the switching transistor (8) into a switched-off state which is maintained over the present half cycle. <IMAGE>