Digital multimeters today have a specific capability for testing diodes, for instance, but analog multimeters that are still prevalent do not. In spite of this, it is quite easy to perform a simple go/no-go test using the simplest of equipment.
Transistors and diodes can be tested for functionality using this technique, but there is no way to provide details about the parameters. This is not a problem because these components will have been tested at manufacture, and when they are used in a circuit they are very rarely inoperative.
The majority of failures result in the component becoming inoperable. This type of test is very simple and allows you to detect problems very quickly and easily.
Most types of diodes can be tested in this way, including diodes such as voltage reference diodes, power rectifier diodes, and diodes for signal transmission.
An Overview of How to Use a Multimeter to Test a Diode
Simple diode tests can be performed. A multimeter is all that is needed to test the diode’s performance.
The diode test determines whether diodes conduct only in one direction or both directions. This means that its resistance in one direction will be different from its resistance in the other direction.
Using the resistance in both directions, it is possible to determine whether a diode is working, as well as which connections are anode and cathode.
In light of the fact that the actual resistance in the forward direction depends on voltage, it is not possible to give exact values for the anticipated resistance as the voltage from different meters will vary – it will even differ between the different ranges on the meter.
Follow These Steps:
- Make sure the meter is on its ohms range – any range should work, but the middle one if there are several should work best.
- The cathode terminal of the diode should be connected to the positive terminal on the multimeter, and the anode terminal to the negative or common terminal.
- The meter should read ohms, and a “lowish” reading should be obtained.
- Replacing the connections in the opposite direction.
- This time, a high resistance value should be obtained.
Notes:
- In step 3, the actual reading will depend on several factors. The main thing is that the meter deflects, possibly to half or more of its original position. The variation depends on several factors including the battery in the meter and the range used. It is important to note that the meter significantly deflects.
- Silicon diodes are unlikely to show any meter deflection whatsoever when tested in the reverse direction. For germanium ones that have a much higher level of reverse leakage current, the meter may show a slight deflection if the range is set to high ohms.
The simple analogue multimeter test of a diode is very useful because it gives an instant indication of whether the diode is actually working. However, it does not allow for complex parameters such as reverse breakdown.
Nonetheless, it is an important maintenance and repair test. In spite of the change in diode characteristics, it is only very rarely the case that it will completely breakdown. This would be apparent immediately with the test.
Therefore, such a test can prove extremely useful for a wide range of electronic testing and repair applications.
Multimeter Test for Transistors
A simple and straightforward confidence check for bipolar transistors can be performed using a diode test using an analogue multimeter. Again, the test using a multimeter provides only a confidence check that the transistor has not blown, but it is still very useful.
Similarly to diodes, transistors are more likely to fail by destroying them than by degrading their performance a little.
An analogue multimeter can be used to perform the diode test between the transistor’s base and collector, and the transistor’s base and emitter to determine the basic electrical properties of the transistor.
There is one more test to be performed. As there are back-to-back diodes, there should be a large resistance between the collector and emitter of the transistor. Nevertheless, a conductor path may be formed between the collector and emitter as well as a diode function at the base if the collector emitter path is blown. Tests must also be conducted on this.
A bipolar transistor cannot be functionally duplicated using two separate diodes because its operation depends on the base, which is the junction of the two diodes, because it is one physical layer and very thin.
Step by Step Instructions:
Instructions are primarily focused on NPN transistors, since these are the most common types in use. There are variations in PNP varieties – these are indicated within brackets (.. .. ..):
- Make sure the meter is on its ohms range – any range should work, but the middle one if there are several should work best.
- Connect the transistor’s base terminal to the multimeter’s positive terminal (usually red)
- The negative or common terminal (usually a black connector) is connected to the collector, and the resistance is measured. Open circuit reading is required (a PNP transistor should show deflection).
- Repeat the measurement with the positive terminal connected to the emitter while the positive terminal is still connected to the base. Again, the reading should be open circuit (the multimeter should deflect with a PNP transistor).
- Reverse the connection to the base of the transistor, this time connecting the negative (black) terminal of the analogue test meter to the base.
- Connect the positive terminal first to the collector and measure the resistance. Then, put it in the emitter. It is indicated that a transistor of either type will open circuit by deflecting (in both cases).
- Next, connect the meter negative or common to the collector and the meter positive to the emitter. Verify that the meter indicates an open circuit. (The meter should display open circuit for both NPN and PNP types.
- Connect the negative and collector of the meter so that both are connected the emitter and collector, respectively. Check again that the meter indicates an open circuit.
- Transistors that pass all the tests are considered to be functional and all junctions are intact.
Notes:
- The last check ensures that the base has not been “blown through” from the collector to the emitter. Occasionally the collector and emitter may still be connected by a diode, but the collector and emitter may be shorted together.
- For germanium transistors, reverse readings will not be as good as they are for silicon transistors. There are some minor carriers present in germanium that allow a small current to flow.
A Look at Analogue Multimeters
The vast majority of multimeters on the market today are digital, but there are still many analogue meters in use. Even though they may not be the latest in technology, they are still ideal for many uses, including measurements such as the ones above. Although the tests detailed above are intended for analogue meters, similar tests can be performed with digital multimeters.
Many digital multimeters offer a special bipolar transistor test function, which is very convenient. A bipolar transistor test function usually operates in a similar way to that described here, although some digital multimeters will report the current gain as well.
A simple test for diodes and transistors is very useful in many service and repair scenarios. An excellent way to determine whether a diode or transistor is functional is to look at its characteristics. The fact that a multimeter can provide this capability in the absence of transistor testers is particularly useful, since these are rarely available. Moreover, the test is very simple to administer.
