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Transistor Test for Terminal Identification, Type & Condition.

How To Perform A Transistor Test For Identifying Terminals, Type (NPN or PNP) & Condition (Good or Bad)

As we know transistor is the most used component in any project, circuit or devices but you cannot use it before the transistor test. The most crucial task in any project or circuit building is to know “How to perform the transistor test“. This transistor test helps you in the terminal’s identification, NPN/PNP & Good/damaged transistors.

How To Perform A Transistor Test For Identifying Terminals, Type (NPN or PNP) & Condition (Good or Bad)

This test is only applicable for BJT transistors. So before any transistor test, we need to know about the structure of the BJT.

Transistor (BJT) 

A BJT (Bipolar Junction Transistor) is a three-terminal semiconductor device. Its made up of two P-N junction diodes fused together forming three layers known as Base, Emitter & Collector.

Transistor

 

There are two types of transistors base on the polarity of its layers.

NPN

In this BJT, the Base i.e. the P-doped layer is sandwiched between N-doped layers known as Collector & Emitter.

The difference between the Collector & Emitter is that the Emitter is the heavily doped layer.

NPN corresponds to two diodes being fused together by the Anode terminal as shown in the figure below.

 

NPN Transistor

Also Read: Basic Electronics Engineering Interview Questions

PNP

PNP transistor is made up of an N-doped layer (Base) sandwiched between P-doped layers known as Collector & Emitter.

PNP transistor corresponds to two diodes, the cathode terminal of these two diodes is fused together as shown in the figure below.

 

PNP Transistor

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This transistor test uses the Diode test function of the Multimeter. So, For the sake of this transistor test, you need to know about the diode test.

diode test mode:

Forward bias P-N junction: Multimeter reads some voltage & beeps.

Reverse bias P-N junction: Multimeter reads OL (Over Limit)   

Terminal’s Identification

The first step in the transistor test is to identify the terminals (Base, Emitter & Collector) of the transistor.

First, you need to mark the terminals of the transistor with numbers 1,2,3. In order to do that, hold the transistor’s flat side facing towards you and start from the left side as shown in the figure below.

Transistor Terminal IdentificationAlso Read: What Is Rectifier? Types Of Rectifiers

Base Terminal Identification

  • Put Multimeter in diode test mode.
  • Place black (common) probe & red probe on any two terminals at a time.
  • Test all possible terminal combinations i.e. 1-2, 1-3, 2-1, 2-3, 3-1, 3-2.
  • Two of these combinations should pass diode test (reading shows voltage 0.5v to 0.8v), the common terminal in these two combinations is the Base terminal.
  • Suppose, 2-1 & 2-3 combinations pass diode test then 2 is the base terminal.

Transistor base Terminal Identification

Emitter & Collector Identification

With the successful identification of the base terminal, two terminals (1 & 3) remain unknown. if you identify the second terminal, subsequently you will also know the third terminal.

  • Set the Multimeter in diode test mode.
  • Record the voltage reading of base terminal with both terminals 1 & 3 one by one.
  • The terminal having a higher voltage between the two is the Emitter.
  • The terminal with lower voltage compared to the other is Collector.

In this example, suppose the 2-1 voltage reading = 0.6v & 2-3 voltage reading = 0.7v

  • So the Emitter is terminal 3 & Collector is terminal 1.

Transistor All Terminal IdentificationAlso Read: How To Test A Diode & Methods Of Diode, LED & Zener Diode Test

Type: NPN or PNP

The next step in the transistor test is to know whether the transmitter is NPN or PNP.

This step depends on the results of the above transistor test.

NPN Test

  • Put Multimeter in diode test mode.
  • Place the Red (positive) probe on Base terminal & the black (common or negative) terminal on Emitter & Collector one by one.
  • If they pass the diode test, it means the junctions are forward bias & it is an NPN transistor.

If you don’t know the terminals.

  • Set the multimeter in diode test mode.
  • Test all the six combinations of terminals for the diode test.
  • Note those two combinations, whose diode test is positive(multimeter Beeps or show voltage).
  • If the common terminal in these two combinations is connected to the red probe of the multimeter, the transistor is NPN.

PNP Test

PNP transistor test is a little different than the NPN transistor test.

  • Put the Multimeter in diode test mode.
  • Connect the Black (common) probe with Base & the Red probe with Emitter & Collector one at a time.
  • If these both combinations pass the diode test, the Transistor is PNP.

If you don’t know the terminals.

  • Check all (six) possible combinations of the terminals for the diode test.
  • Note the two combinations that pass the diode test.
  • If the common terminal in these two combinations is connected to the Black or common probe of the Multimeter, the transistor is PNP.

Testing The Transistor (Good or Damaged)

This transistor test helps us in identifying if the transistor is good or damaged.

Set the Multimeter in diode test mode and test all the possible combination for the diode test. Note down the reading for each combination.

If the transistor satisfies the readings given in the chart below, it is good.Transistor test

 

If the readings don’t match with the chart above, the transistor is damaged & need to be replaced.

 

 

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