Diode is a two terminal semiconductor device. One terminal is called the anode while the other terminal is cathode. Anode needs a high potential while cathode needs a lower potential level for the diode to be forward biased. The term forward biased means the diode terminals are supplied with the right voltage level. When the diode is not supplied by a right voltage, it can either be stay off state or reverse biased. Having basic knowledge on how a diode operates will help easily on how to know a defective diode.
Some Important Facts About Diode
1. Diode is a two terminal electronic device
2. One terminal is called anode and the other is called cathode
3. The diode symbol is
4. For most diodes, the indicator of cathode is that it has a line band. See below example.
5. For diodes with big body, the cathode is indicated as (-) sign while (+) for the anode
6. There are two ways a diode can be biased; first is forward biased and second is reversed biased
7. Forward biased means the anode has higher potential level than the cathode
8. Reversed biased means the anode applied potential is less than the cathode
9. Diode material is can be germanium or silicon. Germanium made diode has around 0.3V forward voltage. On the other hand, a silicon made diode will have a forward voltage of around 0.7V.
10. When forward biased, the diode will be at on state and have a voltage drop of around 0.3V and 0.7V for germanium and silicon semiconductor materials respectively.
11. When reversed biased, the diode will be at off state and the voltage drop across it will not see a level of 0.3V or 0.7V but equal to the circuit supply.
12. At forward biased, the diode will allow current flow
13. At reverse biased, the diode will block current flow
14. In circuit ideal analysis, it is easy to consider a diode operating at forward biased as short while open for a diode operating at the reverse biased.
Troubleshooting – How to Know a Defective Diode
Above facts about diode are very helpful on how to know a defective diode. There are few ways to know if a diode defective. It could be using the diode test feature of a DMM. It can be through actual voltage drop measurement when the circuit is live. Or it could be through visual inspection.
Method 1: Diode Check using DMM
Note: Do this test with circuit unpowered.
a. Get a DMM. In this example I have a FLUKE 175.
b. Rotate the knob to the diode symbol in yellow color
c. Press the yellow button enclosed in red rectangle
d. After doing so, you must see a diode symbol in the display as the blue rectangle indicated and a V DC indicator in the right portion of the display enclosed in green. In addition, when you short the DMM probes, you can hear a beeping sound on most DMM.
e. Now, put the DMM positive probe to the anode of the diode while the negative probe to the cathode of the diode. This is a forward bias connection.
f. You must see a voltage reading of near 0.3V or near 0.7V. In my example below, I am measuring the forward voltage drop of a power diode and I got a reading of 0.311V. This is within the forward voltage level based on the diode datasheet. So, if you can measure here a value that is very far from 0.3V or 0.7V, the diode is defective.
g. Exchange the positive and the negative DMM probe. This time we must see no reading on the DMM as my measurement below. Otherwise, the diode must be defective.
h. There is a diode module that has 2 individual diodes inside. Careful on the identification of the correct terminal. Tapping to wrong terminal will give erroneous judgement. Supposing a diode module like below. When putting the positive probe of the DMM to terminal 2 while the negative probe to terminal 1, the reading would be near 0.3V or 0.7V as the diode is forward biased. Otherwise, the diode module must be defective. Exchanging the DMM probe, there will be no reading. Otherwise, the diode must be defective.
The same observations if the positive DMM probe will be tapped to terminal 1 while the negative probe to the terminal 3.
However, when the DMM positive terminal probe is tapped to terminal 2 while the negative is tapped to terminal 3, the reading should be equal to the sum of each diode forward voltage. For instance, a germanium made diode, the reading would be 2 times 0.3V or around 0.6V. For a silicon made diode, the reading would be 2 times 0.7V or around 1.4V. Otherwise, the diode must be defective.
Method 2: Actual Voltage Measurement while the Circuit is Powered
The second method is to measure the diode voltage information when the circuit is powered. Supposing a very simple circuit below. Power up the circuit with a 10V source and measure the voltage across the diode following below connection. It must read near 0.3V or 0.7V depending on the semiconductor material. If the reading is different, the diode must be defective.
Exchanging the probe connection should read a negative voltage but should still around 0.3V or 0.7V. This is still correct as the negative sign just an indicator only that the DMM probe has not been in correct polarity.
Method 3: Visual Inspection
Sometimes how to know a defective diode will only need a visual inspection. Diodes will burn if too much current is flowing on it. It will break if too much peak reverse voltage applied to it. These are obvious failures that do not need any analysis and tools to detect.