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Oct 30 2020

How to Test a Diode in Circuit with a Multimeter?

Introduction

Semiconductor diode, also known as crystal diode, has obvious unidirectional conductivity. It is a kind of electronic components widely used in electrical equipment for protection, rectification, switching, and many other applications. So it is pretty common to see diodes in daily electronic circuits, such as Zener diodes, light-emitting diodes, photodiodes, etc. Therefore, it is necessary to know how to test whether a diode is properly working or not.

How to Test a Diode Using a Multimeter

Catalog

Introduction

Ⅰ Diode Basics

1.1 To Figure Out Diode Anode and Cathode

1.2 What Would Cause a Diode to Fail?

1.3 Common Diode Failures Analysis

Ⅱ How to Test Diode with a Multimeter?

2.1 Digital Multimeter and Analog Multimeter

2.2 Common Diodes Testing Rules

2.3 Testing Methods of Types of Diodes

Ⅲ Example Analysis

3.1 Test a Diode in Circuit

3.2 Power-off and Power-on Testing Methods

3.3 Conclusion


Ⅰ Diode Basics

1.1 To Figure Out Diode Anode and Cathode

The anode and cathode of diode can be distinguished by screen printing on PCB board, which are as shown in the following:

1) The notched end is the cathode of diode.

2) The end with a horizontal bar is the cathode.

3) The end with white parallel bars is the cathode.

4) One end of the triangle arrow is the cathode.

5) The small end of the plug-in diode is the cathode, and another big end is the anode.

diode symbol

1.2 What Would Cause a Diode to Fail?

The common reasons for a diode failure are open circuit, short circuit and unstable voltage regulation. Among these three types of failures, there may be signs. For example, the power supply voltage rises, the supply voltage drops to zero or the output is unstable. Therefore, specific problems must be analyzed in detail for the diode test.

The common measurement tool for diode is a multimeter, including on-circuit measurement (the diode is on the circuit board) and off-circuit measurement (the diode is not on the circuit board). As for the basic principle of diode measurement, the forward resistance and the reverse resistance of the PN junction are measured, and the basic judgment is based on the values of them. Therefore, to do a good job in diode test, it is necessary to understand the basic structure and working principle of diodes, and then to understand the main fault characteristics of the diode.

 

1.3 Common Diode Failures Analysis

1) open circuit

This means that the positive and negative electrodes of the diode have been disconnected, and the forward and reverse resistance of the diode have become infinite. After the diode is open, the circuit is in an open state.

2) voltage breakdown

This means that there is a path between the positive and negative electrodes of the diode, and the forward and reverse resistance are as large as or close to each other(but not infinite). After a diode breaks down, the action between the positive and negative electrodes may always exit, because there are different manifestations in different circuits.

3) forward voltage

If the forward resistance of the diode is too large, the voltage drop of the signal on the diode will increase, which will cause the output signal to decrease, and the diode will be damaged due to the heat. After the forward resistance becomes larger, the unidirectional conductivity of the diode will become poor.

4) reverse voltage

The reverse resistance of the diode becomes smaller, which means the unidirectional conductivity of the diode be effected.

5) performance degradation

Under this circumstance, the diode does not have obvious failures such as open circuit or breakdown. However, when the situation is getting worse, the stability of the circuit will deteriorate or the output signal voltage of the circuit will drop.

 

Ⅱ How to Test Diode with a Multimeter?

2.1 Digital Multimeter and Analog Multimeter

When using a digital multimeter to test a diode, the red probe connects with the anode and the black probe connects with the cathode. At this time, the measured resistance is the forward conduction resistance of the diode, which is just the opposite of the test result of an analog multimeter.

 

2.2 Common Diodes Testing Rules

(1) The forward resistance of the low-power germanium diode is 300Ω~500Ω, and the silicon diode is lkΩ or more. The former reverse resistance is tens of thousand ohms, and the latter is above 500kΩ (the value of high-power diode is smaller).

(2) The polarity of the diode can be judged according to the resistance values (small forward resistance and large reverse resistance). Set the multimeter to the ohm block (Usually use R×100 or R×1k block, do not use R×1 block or R×10k block. The R×1 block is in a large current, it is easy to burn the tube, while using R×10k block may cause the tube broken down with high voltage). Connect the two polarities of the diode with the test probes respectively, and measure the two resistance values. When the measured resistance value is smaller, the end connected to the black lead is the anode. In the same way, when the measured resistance value is larger, the end connected to the black probe is the cathode. If the measured reverse resistance is small, it means that the diode is short-circuited, on the contrary, if the forward resistance is large, it means that the tube is open. In both cases, the diode can’t be work normally.

