3 Methods of testing a diode with multimeter and oscilloscope

Before connecting a diode into a circuit, it is recommended to test the diode working. A diode may be faulty and unable to work as it expected. As the diode has low forward resistance and high reverse resistance and it can be easily checked by using digital multimeter or ohmmeter. While a defective diode may show open circuit in both forward and reverse bias. Alternatively, a defective diode may show short circuit in both forward and reverse bias.

Testing diode with 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 connected the diode anode to the red lead of meter and cathode to the black lead to make it forward bias. The meter provides voltage from the internal battery and shows the voltage drop. In case of forward bias, for 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 anode connects to black and cathode connected to the red lead. In this case, the diode offers high resistance, so the meter may show 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 forward bias and reverse bias. Alternatively, it may also be said defective, if a diode shows zero voltage drop (short circuit) on forward bias and reverse bias.

Symbol of diode

Testing diode with 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 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 few hundreds for a good diode. For reverse bias, the meter read maximum resistance or out of range for the good diode. You may not get exact readings on ohmmeter but rather an indication of the functionality of the diode.

Diode curve tracing

The VI characteristics can be plotted using 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. 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

  • Diode will exhibit high resistance and voltage drop in reverse bias
  • In forward bias, a good diode offer low resistance and low voltage drop
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