Patent ID: 6972940

Claim:
A power supply protection circuit provided with: a power source line for supplying direct current electric power outputted from a direct current power supply, via relay contacts of a relay, to an electrical equipment; a first and second circuits respectively connected to said power source line in a parallel relation with each other; and a relay operation circuit connected between said first and second circuits, wherein said first circuit is provided with a first semiconductor-switching element and a first excess voltage detecting element, and connected such that if said first excess voltage detecting element detects an excess voltage, said first semiconductor-switching element becomes unconductive, and that if said first excess voltage detecting element does not detect an excess voltage, the path between an input terminal and an output terminal of said first semiconductor-switching element becomes conductive, and wherein said second circuit is provided with a second semiconductor switching element and a second excess voltage detecting element, and connected such that if said second excess voltage detecting element detects an excess voltage, said second semiconductor-switching element becomes conductive, and that if said second excess voltage detecting element does not detect an excess voltage, said second semiconductor-switching element becomes unconductive, and wherein said relay operation circuit is connected with a coil making relay contacts of said relay ON in case an electric current is conducted from one terminal to the other terminal in the coil, a third semiconductor-switching element for controlling conducting said electric current to said coil, and a reduced voltage detecting element that becomes conductive in case of detecting a voltage exceeding a reduced voltage and becomes unconductive in case of detecting a voltage below a reduced voltage, and said third semiconductor-switching element, whose control terminal is connected to the output terminal of said first semiconductor-switching element via said reduced voltage detecting element, is connected such that if said reduced voltage detecting element detects a voltage exceeding a reduced voltage, said third semiconductor-switching element becomes conductive, and that if said second semiconductor-switching element becomes conductive, said third semiconductor-switching element becomes unconductive, and one terminal of said coil is connected to an output terminal of said first semiconductor-switching element, and the other terminal of said coil is connected to an input terminal of said third semiconductor-switching element.