Abstract: A clamp circuit disposed between a receptacle and a circuit to be protected when a connector connects to the receptacle, the clamp circuit including a voltage detector configured to determine a level of a surge voltage in comparison to a threshold voltage, the voltage detector including a plurality of field effect transistors (FETs) of a first conductivity type connected in series, a first FET of a second conductivity type and a first resistor in parallel with the plurality of FETs, a second FET of the first conductivity type in parallel with the first FET, and a discharge circuit to discharge the surge voltage when the surge voltage approaches the threshold voltage.