Troubleshooting a circuit requires sound knowledge of electronics and the way circuit works. You might encounter many circuits which do not work and you have to find the possible cause. Systematic step-by-step procedure needs to be followed while troubleshooting the circuit. If the circuit involves the usage of diode, then you might need to make sure whether the diode is properly functioning or not. In this section we shall see how we can test a diode.
We can check the diode using digital multimeter (DMM). Two modes of DMM which can be used to check the diode are
1. Diode checking mode
2. Ohmmeter mode
Diode checking mode
The image on the right shows a typical digital multimeter. There is a built-in functionality by which we can test a diode. The mode is called diode checking mode. Diode checking mode can be selected by rotating the central knob in the direction of diode symbol.
Before we see how to test a diode, let us briefly understand what goes on inside the multimeter when the diode testing mode is selected. When diode testing mode is selected, the digital multimeter supplies a constant current of approx 2 mA between its test leads. The direction of current is from red lead to black lead. The two test leads are made to be in contact with the two terminals of the diode. Hence current will flow through the diode and there occurs some voltage drop across the diode. The multimeter displays the voltage drop across the diode. We can decide whether the diode is working or not based on the readings displayed by DMM.
When the diode is properly working : Let us assume that the diode under test is properly functioning. If we connect the red (positive) lead with the anode and black (negative) lead with the cathode as shown in the figure below, can u guess the output shown by multimeter?
When the red lead is connected to anode and the black lead is connected to cathode, the internal constant current source from the multimeter forward biases the diode and current starts to flow through the diode. The voltage drop across the forward biased diode is typically 0.7 V for Si diodes. If the diode is working perfectly, the multimeter will show voltage between 0.5 V and 0.9 V. Typical value is around 0.7 V. This is indicated in the above figure.
Now connect the red lead (positive) with cathode and black lead (negative) with anode as shown in the figure below.
Such a connection reverse biases the diode. We know that no current flows through the diode when reverse bias is applied. Hence if the diode is working properly, the multimeter will show OL indicating the equivalence of open circuit. Some multimeter may show the battery voltage of the battery inside the DMM. This indicates that all the voltage of the battery drops across the diode due to its very high resistance.
When the diode is defective : When the diode is defective, there are two possibilities. Defective diodes can be either of the following.
- Defective open diode- The diode behaves as open in both forward and reverse bias.
- Defective short diode- The diode behaves as short in both forward and reverse bias.
Consider the situation when the diode has failed open. In such a condition, the multimeter will show OL in both forward and reverse bias. This is due to the fact that the current flowing through the diode is zero and it is an open circuit equivalent.
When the diode is shorted,the meter will indicate 0 V as there will be no voltage drop across the diode. This is indicated in the figure below. Sometimes the diode behaves as resistive rather than a pure short. In such a case, the multimeter shows some nonzero voltage indicating that there is some voltage drop across the diode.
The resistance of diode in forward bias configuration is very less and the resistance of diode in reverse bias configuration is very high. This makes it possible to test the diode using ohmmeter mode of digital multimeter. Let us see what results are to be expected when the diode is working perfectly. If we connect the red lead with the anode and black lead with the cathode of diode, then relatively low resistance will be displayed by the diode. Typical value is from few hundred ohms to few thousand ohms. The static resistance of diode depends on the value of current and voltage at a point on the V-I curve. Hence the resistance displayed by the multimeter is a function of current supplied by the multimter or the battery voltage.On the other hand, if we connect the red lead with the cathode and black lead with the anode, then relatively high resistance is will be displayed by the multimeter. Typical value is around few mega-ohms.
When the diode has failed open, then multimeter will dispaly OL indicating open circuit equivalent. On the other hand, if the diode is shorted, the multimeter will dispaly very low value of resistance.