A test device includes a transformer, a loop analyzer, and a protection circuit. The protection circuit includes a control unit, a signal acquisition and amplification unit, and an electronic switch. When the loop analyzer outputs a signal, the signal acquisition and amplification unit acquires the signal output from the loop analyzer, amplifies the acquired signal, and outputs the amplified signal to the control unit. The control unit transforms the received signal from analog form to digital form, and compares the digital signal with a reference value. If the digital signal is greater than the reference value, the control unit outputs a control signal to the electronic switch, to turn off the electronic switch. The signal output from the loop analyzer cannot be transmitted to a buck circuit.