| 摘要 |
A switch is provided in a supply line for an operating power supply from an operating power supply generating circuit to a main circuit, An electric current detecting circuit and a comparison evaluating circuit are provided as a controlling circuit portion for controlling the ON/OFF state of the switch. The electric current detecting circuit inputs a voltage in accordance with the value of an electric current supplied from an higher-level system, and inverts and amplifies the voltage to produce an output voltage. The comparison evaluating circuit inputs the output voltage from the electric current detecting circuit, compares the output voltage to a reference voltage, and turns the switch ON if the output voltage is higher than the reference voltage. The reference voltage is set in advance in accordance with a lower limit electric current value for the electric current that must be supplied for the main circuit to operate properly.
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