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How To Test A Diode & Methods Of Diode, LED & Zener Diode Test

How To Test A Diode? Methods Of Diode, LED & Zener Diode Test

In Electronics Engineering projects, the diode is one of the most important components. It is very important & necessary to perform a diode test before placing it into your circuit board. To avoid any disaster or waste your time, it is advisable to do these diode tests before implementing it in any circuit. We carry out these diode tests of diodes, LED & Zener diode to see if it is defected, burned or damaged. Multi-meter is an important tool used to these diode tests.

Before implementing and troubleshooting any diode, LED & Zener diode, one must have the basic knowledge about these components. In this article, we have explained these components as much as we need to perform a diode test, LED & Zener diode.

How To Test A Diode & Methods of Diode, LED & Zener Diode Test

Let’s move forward with the first one as “how to test a diode”.

How To Test A Diode?

Before checking a diode you need to have the basic knowledge of a diode.

What Is A Diode?

 A diode is a unidirectional semiconductor component, which allows the flow of current in only one direction & blocks the flow of current in another direction. if the current lies in the limit of diode’s rating.

A diode has very low (Ideally zero) resistance in one direction and it has very high (ideally infinite) resistance in another direction.

Terminals Of A Diode:

A diode has two terminals called cathode (the negative terminal) and anode (the positive terminal).

The cathode is made of N-type semiconductor and anode is made of a P-type semiconductor.

When the cathode & Anode is connected to the negative & positive terminal of power supply respectively, the diode starts conducting and is said to be in Forward bias. In this configuration, there is a voltage drop of about 0.3v (in case of germanium diode) or 0.7v (in case of silicon diode) across the diode.

And when these connections are reversed i.e. the cathode with positive terminal & anode with negative terminal, the diode breaks the current flow and said to be in reversed bias. In this condition, the diode has very high resistance and the supply voltage appears across it.

Visual Identification Of Anode & Cathode:

Visual identification of Anode and Cathode terminals of a diode is very easy & simple. The white striped side of the diode is Cathode and the non-striped side is Anode.

diode teminals identification

Diode Resistance Test:

In the diode resistance test, you need to use a DMM (Digital Multi-Meter).

  • First, set the DMM in resistance or ohmmeter mode using the knob.
  • Next step is to remove the diode if it is in any circuit.
  • Identify the terminals (Anode & Cathode) using the instructions given above.
  • Place the common probe (Black probe) of DMM on the cathode and Red probe on the Anode of the diode. This configuration is forward bias. Now record the reading.
  • Now swap the probes such that the red probe is on Cathode and black probe (common probe) is on the Anode of the diode. Such configuration becomes reversed bias. Now, record the reading as well.
Analysis:

In forward bias configuration, the diode will become close and the ohmmeter will read very low resistance (Ideally zero).

Diode Resistance mode test forward bias

Note: During diode test, Some ohmmeters may have very low test voltage for measuring resistance, which may not be enough to put the diode in forward bias (diode need 0.7 or more than 0.7v to become forward bias).

In such case, the ohmmeter will read it in open condition and the reading will be very high. For these meters, the next method works perfectly well

Diode Resistance mode reverse bias

In reverse bias configuration, the diode becomes open and will provide very high resistance. So the ohmmeter will read a high resistance (ideally infinite).

Conclusion:
  • In forward bias, if the resistance reading is very low (ideally zero) then the diode is in a healthy condition.
  • In reverse bias, if the resistance is very high (ideally infinite) then the diode is healthy.
  • If in both conditions (forward & reverse bias), the reading is very high then the diode is said to be opened and you need to replace it.
  • If in both conditions (forward & reverse bias), the reading is very low then the diode is probably shorted and it also needs to be replaced.

Using Diode Test Mode:

Diode Test Mode in DMM is specifically designed for diode testing. It may share other function in different meters like continuity test mode etc.

  • Set the DMM in diode test mode.
  • Remove the diode if it is installed in any circuitry.
  • Identify the terminals using the instructions above.
  • Place the common probe (black probe) on cathode & the red probe on the anode of the diode. This makes it forward bias. Record the reading.

Diode Test mode forward bias

  • Now swap the probe’s position on diode such that the black probe connects with the anode & the red probe connects with the cathode. Now it is in reversed bias. Record the reading.

Diode Test mode reverse bias

Conclusion:
  • In forward bias, A healthy diode is closed and DMM will read less than 0.7v (in case of silicon diode) or 0.3v (in case of germanium).
  • In reverse bias, the diode will be in open state and the DMM will show infinite (1 or OL, which reads infinite or Overlimit in DMM).
  • If the readings are somewhat the same as the readings given above then the diode is in healthy & good condition.
  • If the reading does not lie anywhere near that especially in forward bias then the diode is probably burned or damaged & it needs to be replaced.

