The utility model provides an under -voltage and overvoltage crowbar includes: sampling circuit is connected with the alternating current for gather the alternating current signal, the detection circuitry who links to each other with sampling circuit for detect sampling circuit's voltage state, the continuous control circuit of output with detection circuitry for output control signal, with control circuit's the continuous switch motion circuit of output, the output linked switch power supply circuit's of switch motion circuit chip for control signal according to control circuit output makes switching power supply circuit's chip switch on / short circuit. This under -voltage and overvoltage crowbar not only can reduce the consumption of sampling resistor, can also avoid switching power supply to restart the consumption because of what excessive pressure or undervoltage protection produced.