Home Electronics Tutorials How to Identify the 3 Pins of a Transistor correctly: Transistor Testing...

How to Identify the 3 Pins of a Transistor correctly: Transistor Testing Methods in Step-by-step

Bipolar junction transistor (BJT) structure

A transistor is a semiconductor device with 3 terminals which are collector, emitter and base. So identification of the 3 pins of a transistor is an essential part when using transistors for circuits.

Transistors are made of semiconductor materials mainly silicon and some are made out of germanium and other semiconductor materials.

Transistors are smaller in size and for the operation, it consumes less power when compared to the vacuum tubes which are used in the early stages.

transistor diagram
Bipolar junction transistor (BJT) structure

When considering the physical appearance, transistors having a plastic covering and one side of the transistor is a flat surface and other side is curved.

How to identify the 3 pins of a transistor by its symbol

The 3 pins of a transistor symbol can be easily identified by the looking at the direction where the arrow points towards. If the arrow points towards the Emitter, it’s an NPN transistor. If the arrow points towards the base, it’s a PNP transistor.

How to identify the 3 pins of a transistor by its physical appearance

NPN TRANSISTOR PINS
Identification of the 3 pins of an NPN transistor

In most cases considering NPN transistors, when we keep the flat side of the transistor front-facing to us. The pins from the left to right are collector, base and emitter respectively. In most of the PNP transistors, it’s getting reversed. So it will be Emitter, base and collector from left to right.

PNP PINS
Identification of the 3 pins of a PNP transistor

How to identify the 3 pins of a transistor by a multimeter

Identification of the transistor type: NPN transistor or PNP transistor

When analysing the transistor pins, the middle pin of a BJT transistor is always the base and by keeping the positive (Red) probe on the middle pin and negative (black) probe on other two transistor pins, we can identify the transistor type.

  1. Set the multimeter to the diode mode.
  2. Then keep the positive (Red) probe of the multimeter to the middle pin of the transistor and then keep the negative (black) probe on one of the other transistor pins on either side.
  • If there’s a reading showing in the multimeter screen – it’s an NPN transistor
  • If there’s no reading showing in the multimeter screen – it’s a PNP transistor
compare transistor 2

In the diode mode, the multimeter showing a voltage value in a p-n junction where the positive probe is on the anode (p) and the black probe is on the cathode (n)

Identification of the 3 pins of a NPN transistor using Multimeter

  1. Set the multimeter to the diode mode
  2. Then keep the positive (Red) probe of the multimeter to the MIDDLE pin of the transistor
  3. And next, keep the negative (black) probe on the LEFT pin and take the reading from the multimeter.
  4. Then take the multimeter reading by keeping the black probe on the RIGHT pin of the transistor.
compare transistor
Identification of the 3 pins of a NPN transistor using Multimeter

When comparing the two readings, The p-n junction where the higher reading obtained is the base-emitter junction. And the p-n junction where lower value obtained is the collector-base junction.

Same way the above test can be applied to identify the pins of a PNP transistor.

Identification of the 3 pins of a PNP transistor using Multimeter

  1. Set the multimeter to the diode mode.
  2. Then keep the positive (Red) probe of the multimeter to the LEFT pin of the transistor
  3. And next, keep the negative (black) probe on the MIDDLE pin and take the reading from the multimeter.
  4. Then take the multimeter reading by keeping the positive (Red) probe on the right pin of the transistor.

When comparing the two readings, The p-n junction where the higher reading obtained is the base-emitter junction. And the p-n junction where lower value obtained is the collector-base junction.


DO YOU KNOW How to find the Anode and Cathode of a Diode? READ IT HERE >>>

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