Testing a PN Diode

Use Analog Meter To Test Diode

Set your analog meter to x1 ohms range to check for current diode leakage reverse and forward testing. Touch the black probe of your meter to the cathode and red probe to the anode, the diode is reverse biased and should look like an open reading-the meter pointer not moving. Connecting the red probe of your meter to the cathode and black probe to the anode, the diode is forward biased and the meter should read some value of resistance. If you get two readings then most probably the diode is shorted or leaky and you should replace it.

If you don't get any reading either forward or reverse bias, the diode is considered open circuit. The real problem when testing a diode using the diode test function of a digital meter is that an open or leaky diode, the meter sometimes reads okay (0.6). This is due to the digital meter diode test output voltage (which you can measure the output test probe using another meter) is around 500mv to 2v. An analog meter set to x1 ohms range have output about 3V(remember the two 1.5V batteries you installed in the meter!). The 3V voltage is adequate to show you the accurate reading of a diode when under test.

1n5408 diode Even if you have a good reading at x1 ohms range checking a diode, this doesn't mean that the diode is good . You now have to select your meter to x10Kohm range to test the diode again. The output voltage of x10k ohms is about 12 Volt (remember the 9 volt battery in your meter-1.5 volt + 1.5 volt + 9 volt = 12 volt). Again the diode under test should show only one reading. This is exception to Schottky diode where it have two readings but not shorted reading.

If the meter showed one reading then the diode under test is good. If it has two readings then most probably the diode is either shorted or leaky. The digital meter can't test it because the output from the meter is only about 500mv to 2 volt.