PURPOSE: To obtain a simple and inexpensive surge protector, by reducing allowable loss of a resistor and a Zener diode.

CONSTITUTION: Upon application of a surge voltage VP at point A, potential at point B is cut through Zener voltage VZ. Potential at point C is a sum of base-emitter voltage VBE and a Zener voltage Y2. When the Zener voltage V2 is set at a level about 0.6∼0.7V lower than the maximum input voltage of a voltage regulator 2, potential at point C does not exceed the maximum input voltage. Normally, consumption current in 5V system flows from point A through collector and emitter of a transistor 5, point C to the voltage regulator 2. Current level is subjected to limitation of collector current, but no trouble occurs in the consumption current of 5V system when the transistor is selected properly. Resistor 6 may have high resistance for feeding base consumption current of 5V system, and thereby allowable loss can be reduced.