Test The Diode

Sunday, January 7th, 2018 - Troubleshooting

To test the diode, we must understand the working principles of the diode. When a diode is forward- biased, the negative charge of the battery repels the negative charges in the diode, forcing them into the positive side of the diode. The positive force of the battery pulls these electrons on through the diode.

Principle Of The Diode

When the battery is reversed, the diode is reverse-biased, because the positive force of the battery and the positive holes of the diode attract each other, allowing no current flow to take place. A reverse-biased diode is shown in the following picture.

Principle Of The Diode

The p (positive) side of a diode is called the anode, and the n (negative) side is called the cathode. It is important for the troubleshooter to know which side is which. The arrow indicates the p side. The line indicates the n side. A line or dot placed on the diode by the manufacturer indicates the cathode side. Notice that the manufacturer marks the cathode side (negative) of the diode with a plus sign or band. When the end of the diode with the band is connected to the positive side of the power source, it will be reverse-biased.

Test The Diode With Multimeter

To test a diode, the troubleshooter can use either an ohmmeter DVM or a diode or transistor checker. When checking a diode with an ohmmeter, you can use the low-/high-resistance reading technique. You place the selector switch on R × 100 and you place the leads across the diode. In the forward-biased direction, the ohmmeter should read less than 100 Ω (i.e., low-resistance reading). In the reverse-biased direction, the ohmmeter should read about 5000 Ω (i.e., high-resistance reading). These low/high readings indicate that the diode is probably good. If you obtain a low/low or a high/high resistance reading, the diode is probably defective. The following picture shows the correct testing of a diode with an ohmmeter.

Methods For Test The Diode With Ohmmeter

Methods For Test The Diode With Ohmmeter

Most types of diodes can be checked by using an ohmmeter. Keep in mind that the actual resistance value of the diode is not very important as long as you get a low/high reading when the ohmmeter polarity is switched. If any doubt exists after the ohmmeter check, it is recommended that a new diode be substituted. Also, keep in mind that a low resistance reading both ways is common when the diode is tested in the circuit. To be sure that the diode is good, unsolder one lead and check the diode again with the ohmmeter. Keep in mind, when you replace a diode, that a diode will only block a certain amount of voltage in the reverse direction. This is called the piv rating, where piv is the acronym for peak inverse (reverse) voltage. Never exceed this voltage rating, or else the diode will be destroyed.

Although there are numerous types of diodes—zener, light-emitting, photoconductive, varactor, and tunnel—each has its own unique characteristics. When you are in doubt about the diode’s quality, the best testing method to use is substitution. For example, zener diodes are special diodes that can operate in reverse bias. In reverse bias the zener diode does not conduct until the voltage threshold, or breakdown voltage, is reached, and then it conducts at a relatively constant voltage. This operation allows the zener diode to act as a voltage regulator that can be used for regulated voltage power supplies.

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