Testing Diode

Testing diode is different from testing resistors because you need a good method to check it. If you don't know or wrongly test a diode you will not be able to repair the equipment. A defective diode you may think it is good and will take up your precious time. A diode can fail in one of the four ways. It can become:
-Open circuit
-Shorted
-Leaky and
-Breakdown when under full load

An analogue multimeter or digital multimeter can be used to measure for all the first three conditions except the last one where the diode breakdown under full load. From my of experienced gained from the service line, i found that measuring a diode using analogue multimeter is more accurate than using a digital multimeter. I could explain to you why i preferred analogue multimeter. I do not know about you because i really came across a lot of diodes where it measured ok using digital meter but failed when check with analogue meter.

The first step on how to test a diode is to solder out one of the diode leg. You can't always be certain if a diode is good or defective if you perform in-circuit test, because of back circuits through other components. To be measure it accurately, you need to lift, or disconnect, one diode leg from the circuit to avoid back circuits.
Unless you are very sure about the board you are testing. Sometimes i do found bad diodes when checking the diodes on board. Your experienced will tell you when to test a diode on-board or off-board. If you are new to electronic repair, i strongly suggest that you check a diode with a leg removed from the board. I will set my analogue meter to x1 ohms range 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 multimeter to the cathode and black probe to the anode, the diode is forward biased and the multimeter should read some value of resistance. If you have two readings then most likely the diode is shorted or leaky and you should replace the diode. If you don't get any reading either forward or reverse bias, the diode is considered open circuit and need to be replace.

The big problem when testing a diode using the diode test function of a digital tester is that an open or leaky diode, the tester sometimes reads ok (0.6). This is due to digital tester diode test output voltage (which you can measure the output test probe using another meter) is around 500milivolt to 2volt. An analogue meter set to x1 ohms range have output about 3Volt(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 range doesn't mean that the diode you are measuring is good . You now have to set your meter to x10K ohm range to check the diode again. The output voltage of x10k ohms is about 12Volt(remember the 9v battery in your meter-1.5v+1.5v+9v=12volt). 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 tester can't test it because the output from the meter is only 500mv to 2Volt. After reading this article you may test it on your own with the analogue and digital meter.

If a diode breakdown when under full load, there is no way to testing a diode (unless you have a very expensive diode tester which is specially designed to find this type of problem).Substituting with a known good diode is often the only way to prove that an intermittent diode is the cause of the 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 the meter or circuit damage could result. Conclusion-In order to accurately test a diode you need to set the analogue meter to x1 ohms and x10K ohms range. One more thing you need to remember is Schottky diode test under 10k ohms range should have two reading but not shorted reading. In other words, find from your semiconductor data book for the diode part number before performing any test using the analogue multimeter.