Using Voltmeter

In order to test a diode using a voltmeter, you need to connect a diode with a battery and a resistor as shown below.

Voltmeter Test mode forward bias

  • You need to place a resistor in series to limit the current flowing through it.
  • Supply the voltage by connecting it to a battery in forward bias.
  • Measure the voltage across the diode.
  • Now swap the terminals of the battery to put the diode in reverse bias.

Voltmeter Test mode reverse bias

  • Measure the voltage across the diode.
Conclusion:
  • In forward bias, the voltmeter should read 0.7v (for silicon diode) or 0.3v (for germanium diode).
  • In reverse bias, the voltmeter is suppose to read the same voltage as the supply does.
  • In forward bias, if the voltmeter reading is 0v then the diode is short. If the voltmeter reads same as supply voltage then the diode is open. In both cases, the diode is defected & needs to be replaced.
  • In reverse bias, if the voltmeter is reading anything other than the supply voltage, the diode is defective.

Testing For Defective Open Diode:

The diode is open & defective and needs to be changed if the following cases happen:

  • In the resistance test, the readings in ohmmeter read very high resistance in both configurations (forward & reverse).
  • In diode test mode, if the reading is infinite (1 or 0L) in both configurations.
  • In the voltmeter test, if the voltmeter reads equal to supply voltage in both configurations.

Testing For Defective Close or Shorted Diode:

The diode is close and defective & need to be changed if the following cases happen:

  • During the resistance test, If the readings are very low in both configuration (forward & reverse).
  • In diode test mode, if the readings are somewhat the same in both configurations.
  • In the voltmeter test, if the voltmeter reads 0v in either one or both configuration.

How To Test An LED?

What Is LED?

LED stands for light-emitting diode. It is a type of diode that emits light when current pass through it. Just like a diode, LED has specific forward voltage drop ranging from 1.8v to 3.3v depending on its color. LED has two terminals known as Anode & Cathode.

Visual Identification Of LED Terminals:

Usually, in LED manufacturing, the Anode terminal leg is made longer and the cathode terminal leg is made shorter. So this is one way to recognize the terminals.

LED terminals identification

The second way is to look inside the LED. The flat terminal compares to the other terminal is the cathode while the other thin terminal is Anode.

Using Diode Test Mode:

  • Put the DMM in diode test mode using the knob.
  • Disconnect the circuit supply to the LED if it is in an energized circuit.
  • Identify the terminals as shown in the instructions above.
  • Place the common probe (Black probe) on the cathode and the red probe on the Anode of LED.

LED Test using multimeter

  • LEDs do not work in reverse bias, so there is no need for checking in reverse bias.

If the LED glow, then it is good. If it doesn’t glow then the LED is probably burned or damaged.

How To Test A Zener Diode:

What Is A Zener Diode:

A Zener diode is a special type of a diode that is usually operated in reversed bias. In forward bias, it acts as a common diode. But what differentiates it from the common diode is that it also allow current in reversed bias when the supply voltage reaches Zener voltage (breakdown voltage).

Terminals Identification:

Just like a common diode, the Zener diode has the same strip over the cathode & the non-striped side is the anode.

Zener diode Terminal Identification

Testing Zener Diode:

A Zener diode can be tested using the following two methods. The second method is very important.

Using Diode Mode:

As we have discussed that the Zener operates same as a common diode so the diode test mode method will be the same for Zener.

The forward bias will show less than 0.7v & the reverse bias will show infinite (1 or OL) because it will block the current unless the meter can provide more voltage than its Zener breakdown voltage.   

Using Voltmeter:

This is an important method to check for Zener diode in reverse bias. You need to know about the Zener breakdown voltage from its datasheet before testing it.

Suppose, our Zener diode has 9v breakdown voltage. And we are supplying it 12v in reverse bias. The Zener should build 9v across it and do not exceed it.

In order to test Zener, we are going to make a circuit with a resistor to limit the current & connect it as shown in the figure below.

Zener Diode Test using voltmeter

  • Using a voltmeter, we will check the voltage across it.
  • If the voltage reading matches its Zener breakdown voltage from its datasheet then the Zener is in good condition.
  • If the voltage increase from it rated breakdown voltage then the Zener is damaged and need to be replaced.

Diodes are of many types & all diodes have the same properties. So these diode tests can be used to test any type of diode. 

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