| 摘要 |
PROBLEM TO BE SOLVED: To fully use the capacity of an internal power supply, to prevent the internal power supply from being charged by an external power supply, to prevent a circuit from being damaged even if the internal power supply is set inversely, and to miniaturize the circuit by reducing a packaging area. SOLUTION: A switch means 6 is provided between a connection line L4 to the -side of a primary battery 1 and a ground line. The switch means 6 is composed of FETs Q1 and Q2. The gate of Q1 is connected to that of the Q2. A resistor R3 is connected between the gate and a power-feeding line L3. The drain of an FET Q3 is connected to the connection point between the resistor R3 and the gates of the Q1 and Q2. When the primary battery 1 is set inversely, a connection point PA is biased inversely and the Q1 and Q2 are turned off. When an external power supply 2 is connected, the Q3 is turned on, the connection point PA is biased inversely, and the Q1 and Q2 are turned off.
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