PURPOSE: To enable operation even when a power source is connected reversely by connecting the anodes of first and second diodes to the input terminal and the cathodes thereof (+) terminal while connecting the cathodes of second and fourth diodes to the input terminal and the anodes thereof to (-) terminal.

CONSTITUTION: When a DC power source 1 is connected correctly, current (i) flows through the first input terminal T1, a first diode D1, (+) terminal, an electronic circuit 2, (-) terminal, a second diode D2 and the second input terminal T2. When the DC power source 1 is connected erroneously, current (i) flows from the positive terminal of the DC power source 1 through the second input terminal T2, a third diode D3, (+) terminal, the electronic circuit 2, a fourth diode D4 and the first input terminal T1 to the negative terminal of the DC power source 1, and the current flows in normal direction through the electronic circuit 2. Consequently, the electronic circuit 2 functions normally regardless of the connection of the DC power source 1.