A protection circuit suitable for electrical instruments includes a software detecting circuit, a logic AND circuit, a driver and control circuit for engine operating power components, and a hardware detecting circuit connected with the logic AND circuit. Both the software detecting circuit and the hardware detecting circuit monitor the state of an operating switch and provide signals to the logic AND circuit. Only when the state of the switch changes from opened to closed and both detecting circuits determine that this state is correct will the driver and control circuit signal the engine to operate. Furthermore, by connecting two or more power components in serial at the same time, the engine will not work by accident and the machine will not be out of control when any one of the power components is damaged. A circuit using capacitors of suitable specification can also be provided to isolate the controlling pin of a power component in the form of an SCR to thereby avoid failure and damage of the entire system.