A resistance determining system for an over voltage protection (OVP) circuit, includes an external power source, a microcontroller, a digital rheostat and a display unit. The external power source supplies an external voltage to the OVP circuit. The microcontroller stores an over voltage value. The microcontroller is connected to the external power source and configured to detect the external voltage and compare the external voltage with the over voltage value. The digital rheostat is connected to the microcontroller and includes a first rheostat having two connection terminals respectively connected to two first connection ends of the OVP circuit. The microcontroller adjusts the first rheostat to be a first resistance value to activate the OVP circuit when the external voltage is substantially equal to the over voltage value. The display unit is connected to the microcontroller and configured to display the first resistance value.