What is Diode Test : Working & Its Applications

For beginners, they must have knowledge of electronic components & their working to troubleshoot any circuit or project. Similarly, having good knowledge of how to test different components is also one of the troubleshooting skills. To avoid undesired results, it is advisable for beginners to test all the basic electronic components like resistors, capacitors, diodes, and transistors before connecting to the printed circuit board. If we do not perform any tests on components before connecting them to the printed circuit board, then it is very difficult to recognize the source of the issue to check all the components. So this article discusses an overview of how to test a diode or diode test and its working.

What is Diode Testing?

Diode testing definition is that it is an important property of the diode which can be used to test the diode to know whether the diode is working properly or not. The diode testing can be done by using a digital multimeter to measure the resistance across the diode terminals.

Diode Tester Circuit Diagram

The simple diode tester circuit using IC741 is shown below. This circuit can be built with different electrical and electronic components like 741 Op-Amp IC1, R2 & R3 resistors – 1M, R1 is 6.8K, R4 is 470 Ohms, C1 capacitor – 100 uF, D2 red color LED -1, D3 green color LED, two 9V B1 & B2 batteries and 1switch.

The main function of this diode tester circuit is to check whether the diode is working in good condition or not & also it decides its polarity. So this can be possible by using a 741 op-amp as an oscillator.
Here in the circuit, the oscillator uses three resistors like R1, R2 & R3, and capacitor C1. The output of the oscillator is a square wave which makes the D2 & D3 LEDs activate and deactivate by showing the condition of the diode under test.

Diode Tester Circuit Working

The diode which needs to be tested in the circuit can be arranged in between two points like A & B which is shown below. This circuit can be worked through dual 9V batteries and it works in two cases which are discussed below.

In the first case, the anode terminal of the diode is connected to point A, and the cathode terminal is connected to point B as shown in the above figure. Once the output of the oscillator is at a high voltage range then the diode is forward biased, so the D3 LED will be turned ON. Similarly, when the output of the oscillator is low then the diode is reverse biased as a result, no LED will be turned ON.

In the second case, the cathode terminal of the diode is connected to point A & anode terminal is connected to point B. Once the output of the oscillator is high then the diode will be reverse biased, so LED will not turn ON. Similarly, if the output of the oscillator is at less voltage level then the diode will be forward biased so the D2 LED will be turned ON.

If the diode test meets the above two conditions then the diode is said to be in good condition. So we can determine its polarity otherwise it is damaged. If the diode in the test is turned ON then it is shorted and if it is not turned ON then it is open. The R4 resistor in the circuit is connected in series with LEDs to restrict the current flow.

How to Test a Diode?

The diode is a semiconductor device including two terminals. The main function of the diode is to allow the current flow only in one direction and it is used in different applications like clippers and clampers, rectifiers, etc. The diode testing is mainly done to identify its working conditions in both biasing modes. Before diode testing, we need to identify its terminals first to know the anode and cathode.

Most of the diode’s body contains the silver band or other color bands which are known as cathode & the remaining terminal is the usual anode. So, performing the basic diode testing is extremely simple and can be done in different ways by using an analog multimeter, digital multimeter, and ohmmeter.

Diode Testing using Analog Multimeter

The diode testing using an analog multimeter can be done like the following. In this meter, we use resistance mode to test the diode because it doesn’t include a dedicated diode test mode so it is related to the diode testing with DMM ohmmeter mode.

• Before testing a diode, we need to keep the selector switch of the analog multimeter in the resistance mode.
• Analog multimeter includes two probes black and red where a black color probe is a cathode and a red color probe is an anode.
• After that connect the two probes of the meter to the positive & negative terminals of the diode.
  If the analog multimeter shows a low-resistance value, then the diode is said to be healthy.
• Now the selector of the meter needs to be placed in high resistance mode & reverse the diode terminals and connect them with the two probes of the multimeter in reverse bias.
• If the analog multimeter shows a very high resistance or OL on the display then the diode is said to be defective. So this is the procedure to test a diode using an analog multimeter.

Diode Testing using Digital Multimeter

The testing of a diode with a DMM or Digital Multimeter can be done by using two methods like diode mode and ohmmeter mode or resistance mode.

Diode Mode Testing

This type of testing is the best way to check a diode because it depends on the diode’s characteristics. In this testing, the diode is connected in forward biased to measure the voltage drop across the diode with a digital multimeter.

• First, the center knob of the digital multimeter is set to diode checking mode by turning to that position where the symbol of the diode is indicated.
• In this diode mode, the digital multimeter supplies 2mA of current in between the two leads.
  Connect the multimeter’s red color probe to the anode terminal of the diode and the black color probe is connected to the cathode terminal of the diode. So the diode is said to be forward-biased.
• Check the readings on the display of the digital multimeter. If the voltage value ranges between 0.6 – and 0.7 then the diode is perfect. For Si diode, this value will be 0.7V whereas, for Germanium diode, this value ranges between 0.25 – 0.3V.
• Now the diode terminals need to reverse and connect with the multimeter in reverse bias which means the red color probe is connected to the cathode and the black color probe is connected to an anode of the diode. In this condition, there is no flow of current throughout it. So the digital multimeter must display OL or 1if the diode is good.
• If the digital multimeter displays no relevant values, then this diode is said to be defective. So the defect within the diode is either short or open.
• If the diode is open, it means the diode behaves like an open switch within both biasing conditions. So there is no flow of current in any biasing conditions. So, the digital meter will specify OL or 1 in both biased conditions.
• If the diode is shorted, it means the diode behaves like a closed switch within both biasing conditions. So there is a flow of current in any biasing conditions. So, the voltage drop across the diode will be displayed on the display between 0V – 0.4V. Thus, the digital multimeter will show a 0V value, however in some cases, it will be displayed with very little voltage (0.4V).

