Testing diodes is different from testing a resistor because you need a skill to do it. If you do not know or wrongly test a diode you will be unable to repair the equipment. A bad diode you think it is good. This will definitely waste your precious time.

A rectifier diode can fail in one of the four ways. It can become:

-Open
-Shorted
-Leaky
-Breakdown when in full operating voltage

An analog multimeter or digital multimeter can be used to check for all the first three conditions except the last one where the diode breakdown in full operating voltage. From my of experienced in the electronic repairing line, i found that checking diode using analog multimeter is more accurate than using a digital multimeter. I could explain to you why i preferred analog meter. I do not know about you because i really came across quite a number of diodes where it checked ok using digital multimeter but failed when test with analog meter.

The first step on how to test a diode is to remove one of the diode lead. You can't always be certain if a diode is good or bad if you perform in-circuit test, because of back circuits through other components. To be absolutely sure, you will need to lift, or disconnect, one diode lead from the circuit to avoid back circuits. Unless you are very sure about the board you are checking. Sometimes i do found bad diodes when checking it on board. Your experienced will tell you when to check a diode on-board or off-board. If you are new to electronic repair, i highly recommend that you check a diode with a lead removed from the board.

I will set my analog meter to x1 ohms to check for current diode leakage reverse and forward testing. Connecting 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. 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 have 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.

The real problem when checking a diode using the diode test function of a digital meter is that an open or leaky diode, the meter sometimes reads ok(0.6). This is due to 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 have output about 3V(remember the two 1.5V battery you installed in the meter!). The 3V voltage is enough to show you the accurate reading of a diode when under test.

Even if you have a good reading at x1 ohms doesn't mean that the diode is good . You now have to set your meter to x10K to test the diode again. The output voltage of x10k ohms is about 12V(remember the 9v battery in your meter-1.5v+1.5v+9v=12v). 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 500mv to 2V.

If a diode breakdown when under full operating voltage, there is no way to testing diode (unless you have a very expensive diode checker which specially designed to locate this type of problem).Substituting with a known good diode is often the only way to prove that an intermittent diode is causing a particular problem. Sometimes an intermittent diode could be locate using a coolant spray.

Caution: Be certain that power is removed from any circuit before performing any of the following diode checks, otherwise meter or circuit damage could result.

Conclusion-In order to correctly test diode function you need to set the analog meter to x1 ohms and x10K ohms range.

Jestine Yong is a electronic repairer and a writer. For more electronic repair information please visit his website at http://www.noahtec.com/electronic-repair-articles.htm