When the positive pole of the power supply is connected to the positive pole of the diode, and the negative pole of the power supply is connected to the negative pole of the diode, the voltage at this time is positive voltage.When the positive pole of the power supply is connected to the negative pole of the diode, and the negative pole of the power supply is connected to the positive pole of the diode, the voltage at this time is the reverse voltage.Diodes are the basis of semiconductor devices, semiconductor diode devices.When the positive voltage is added at both ends of the PN junction (that is, the positive pole of the p-side power supply and the negative pole of the n-side power supply), the resistance of the PN junction is very low and the positive current is large (the PN junction is in the conduction state).Equivalent to at both ends with a positive voltage, it is in the conduction state, the resistance is very small, this resistance is called the forward resistance, only a few ohms, if there is a bulb and it in series, the bulb will give light;When the reverse voltage is added at both ends of the PN junction (that is, the negative pole of the power supply at the P side and the positive pole of the power supply at the N side), the PN junction presents high resistance and weak reverse current (the PN junction is in the cut-off state), which is the unidirectional conductivity of the PN junction.When the reverse voltage is applied at both ends, it is in the cut-off state, and the resistance is very large. This resistance is called the reverse resistance, which has several thousand ohms.