Resistance Mode Testing or Diode testing using Ohmmeter

In this mode of testing, the diode’s resistance in both the forward & reverse bias can be measured. For a perfect diode, the resistance in forward bias must range from a few 100 Ohms to a few 1000 ohms whereas, in the reverse bias, the resistance must be very high as an open-loop within a multimeter.

To test the diode in this mode, first, the central knob of the meter must be placed where the resistor values or ohm symbol are specified on the meter. For forward bias, we have to keep the knob in low resistance (1K ohm) mode whereas, for reverse bias, we have to keep the knob in high resistance mode (100K ohm).

After that, connect the red color probe to the anode and the black color probe to the cathode terminal of the diode, then this is called the diode is forward-biased. Once this diode is connected in forward-biased, then the diode’s resistance will be very small like a few tens of ohms which is displayed on the display meter. So the diode is said to be not good.

Similarly, if the resistance reading of this diode is high like a few hundred ohms then the diode is said to be good & properly working.

Now we need to reverse the two terminals of the diode and connect the red color probe of the meter to the cathode and the black color probe of the meter to the anode, then the diode is said to be reverse biased.

Now change the two probes of the meter as a red color probe is a cathode and a black color probe is an anode such that the red probe is connected to the cathode and a black probe is to the anode. So the diode is reverse biased.

If the digital meter displays OL or a high resistance value on the display of the meter, then the diode will work properly. Since the diode in reverse biased condition gives a very high resistance.

From the above information, we can conclude that for proper diode working the digital multimeter must-read low resistance within the forward-biased condition whereas, in the reverse bias condition, it must read OL or high resistance.

In both biased conditions, if the digital meter displays OL or very high resistance, then this diode is known to be opened. Similarly, in both biased conditions, if the DMM displays a very low resistance then the diode is shorted.

Diode Testing Applications

The applications of a diode test include the following.

• The diode test using a digital multimeter or DMM can be performed to know whether the diode is working properly or not.
• A diode test is used to measure the voltage drop across the diode once it is connected forward-biased. So the diode in forward bias performs as a closed switch by allowing the flow of current.
• The diode test is applicable in different electrical and electronic projects to check the proper working of the components.

Diode Test Advantages & Disadvantages

The advantages of the diode test include the following.

• The advantage of a diode test using a multimeter is a very easy & speed method.
• Multimeters have high accuracy, high input impedance, no loading effect, clear reading, and less cost due to the incorporated technology.
• The properly working diode has one property it provides a large resistance value in reverse biasing & provides less resistance in forward biasing.
• Diodes can be used as a rectifier for converting AC to DC.
• These components defend electronic components which are in danger to reverse voltages.
• Diodes operation is fast & it doesn’t need any warm up time.
• Diodes generate less noise.

The disadvantages of the diode test include the following.

• A defective diode will have a high resistance value in forward & reverse bias conditions.
• The diode has the most common fault such as a failure of a diode.
• The diode is very responsive to temperature.
• A Digital multimeter is expensive as compared to an analog multimeter.
• The LCD display of the meter mainly depends on an external power source or a battery.
• These components are very sensitive to temperature.
• Diodes have impedance & high reverse current.

How Do You Check a Diode?

The diode can be checked by using a multimeter by connecting the meter’s red color probe to the anode terminal of the diode and the black probe is to the cathode of the diode. So the diode is said to be forward-biased then check the readings on the display of the multimeter. If the resultant voltage on the display is between 0.6 to 0.7V then the diode is said to be perfect & healthy.

What are the Two Faults of Diode?

The two faults of a diode are large reverse voltage and excessive forward current.

Why is Heat produced in a Diode?

Diode efficiently generates heat as resistance is generated within them. So this is effective in changing present energy within the atmosphere into heat energy.

What is the Current through the Diode?

The flow of current throughout a diode is only in one direction, so this is called forward bias.

What is Voltage Breakdown?

It is a diode parameter that defines the highest reverse voltage that can be given without causing an exponential rise in the leakage current of the diode.

What is a Digital Multimeter used for?

A Digital multimeter is one kind of measuring instrument used to measure different quantities like resistance, voltage, and current. Once these values are measured, they will be shown on the display of the multimeter to read easily by the user.

Thus, this is all about an overview of a diode test using multimeters. This diode test is one of the foundations of electronics to measure the resistance of a diode across its terminals and also it checks whether a diode is working properly or not. When the diode is connected in forward bias then it has fewer resistance values whereas it is connected in reverse biased then it has a high resistance value. Here is a question for you, what are the applications of a multimeter?