(3) Silicon diodes generally have a forward voltage drop of 0.6V~0.7V, and the forward voltage drop of a germanium diode is 0.IV~0.3V. By measuring the forward voltage of the diode, it can be judged that the tested diode is a silicon tube or a germanium tube. This method is to connect a resistor (lkΩ) behind the power supply, and then connect with the diode according to the polarity characteristic to make the diode forward conducting. At this time, use a multimeter to measure the tube voltage drop. In addition, it is more convenient if it is used in energized dynamic measurement.

 

2.3 Testing Methods of Types of Diodes

  • Zener Diodes

How to test a Zener diode? The following here is to give some ideas.

(1) Generally use the low-resistance block to test the Zener diode with a multimeter. Since the battery in the meter is 1.5V, this voltage is not enough to make the Zener diode reverse breakdown. So the forward and reverse resistance should be the same as a normal diode.

(2) Measurement of the voltage stabilization value Vz of the Zener diode. When measuring diode, the power supply voltage must be greater than the stable voltage of the tube under test. In this way, the high-resistance block of the multimeter (R×10k) must be used. At this time, the battery in the meter has a higher voltage. When the multimeter's range is set to high barrier, measure diode reverse resistance. If the measured resistance is Rx, the voltage regulation value of the Zener diode is:

diode equation

In the formula, n is the override of the gear used. For example, if the highest electrical barrier of

R0 is the central resistance of the multimeter.

E0 is the highest battery voltage value of the multimeter used.

Zener Diode Image

Example: Use an MF50 multimeter to measure a 2CW14 diode.

R0=10Ω, the highest electrical barrier is R×10k.

E0=15V, the measured reverse resistance is 75kΩ, then its voltage regulation value is:

diode equation

If the measured resistance is very large (close to infinite), it means that the voltage Vz under test is greater than E0, therefore, tube will not break down. If the measured resistance is very small (0 or only a few ohms), it means that the test probes are connected reversely, and then just swap the test probes.

 

  • Light-emitting Diodes (LED)

A light-emitting diode is a semiconductor device that converts electrical energy into light energy. It has the characteristics of small size, low working voltage and low working current.

(1) There is a PN junction inside the light-emitting diode, so LED has the same characteristic of unidirectional conductivity. Its detection is similar to the measurement of ordinary diodes.

(2) Use the R×1k or R×10k gear, and the forward and reverse resistance values are measured. Generally, the forward resistance is less than 50kΩ, and the reverse resistance is greater than 200kΩ.

(3) The working current of the light-emitting diode is an important parameter. If the working current is too small, the light-emitting diode will not light up, and it is too large, the light-emitting diode will be easily damaged.

(4) The forward turn-on voltage of the light-emitting diode is 1.2V ~ 2.5V, and the reverse breakdown voltage is about 5V.

 

  • Photodiodes

Photodiode is a semiconductor device that can convert light intensity into electrical signals.

(1) There is a window on the top of the photodiode that can inject light, and the light irradiates the die through it. Under the excitation of the light, a large number of photoelectric particles are generated in the photodiode, which greatly enhances its conductivity and reduces internal resistance.

(2) The photodiode is similar to the Zener diode. It also works in the reverse state, with reverse voltage.

(3) The forward resistance of the photodiode does not change with the light. Its reverse resistance is larger when there is no light, and becomes smaller when it is exposed to light. That is, the stronger the light, the smaller the reverse resistance. Without light, the reverse resistance will return to the original value.

(4) According to the related principle, use a multimeter to measure the reverse resistance of the photodiode. Change the light intensity when measuring, and observe the change of the reverse resistance of the photodiode. If there is no change or less change of the reverse resistance when light changes, it indicates that the tube has failed.

 

  • High-speed Switching Diodes

The method of detecting high-speed silicon switching diodes is the same as that of ordinary diodes. The difference is that the forward resistance of this tube is relatively large. Measuring with Rxlk block, the forward resistance value is 5k ~ 10k in general, and the reverse resistance value is infinite.

 

  • Fast Recovery Diodes / Ultrafast Recovery Diodes

Detecting fast recovery and ultra-fast recovery diodes with a multimeter is basically the same as that of detecting plastic-encapsulated silicon rectifier diodes. That is, first use the Rxlk block to test its unidirectional conductivity. Generally, the size of forward resistance is about 4 ~ 5k, and the reverse resistance is infinite. And then use the Rxl block to repeat the test, at this time, the forward resistance is several ohms, and the reverse resistance is still infinite.

