Abstract: An overcurrent detection circuit comprises: a switch provided between a power supply and a load for switching connection on/off between the power supply and the load; a current detector for detecting a load current flowing in the load; a reference value generator which outputs a reference value of a predetermined first level at the ON-time of the switch means and reduces the level of the reference value with passage of time from the ON-time so as to converge the level of the reference value to a predetermined second level; and an overcurrent signal generator which compares the detected load current with the reference value and outputs an overcurrent signal when the load current reaches or exceeds the reference value.

Abstract: A microcomputer 4 responds to an instruction signal to turn the load L on or off to control the FET 2 through the drive circuit 5. The microcomputer 4 samples an electric current I.sub.L which is supplied through the A/D converter 6 and which flows in the load L at every predetermined sampling time T.sub.S and subjects the sampled electric current I.sub.L to a comparison with a known rated current value I.sub.R for the load L. When I.sub.L .gtoreq.I.sub.R is satisfied, the microcomputer 4 starts counting energizing time T. When the accumulated value of I.sub.L .times.T.sub.S is enlarged to a predetermined value S.sub.O, the microcomputer 4 turns the FET 2 off. When the detected electric current I.sub.L is larger than an upper limit I.sub.O (>I.sub.R) determined previously, the microcomputer 4 immediately turns the FET 2 off through the drive circuit 5.