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How To Test Rectifier Diode

Diodes are one of the commonly used components in electronic devices. Thus, for ensuring that the diode is apt for the particular (as per requirement) use, to test a diode is important. We can test ordinary diodes and Zener diodes using the digital or analog multimeter.

As diodes are used in circuits for protection, rectifications, etc thus it is the one which gets damaged first in case of any fault in the system. Few example circuits can be a full-wave rectifier, half-wave rectifier, LED driver circuit. This reason gives even stronger reason to check a diode always before its use. Also, we have two modes of diode namely, forward conducting mode and reverse blocking mode. Thus both of these need to be tested separately.

How to test a diode

It can be tested using a multimeter. In a practical diode, we have resistance in both forward and reverse directions. It is always better to check a circuit before assembling it. But if we don’t do so and results are also not as per our expectations then we may get confused that whether there is an issue in the circuit or the components (diode, other electronic devices) are not performing desirably.

A diode is best tested when it is forward biased. The voltage drop due to its forward resistance is calculated. In forward-biased condition, the diode acts as a switch (if resistance is ignored). Let us now learn how to test the diodes.

Testing of Diodes

With Digital Meters

Nowadays most digital multimeters are provided with special ‘diode test’ range. It is done to ensure perfect measurement as other voltages may not overcome the diodes forward junction potential (and thus no conduction in the forward direction). 

But one question that arises here is that what if we don’t have a diode test range in a digital multimeter!

Well, we have another method that can help test the health of the diode. We could set the multimeter to the resistance mode (ohmmeter method) and then proceed. 

Let us understand the procedure of conducting the health check-up test for diodes by both the ways.

With Diode Test Range in Multimeter 

The following procedure is used to test the diode:

The next step is how to analyze the data and decide whether the diode is ready to be a part of the circuit or not. We check if it is good or bad!

Diode testing

Analysis of Diode Test Conducted 

Testing Diode in Resistance Mode

Let us see, how to decide if the diode is good, open (OL), or short. Follow the below steps for conducting the test.

VAnode > VCathode – forward bias

VAnode < VCathode – reverse bias

in forward mode, resistance ranges between 1K to 10M 

& in reverse mode, the digital meter shows OL

both have the same or nearby values. If the readings are contrary to the above conditions then also it’s bad.

This resistance method of testing can be made more effective if readings are compared with an already tested good diode.

Let us now learn about the testing of some particular diodes.

Zener Diode Test

Zener diode is the one that conducts in reverse bias also (if the reverse voltage is greater than the Zener breakdown voltage). This demands for some modifications in the previous testing circuit. Following is the procedure to test a Zener diode:

Zener diode test

Procedure to Test a Diode

For instance, if the breakdown voltage is 3V and you are giving a 10V supply then also the meter will read a value around 3V only.

LED (Light Emitting Diode) Test

This light-emitting diode is somewhat different than the one we have studied so far (in terms of physical appearance). Therefore, to decide its anode and cathode terminals we need to see its length. The longer leg (lead) is the anode and the shorter one is called the cathode. One another way of checking the terminals is by seeing the surface of the LED. The side with a flatter surface is the cathode and the other side is but obvious anode.

LED testing

Procedure to Test a Diode

Now, tell me can we check the LED in reverse bias? Think!!

Of course not. Simply because LED does not work in reverse bias.

Schottky Diode Test

Similar to other common diodes, it also restricts the current flow to one direction. But it has a quicker response time when compared with others of the same family of diodes. 

Schottky diode testing

Procedure to Test a Schottky Diode

Testing of Small Signal Diodes

Small Signal Diodes

Signal diodes are the one which works with lower power and higher frequency. This makes them more useful for switching purposes. The testing of these small signal diodes is quite similar to that of the methods discussed above. The only difference being the lesser value at the digital multimeter whenever the input is given. Also, the range of input that can be given to these diodes is lesser when compared to the large signal diodes. 

Testing of Large Signal Diodes

Large signal diodes are the one which has comparatively more power and somewhat lesser frequency when compared with the small signal diodes. Therefore, while doing the testing of the diode the voltage range is higher, and also the input that can be given at the input terminals has a wider range.

Procedure to Test a Small/Large Diode

Let us now learn to test a diode using an analog meter.

How to Test a Diode Using an Analog Multimeter

One important point to note here is that the zero on voltage and resistance scales are reversed in an analog meter. Therefore, we need to reverse the probes of the meter. Like to check the diode in the forward bias we need to connect the red probe to the cathode and the black one to the anode. Similarly, we can reverse the probes to get the reverse bias. This is the main difference while testing a diode with a digital meter and an analog meter. 

Testing with analog meters

The manufacturer gives the analog range of the meter so that it can be used or already proven good diode values can be taken as a reference. One more important point to note is that some meters use resistance and some use junction voltage. So you must have an eye on that as well before starting your test.

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