 

  • DIAC (Diode for Alternating Current) Diodes

Use the Rxlk block, and measure the forward and reverse resistance values of diac, which should be infinite. If the test probes are exchanged to measure, the pointer swings to the right, which indicates that the test tube has a leakage fault. Another method is placing the multimeter in the DC voltage block. During the test, shake the megohmmeter, and the voltage value indicated by the multimeter is the VBO value of the tube. Then change the two pins of the tested tube, and measure the VBR value in the same way. Finally, compare VBO and VBR. The smaller the difference between the absolute values of the two, the better the symmetry of the diac diode.

 

  • TVS Diodes

For the dual TVS, resistance values between the two pins should be infinite when the red and black test probes of multimeter are exchanged at random. Otherwise, the tube has poor performance or has been damaged.

 

  • High-frequency Varistor Diodes

a. Identify Diode Polarity

The difference between high-frequency varistor diodes and ordinary diodes is that their color code is different. It is generally black of ordinary diodes, while high-frequency varistor diodes’ is light. Its polarity rule is similar to that of ordinary diodes. That is, the end with the green ring is the cathode, otherwise it is the anode.

b. Measure Forward and Reverse Resistance

The specific method is the same as the method of measuring ordinary diodes. Using the Rxlk block of a AM-500 multimeter, the forward resistance is 5k~55k, and the reverse resistance is infinite.

 

  • Varactor Diodes

Using Rx10k block, no matter how the red and black test leads are exchanged for measurement, the resistance between the two pins of the varactor diode should be infinite. During the measurement, if the multimeter swings slightly to the right or the resistance value is zero, it means that the varactor diode under test has a leakage fault or has been broken down. No matter the loss of varactor diode capacity or internal open-circuit fault, it is impossible to detect them with a multimeter. When necessary, the replacement method can be used for inspection to make judgment.

 

  • Infrared Light Emitting Diodes (IRED)

Put the multimeter in the Rxlk block and measure the forward and reverse resistance of the IRED diode. Generally, the forward resistance should be about 30k, and the reverse resistance should be above 500k. It means the tube can work normally. The larger the reverse resistance, the better.

 

  • IR Receiver Diodes

a. Appearance Identification: Diode Cathode / Anode

(1) Common infrared receiving diodes are black in appearance. In addition, there is a small oblique plane at the top of the tube body of the infrared receiving diode. Usually, the pin with one end of the oblique plane is the negative pole and the other end is the positive pole.

(2) Use the Rxlk block to test the resistances between two pins. When a diode works normally, the resistance value of two pins are different. And exchange the test leads several times to get some pairs of values. According to the smaller resistance value, the pin connected to the red probe is the cathode, and the pin connected to the black probe is the anode.

b. Performance Detection

Use a multimeter to measure the forward and reverse resistance of the infrared receiving diode. According to the resistance values, whether the diode is damaged can be judged preliminarily.

 

  • Laser Diodes

Use the Rxlk block of multimeter, and determine the order of the pins of the laser diode according to the method of detecting ordinary diodes. Because the forward voltage drop of the laser diode is larger than that of the ordinary diode, when detecting the forward resistance, the pointer of the multimeter is slightly deflected to the right, and the reverse resistance is infinite.

 

  • Unijunction Transistor (UJT)

a. Discrimination of Electrodes

Based on the R×1k block, use two meter pens to measure the forward and reverse resistance between any two of the three electrodes ( base B1 and base B2, and emitter E) of the ujt diode. The measured resistance values between the two electrodes are both 2~10kΩ, in addition, B1 and B2 will be different.

b. Performance Judgment

The performance of an ujt diode can be judged by measuring whether the resistance between its pins is normal. Use the R×1k barrier, the black test lead connect to the emitter E, and the red test lead connect to the two base electrodes in turn. Normally, a resistance value should be several thousand to ten thousand ohms. On the contrary, the red test lead connects to the emitter E, and the black test lead connects to the two base electrodes in turn, and the resistance should be infinite under normal conditions. The forward and reverse resistance values between the two bases are both in the range of 2~10kΩ. If they differ greatly from the normal value, the diode is damaged.

 

Ⅲ Example Analysis

3.1 Test a Diode in Circuit

a. Diode Test UsingAnalog Multimeter

The following measurements are all based on silicon diodes. If it is a germanium diode, the forward and reverse resistance of the diode will decrease.

