Protection circuit for power transistor supplying cooling fan motor - uses voltage divider including positive and negative temp. coefficient thermistors to supply coil of relay which can bridge power transistor

摘要

The fan is controlled by a power transistor connected in series with the motor. If the fan is obstructed and the motor speed falls, or during starting of the fan motor, the power transistor may be overheated and thereby destroyed. An electromagnetic relay (6) is connected between collector and emitter of the power transistor (3) so that under abnormal conditions the transistor may be bridged through the relay contacts (7,9). The relay coil is supplied through a voltage divider circuit (10,11,12) incorporating two thermistors (11,12) one positive temp. coefficient, the other negative. Both thermistors are placed in the air stream (6) from the fan (2) and their characteristics are such that the relay is opened after starting and closed if the air flow becomes too small. The threshold temp. for the thermistors (11,12) is set to 1300 degrees C.