Input protection circuit of a handheld electric device

An input protection circuit of a handheld electric device for protecting internal circuitry of the handheld electric device, the internal circuitry having a positive input node and a ground node, the input protection circuit comprising:

• a power socket having a positive input node and a ground node for electrically connecting two output nodes of a direct current (DC) power supply, the ground node of the power socket being electrically connected to the ground node of the internal circuitry;
• a bipolar junction transistor (BJT) having an emitter electrically connected to the positive input node of the power socket, a collector electrically connected to the positive input node of the internal circuitry, and a base;
• a metal-oxide semiconductor (MOS) transistor for controlling on and off of the BJT, the MOS transistor having a source electrically connected to the ground node of the internal circuitry, a drain electrically connected to the base of the BJT, and a gate;
• and
• an overvoltage protective circuit having two input nodes and an output node for controlling on and off of the MOS transistor, the two input nodes being electrically connected to the positive input node and the ground node of the power socket, the output node being electrically connected to the gate of the MOS transistor;
• wherein when a reverse DC voltage or a DC voltage exceeding a threshold inputs from the positive input node and the ground node of the power socket, the overvoltage protective circuit will turn off the MOS transistor thereby turning off the BJT to prevent damages of the internal circuitry;
• and when a DC voltage below the threshold inputs from the positive input node and the ground node of the power socket, the overvoltage protective circuit will turn on the MOS transistor thereby turning on the BJT so as to input the DC voltage to the internal circuitry through the BJT.