1) Measure forward resistance FR

The following figure is a wiring schematic diagram for measuring the forward resistance of a diode with an analog multimeter:

forward resistance test

Give the result as follows:

Indicator

Description

Pointer indication

Use the R×1k block to measure the diode, the forward resistance is several thousand ohms, and the pointer indicates stability. If the pointer swings slightly, it indicates that the thermal stability of the diode is poor.

Pointer indication

If the pointer indicates hundreds of kiloohms when measuring the forward resistance, it means that the diode is open.

Pointer indication

If the pointer indicates tens of kiloohms, it indicates that the diode has a large forward resistance and poor diode performance.

 

Description of measurement of forward resistance:

Forward Resistance (FR)

Description

Thousands of ohms

Normal

Zero or much less than a few thousand ohms

Breakdown

Hundreds of kilos

Large FR, the diode is open

Dozens of kilohms

Large FR, bad forward characteristics

The pointer is unstable

Poor stability


 

2) Measure reverse resistance RR

The following figure is a wiring schematic diagram for measuring the reverse resistance of a diode with an analog multimeter:

reverse resistance test

Give the result as follows:

Indicator

Description

pointer indication

When measuring the reverse resistance, the value should be several hundred kiloohms. The larger the resistance value is, the better the indicator should be stable.

pointer indication

If the reverse resistance is only a few thousand ohms, it means that the diode has broken down and has lost its unidirectional conductivity.

 

Description of measurement of reverse resistance

Reverse Resistance

Description

Hundreds of kilos

Normal

Zero

Breakdown

Much less than a few hundred thousand ohms

Diode’s reverse characteristic is not good.

Pointer does not move

The diode is open. Note: The reverse resistance of some diodes is very large, at this time, it is not certain that the diode is open, so that its forward resistance should be measured. If the value is normal, it means that the diode is not open.

Pointer is unstable

The pointer cannot be stabilized at a certain resistance value during measurement, indicating that the diode has poor stability.


 

3.2 Power-off and Power-on Testing Methods

Diode in-circuit measurement is divided into two situations: Power-off and Power-on state

a. Power-off Measurement

There are something should be noted the method of this test.

  • The influence of the external circuit on the test result is the same as the resistance and capacitance measured of internal circuit. And the influence of the measured forward resistance by the external circuit is lower than the reverse resistance.
  • If there is any doubt about the measuring result, the diode should be removed from the circuit and measured separately.

diode voltage drop test

 

b. Power- on Measurement

When the circuit board is powered on, the test point is the tube voltage drop. Because the diode has a very important characteristic: when it is turned on, the tube voltage drop is basically unchanged. So the voltage drop is normal after being turned on, that is to say, the diode is normal.

Measurement method: The diagram below shows the connection diagram of the tube voltage drop after the diode in the DC circuit. Setting multimeter in DC voltage 1 V block, the red probe is connected to the cathode of the diode, and the voltage indicated is the forward voltage drop of the diode.

 

Diode forward voltage drop measurement results are analyzed as follows:

Diode

Description

Silicon diode

0.6V

The diode is normal and in a forward conducting state.

> 0.6V

The diode is not in the conducting state.

Close to 0

The diode is in a breakdown state, the current in the loop will increase.

Germanium diode

0.2V

The diode is normal and in a forward conducting state.

> 0.2V

The diode is off or is faulty.

Close to 0

In the breakdown state, the loop current increases significantly, without unidirectional conductivity.


 

3.3 Conclusion

The following points should be noted when measuring diodes:

1) The diode in AC is in the cut-off state, because the diode is in the reverse state, and the reverse voltage at both ends is very large. The average voltage across the diode measured by the DC block is negative at this time.

2) Use different blocks of the same multimeter to measure positive and negative resistance of one diode, their values will different. The forward and reverse resistances of the same diode measured with different multimeters are also different.

3) When measuring the forward resistance of a diode, if the pointer cannot stop at a certain resistance value and constantly swings, it indicates that the thermal stability of the diode is not good.

4) Some multimeters will provide a “diode check” function that displays the actual forward voltage of the diode when its conducting current. Such meters typically indicate a slightly lower forward voltage than what is “nominal state” for a diode, due to the very small amount of current used during the measurement.

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pinglun 1 comment

    • pingluntus
    • Ginachen on 2020/11/20 17:51:59

    Hey, there How do you test a germanium diode?

      • pingluntu
      • author on 2020/11/21 9:38:01
        author

      Re:

      The cathode terminal is on the side with a band. You then connect the red probe to the anode terminal. If your meter is ON and set to the diode test mode, it should provide you with readings in volts. If the figure is 0.3 V or less, then the diode is a germanium type.

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