This article covers diode testing and troubleshooting of diodes, emphasizing the importance of correct polarity and biasing in their functionality. It explains how to perform diode testing using an ohmmeter, including recognizing signs of shorted or open diodes, and highlights the role of voltage supply in accurate testing.
Troubleshooting Diodes
Diode operation is determined by its connection in a circuit. Current conduction is directly dependent on the polarity of the device. Heavy conduction occurs when it is forward biased, and very little conduction occurs when it is reverse biased. Connecting a diode backward in a circuit reverses its conduction. This can destroy the diode and possibly damage several other circuit parts. A person working with diodes must be absolutely certain that a diode is connected properly in a circuit. They must also be able to test the condition of a diode.
Diode Schematic Symbol
A schematic symbol of a diode is commonly used to identify its polarity within a circuit. Figure 1 shows a diode symbol, element names, and the polarity of the crystal material. The term anode is commonly used to describe the electrode that attracts or gathers in electrons. The P-type material of the crystal serves as the anode in a diode. The term cathode is used to denote the electrode that gives off or emits electrons. The N-type material of the crystal serves as the cathode. Note that forward current passes through a diode from the cathode to the anode. A diode symbol indicates this movement from the bar to the point of the arrow.
Figure 1. Diode Symbol and Current Flow Direction
Because diodes are available in many different package types and styles, there is often some confusion about lead identification. A person working with diodes is frequently called on to identify leads. The ohmmeter function of a multimeter is used to perform this operation.
The polarity of the ohmmeter voltage source must be noted for this test to work. Ohmmeters typically have a black, or common lead, as negative and the red lead is positive. A diode connected to the ohmmeter will show a relatively low resistance when forward biased and infinite resistance when reverse biased. Forward biasing occurs when the polarity of the ohmmeter matches the crystal polarity of the diode. The red lead is, therefore, connected to the anode and the black lead to the cathode. Lead identification is simply a matter of determining the forward-bias direction of a diode and noting the polarity of the ohmmeter leads.
Diode Testing
An ohmmeter can be used to test a diode as shown in Figure 2 and 3. In preparation for this test, the meter should be placed in the $R \times 100$ or $R \times 1-K$ Ohm range. The meter leads are then connected to the diode. Exact resistance readings are not particularly important. Only high-resistance or low-resistance indications are used. Figure 2 shows how an ohmmeter is connected to forward bias a diode. This test normally causes the meter to show a relatively low resistance. Reverse biasing of a diode is shown in Figure 3. An infinite resistance is indicated by the ohmmeter during this test. A good diode normally shows low resistance when forward biased and high resistance when reverse biased. If a diode does not respond in this manner, it is probably defective. In many digital meters, a diode symbol may appear on the ohmmeter’s switching range. This symbol indicates that the ohmmeter will supply the necessary voltage needed to cause the diode to respond when forward biased. Other ranges may not provide the voltage needed to produce conduction when forward biased. When a digital meter is used, look for the diode symbol and place the function switch in this position when evaluating a diode.
Figure 2. Testing of Diode in Forward Direction
Figure 3. Testing of Diode in Reverse Direction
A diode that shows low resistance in both directions is considered to be shorted. Shorting usually occurs when the maximum current or voltage ratings of the device are exceeded. Shorted diodes often cause damage to other circuit components. Blown fuses, tripped circuit breakers, or overheated resistors are generally good indicators of a shorted diode. It is a good practice to test other circuit components before replacing a shorted diode.
When a diode shows infinite resistance in both directions, it is considered to be open. Open diodes generally cause a circuit to be nonconductive. This condition rarely causes damage to other circuit parts. Operation simply stops, and there is no current flow through the diode. Open diodes do not occur very often in electronic circuits. A quick ohmmeter check will readily detect this type of fault should it occur.
The value of an ohmmeter’s voltage supply is an important consideration when testing diodes. If the supply voltage is less than 0.2 V, it will not be large enough to forward bias all diodes. This could cause a good diode to appear to be open. The ohmmeter would, therefore, show an infinite resistance in both directions. You may recall that it takes at least 0.3 V to cause conduction in germanium diodes and 0.7 V in silicon. Many electronic multimeters, especially digital meters, respond in this way. The voltage supply of this type of instrument must be evaluated to see if it can be used to test a diode.
The supply voltage of some ohmmeters may also be extremely large. This type of meter could actually develop enough current or voltage to damage certain diodes. As a rule, high-frequency detection diodes are very susceptible to being damaged in this way. To avoid this kind of problem, do not attempt to test high-frequency diodes with high-voltage ohmmeters.
Review Questions
- When a diode is tested with an ohmmeter, conduction occurs when it is forward biased.
- A good diode normally shows resistance when forward biased and resistance when reverse biased.
- A diode that shows low resistance in either direction of biasing is .
- A diode that shows infinite resistance in either direction of biasing is considered to be .
- The of the voltage source of an ohmmeter must be known to properly identify the leads of a diode.
Answers
- heavy
- low, infinite
- shorted
- open
- polarity
Diode Testing and Troubleshooting Key Takeaways
Proper diode testing and troubleshooting are essential for ensuring the reliable operation of electronic circuits. Understanding the importance of correct biasing, the function of diodes in both forward and reverse bias, and the use of ohmmeters for testing helps in diagnosing faults such as shorted or open diodes.