The continuous diode is connected in parallel at both ends of the coil, and the coil will generate induction electromotive force at both ends when passing the current. When the current disappears, its induced electromotive force will reverse voltage to the components in the circuit. When the reverse voltage is higher than the reverse breakdown voltage of the element, the components such as triode and thyristor can be damaged. When the current in the coil is lost, the induced electromotive force generated by the coil is consumed by the circuit of the diode and coil. This protects the safety of other components in the circuit. Fly-wheel diode reverse parallel in the circuit in the relay or the ends of the inductance coil, when power inductor at both ends of the electromotive force does not disappear immediately, residual electromotive force through a diode released at this time, the role of diode called fly-wheel diode.