초록 ▼

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

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.

대표청구항 ▼

[ What is claimed is:] [1.] A switch circuit having an excess-current detection function, comprising:a semiconductor switching device disposed between a load and a power source and arranged to turn on or off the connection between said load and said power source in accordance with a control signal s

[ What is claimed is:] [1.] A switch circuit having an excess-current detection function, comprising:a semiconductor switching device disposed between a load and a power source and arranged to turn on or off the connection between said load and said power source in accordance with a control signal supplied to a control terminal thereof;switch control means arranged to receive an instruction signal output from an instruction-signal output portion and to output the control signal to said control terminal of said semiconductor switching device so as to control supply of electric power from said power source to said load;electric-current detection means for detecting a load electric current which flows in said load;excess-current control means for changing on/off control of said semiconductor switching device when detection of said load electric current satisfies at least one of a plurality of an excess current determining conditions previously determined in accordance with the electric characteristics of an electric line between said load and said power source, or electric characteristics of said load; andan A/D converter connected with said electric-current detection means for converting the detected load electric current from an analog value to a digital value,wherein said excess-current control means has comparison means for subjecting said digital value of said detected load electric current and a reference electric current determined previously in accordance with the electric characteristics to a comparison, to determine an excess current, said excess current being a load electric current that is greater than said reference electric current and count means for counting energizing time in which said excess current flows in said load, and said excess current determining condition is determined by using said excess current and said energizing time; andwherein said excess-current control means is arranged to determine the excess current when a condition is satisfied such that a predetermined electric current larger than said reference electric current flows in said load for time determined previously in accordance with said electric characteristics and provided with said plurality of excess current determining conditions obtained by combining different set electric currents and set time periods arranged in such a manner that the set time is shortened in inverse proportion to the set electric current, said comparison means further subjects said digital value of said detected load electric current and each of said set electric currents to a comparison, and said count means further counts energizing time in which an electric current larger than each of said set electric currents flow in said load.