How to Test a Diode

To test a diode,  a multimeter can be used as fast and simple way to test a diode.  A good diode will show an extremely high resistance  ( ideally an open ) with reverse bias and a very low resistance with forward bias.  A defective open diode will show an extremely high resistance ( or open) for both forward and reverse bias.  A defective shorted or resistive diode will show zero or a low resistance for both forward and reverse bias.  An open diode is the most common type of failure.

  • Note:  The DMM Diode Test Position:
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DMM (Click to Full Screen)

 Many DMMs (Digital Multimeters) have a diode testing position that provides a convenient way to test a diode.  A typical DMM, as shown here has a small diode symbol to mark the position of the funtion swith.  When set to diode test, the meter provides an internal voltage sufficient to forward bias and reverse bias a diode.  This internal voltage may vary among different makes of Digital Multimeter (DMM, but 2.5 V to 3.5 V is a typical range of values.  The meter provides a voltage reading or other indication to show the condition of the diode under test. ( Note Ends )

Procedures of Diode Testing

Indications when diode is working while testing:

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Forward Bias Diode Test

 In first testing procedure of diode, we connect positive lead of Digital Multimeter with Anode of diode and negative lead to cathode of diode. If the diode is good, we will get a reading of between approximately 0.5 V and 0.9 V, with 0.7 V being typical for forward bias. This was forward bias result, now we see what happens when diode is in Reverse Bias?See below…

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Reverse Bias Diode Test

Here diode is connected in reverse bias, positive lead is connected to cathode and negative lead is connected with anode of diode so diode becomes reverse biased. So, if the diode is working properly , we will get a voltage reading based on the meter’s internal voltage source.  The 2.6 V shown in the figure represents a typical value and indicates that the diode has an extremely high reverse resistance with essentially all of the internal voltage appearing across it.

Indications when Diode is Defective:

When a diode has failed open, we get an open circuit voltage reading (2.6 V is typical) or “OL” indication for both the forward bias and the reverse bias condition.  If a giode is shorted, the meter reads “0″ V in both forward and reverse bias test.  Sometimes, a failed diode may exhibit a small resistance for both bias conditions rather than a pure short.  In this case, the meter will show a small value of voltage much less than the correct open voltage.  For example, a resistive diode may result in a reading of 1.1 V in both directions rather than the correct readings of 0.7 V for forward bias and 2.6 V for reverse bias.

Most Common method is Ohms Function which is adopted while testing / checking a diode for its serviceability. So, lets know about this simple Method.

Checking a Diode with the Ohms Function:

DMM (Digital Multimeter)  that do not have a diode test position can be used to check a diode by setting the function switch on an Ohms range.  For a forward bias check of a good diode, we will get a resistance reading that can vary depending on the meter’s internal battery.  Many meters do not have sufficient voltage on the Ohms setting to fully forward bias a diode and we may get a reading of from several hundred to several thousand ohms.  For the reverse bias check of a good diode, we will get some type of out of range indication such as “OL” on most DMMs because the reverse resistance is too high for the meter to measure.

Even though we may not get accurate forward and reverse resistance readings on a DMM, the relative reading indicate that a diode is functioning properly , and that is usually all we need to know.  The out of range indication shows that the reverse resistance is extremely high, as we expect.  The reading of a few hundred to a few thousand ohms for forward bias is relatively small compared to the reverse resistance.  Indicating that the diode is working properly.  The actual resistance of forward biased diode is typically much less than 100Ω

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