Using a Multimeter to Measure the Polarity (Positive and Negative Poles) and Quality of a Diode
Polarity of a Diode (Positive and Negative Poles)
Generally, diodes directly indicate their positive and negative poles. When you see a section with a white coil on the tube shell, that is the negative pole. Or the side with the shorter lead is the negative pole. But what if there are no such features?
A multimeter is a tool that every electrician has. When using the ohm range (for measuring resistance) of a multimeter to measure the resistance of the positive and negative poles of a diode, since there is a built - in battery inside the multimeter, you need to note that the terminal marked with "-" on the multimeter shell is connected to the black test lead, and the terminal marked with "+" on the shell is connected to the red test lead. The current flows out from the red test lead and flows back through the black test lead. Additionally, you should choose the Rx1000 ohm range for measurement. The Rx1 range has too large a current and the Rx10K range has too high a voltage, both of which can easily damage the diode, so they are not suitable.
The specific testing method is shown in the right - hand figure. Connect the two test leads of the multimeter to the two pins of the diode respectively. The forward resistance of a diode is very small, generally ranging from tens of ohms to hundreds of ohms, while the reverse resistance is very large, generally between tens of thousands of ohms and hundreds of thousands of ohms. If in the two tests in the figure, the test on the right shows a small resistance and the test on the left shows a large resistance, you can conclude that the pin connected to the red test lead on the right is the positive pole of the diode, and the other pin is the negative pole.
Some modern digital multimeters may have a function for judging the quality of a diode (continuity range). Set the multimeter to this range for measurement. If there is a reading, the end of the red test lead is the positive pole. If there is no reading or it shows "1", the black test lead is connected to the positive pole.
Judging the Quality of a Diode
Still use the above - mentioned method of using the resistance range of a multimeter for judgment. If the difference between the measured forward and reverse resistances is very large, it indicates that the one - way conductivity of the diode is good. If the two measured resistance values are both very small or both very large, it means that the diode has lost its one - way conductivity, and it is a defective diode with quality problems.
