Before connecting a diode to a circuit, it is recommended to test the diode working. A diode may be faulty and unable to work as expected. The diode has low forward resistance and high reverse resistance and it can be easily checked by using a digital multimeter or ohmmeter. While a defective diode may show an open circuit in both forward and reverse bias. Alternatively, a defective diode may show a short circuit in both forward and reverse bias.
Testing a diode with a digital multimeter
Nowadays, the digital multimeter has a diode testing option where you test the diode. First of all, move the knob to the small diode symbol for testing a diode. Then the diode anode to the red lead of the meter and the cathode to the black lead to make it forward bias. The meter provides voltage from the internal battery and shows the voltage drop. In the case of the forward bias, for a good diode, the typical voltage drop is between 2.5 to 3.5 volts. Other readings, in this case, indicate a faulty diode.
In the second part, the terminal of the diode is reversed such that the anode connects to black and the cathode is connected to the red lead. In this case, the diode offers high resistance, so the meter may show a maximum voltage drop for a good diode. The maximum voltage will depend upon the battery used in the multimeter, which may vary from manufacturer to manufacturer.
A diode will be said faulty if it shows maximum voltage drop (open circuit) on both the forward bias and the reverse bias. Alternatively, it may also be said defective, if a diode shows zero voltage drop (short circuit) on the forward bias and reverse bias.
Testing a diode with an ohmmeter
Some multi-meter may not have the diode testing portion, in that case, the diode can be tested by moving the knob to the ohm range. In the forward bias check, the diode shows a resistance value that depends upon the voltage of the internal battery. Typically, the internal voltage of DMM is not able to forward bias the diode completely, so, resistance may read in a few hundred for a good diode. For reverse bias, the meter reads maximum resistance or out of range for the good diode. You may not get exact readings on the ohmmeter but rather an indication of the functionality of the diode.
Diode curve tracing
The VI characteristics can be plotted using an oscilloscope and function generator. For curve tracing, use the function generator to a sinusoidal wave of 4Vpp and measure the current and voltage across the diode. To plot the current graph, a current measuring probe will be required. The oscilloscope usually shows the plot of Voltage vs time and current vs time. To change the plot to voltage vs current (VI characteristics), X-Y mode function. The process of curve tracing is explained in the video link below.
Conclusion
- A diode will exhibit high resistance and voltage drop in reverse bias
- In forward bias, a good diode offers low resistance and low voltage drop