FAIL SAFE CIRCUIT OF COOLING CONTROL UNIT FOR PREVENTING COOLANT FROM OVERHEATING BY DRIVING FORCIBLY COOLING FAN IF MICROCOMPUTER IS BROKEN DOWN

摘要

PURPOSE: A fail safe circuit of a cooling control unit is provided to stably drive a cooling fan if a power transistor is broken down by driving the cooling fan with a relay and to remove basically the generation of heat caused by using the power transistor. CONSTITUTION: In a cooling control unit(20) having a microcomputer(21) for driving a cooling fan motor(30) by operating a power transistor according to a control signal output from an engine control unit(10), a fail safe circuit for the cooling control unit is composed of a position watchdog signal generating circuit for generating a position watchdog signal with constant frequency if the microcomputer works normally; a differentiator circuit(22) for differentiating and outputting the position watchdog signal into a reset signal; a ramp signal generating circuit(23) for generating a ramp signal with a time constant by discharging charged voltage according to the reset signal; a comparator(24) for comparing the ramp signal with a reference voltage value and outputting a low-level signal if the position watchdog signal is not generated from the position watchdog signal generating circuit; and a relay(26) for driving the cooling fan motor if the low-level signal is output from the comparator.