To limit consumption power in the case of excessive temperature rise, and ensure safe operation of a unit, by generating a control voltage in such a manner that a power source voltage is kept constant when a temperature is at most a previously determined threshold value, and is decreased as the temperature exceeds the threshold value and becomes higher.

In a circuit 17 for generating a control voltage for the power source of a unit, a resistance bridge is formed by using resistors 2, 3, 4 and resistors 5, 7. The temperature coefficient of the resistor 7 is negative. When a temperature is at most a previously determined threshold value, the output voltages of two amplifiers 10, 13 are high, transistors 9, 12 are turned off, and the voltage of an output terminal 19 becomes equal to the voltage of a reference voltage source 6. When the temperature rises and reaches a first threshold value, the transistor 9 is turned on, the voltage of the output terminal 19 is decreased. When the temperature further rises and reaches a second threshold, the transistor 12 also is turned on, and the voltage of the output terminal 19 is further decreased.