Patent ID: 8217608

Claim:
An electric power supply device for switching supply and stop of electric power to a motor load by performing an on-off operation of an electronic switch which is disposed in a wiring path for connecting a power source to the motor load, the electric power supply device comprising: a current detection section that detects a current flowing in the motor load; and a switch control section that performs control in which an operation of turning off the electronic switch for only a second predetermined time after turning on the electronic switch for only a first predetermined time for which lock torque can be caused is repeatedly executed when the current detection section detects a lock current flowing at the time when the motor load locks, and the electronic switch is returned to a normal operation when the current detection section detects a current smaller than the lock current, wherein the current detection section has a comparison section for comparing a variable determination voltage A with a voltage (Vds) between both ends of the electronic switch; wherein the switch control section sets a standard determination voltage (v 1 −V 3 S) which is slightly larger than the voltage between both ends of the electronic switch in a normal use state of the current flowing in the motor load, sets a first determination voltage (v 1 −V 31 ) larger than the standard determination voltage and smaller than the voltage between both ends of the electronic switch at the time when the lock current flows and a second determination voltage (v 1 −V 32 ) larger than the voltage between both ends of the electronic switch at the time when the lock current flows, changes the determination voltage A to the second determination voltage when the voltage between both ends of the electronic switch exceeds the determination voltage A in the case of setting the determination voltage A at the standard determination voltage, changes the determination voltage A to the first determination voltage when a state, in which the voltage between both ends of the electronic switch is smaller than the determination voltage A, continues for the first predetermined time in the case of setting the determination voltage A at the second determination voltage, and determines that the lock current flows in the motor load when the voltage between both ends of the electronic switch is larger than the determination voltage A in the case of setting the determination voltage A at the first determination voltage.