Patent ID: 7561395

Claim:
A voltage control circuit comprising: first, second and third transistors, a relay, and first, second, and third resistors; the emitter of the first transistor connected to a first power supply, the collector of the first transistor connected to a first contact of the relay, a second contact of the relay grounded, third and fourth contacts of the relay respectively connected to positive and negative electrodes of the second power supply, fifth and sixth contacts of the relay connected to a main system; the collector of the third transistor connected to the first power supply via the third resistor, and also connected to the base of the first transistor, the emitter of the third transistor grounded; the first and second resistors connected in series between the positive electrode of the second power supply and ground; the base of the second transistor connected to a node between the first and second resistors, the collector of the second transistor connected to the base of the third transistor, and the emitter of the second transistor connected to the first power supply.