How to Test a Diode with a Fluke Multimeter

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You are an electrician, a technician or an engineer and the diode in a circuit board gets bad. How do you test this diode to confirm it is bad?

Well if you have a multimeter, it is a very simple exercise. Let’s find out how to do this. The diode test can be carried out in either forward or reverse bias.

Forward Bias Test

Good Diode

To begin the test in forward bias, switch the knob on the multimeter to the “DIODE” selection and test as shown below:

What is a Diode?

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The diode is fabricated of a semiconductor material, usually silicon, which is doped with two impurities. One side is doped with a donor or n-type impurity which releases electrons into the semiconductor lattice. These electrons are not bound and are free to move about. Because there is no net charge in the donor impurity, the n-type semiconductor is electrically neutral. The other side is doped with an acceptor or p-type impurity which imparts free holes into the lattice. A hole is the absence of an electron which acts as a positive charge. The p-type semiconductor is also electrically neutral because the acceptor material adds no net charge. When a P-type semiconductor material is combined with an n-type semiconductor material, a p-n junction is formed. This p-n junction is called a diode.

Thus the diode has two terminals or electrodes (di-ode),that act like an on-off switch. When the diode is “on”, it acts as a short circuit and passes all current. When it is “off”, it behaves like an open circuit and passes no current. The two terminals are different and are marked as plus(+) and minus(-) in the schematic below: