Why the value is not displayed when measuring the forward and reverse resistance of the diode using a digital multimeter
The unidirectional conductivity of the diode is conditional, that is, the PN junction of the diode is forward biased before it is turned on. Simply use the resistance gear to measure the resistance, the diode itself is not conducting, so the resistance cannot be measured. The diode gears of the multimeter are all with voltage, generally around 1.5-2.8V. When measuring the diode, the voltage is actually added to both ends of the diode to make the PN junction forward biased. Only then can we see about 0.7V on the multimeter. The voltage drop proves that the diode is good. The resistance of about 10K measured by the pointer meter is not accurate, and the values obtained by measuring different diodes are also different.
How to measure a diode with a multimeter
Diodes must be measured with the diode range of the multimeter. Turn the gear of the multimeter to the diode gear, insert the red test lead into the positive end, and the black test lead into the negative end.
Use the red test lead and the black test lead to touch the two electrodes of the diode under test. If the diode shows a voltage drop of about 0.7V, it means that the end touched by the red test lead is the positive pole of the diode, and the other side is the negative pole of the diode.
Change the direction of the diode under test, if the voltage value cannot be measured, it means that the red test lead is in contact with the cathode of the diode, and the black test lead is in contact with the anode of the diode.
After the above two measurements, the positive and negative poles of the diode can be confirmed, and the quality of the diode can be confirmed.
When the digital multimeter is in the resistance mode, the voltage between the two test leads is low, which is lower than the forward conduction voltage of about 0.6-0.7V of ordinary silicon diodes, and the diodes are in the cut-off state. The resistance value in the cut-off state is very large, so when measuring the diode with a digital multimeter, whether it is forward or reverse, the displayed value overflows. All digital multimeters have a diode measurement gear, which should be used to measure the forward and reverse on-off of the diode.
In addition, the diode is a nonlinear element, and the resistance value is not fixed. When the forward conduction resistance of the diode is measured with the resistance gear of the pointer multimeter, it can be seen that the resistance value of different gears is very different. Therefore, it is not meaningful to use a multimeter to measure the resistance value of the diode. As long as the forward conduction and reverse cut-off can be measured, or the positive and negative poles can be distinguished, and whether there is a breakdown or burnout, it is enough. There is no need to read the specific resistance.
The digital multimeter has a dedicated diode test file for measuring diodes. If you have to use a resistance file to measure, the diode has a resistance value of more than tens of K in the reverse direction, so the meter shows an overflow of 1. There are hundreds of ohms to several K in the forward direction, because the resistance value is different because the voltage applied to both ends of the semiconductor diode is different. Therefore, the diode is generally measured with a special file. The reverse is generally 1 (overflow), and the forward is between 300--750 (this is actually a voltage drop). Most of them are around 600 (silicon) and below 500 (germanium) tubes .
