What is a Transistor and What does it do?

What is a transistor?

A transistor is an electronic device built with semi conductor materials. transistors are very widely used in both analogue and digital electronics. an integrated circuit (IC) or Chip, can contain many many transistors, maybe hundreds of them. transistors can appear in many forms, sizes (smd), big or small, more leads or pins.

A common ordinary transistor has 3 pins or terminals known as – base (B), collector (C) and emitter (E),  there are different kinds of transistors, some are NPN,PNP,FET and many…

What does a transistor do, what are its uses?

  • It can switch electronic signals or power
  • It can Amplify signals

Transistors are used in amplifiers, for switching power etc. for example, a little current in the base of a transistor can control a large current of the collector. also it can be used as a switch. all ordinary transistors have 3 pins, they are known as base (B), collector (C) and emitter (E)


Transistorer (cropped) free to use image from wikipedia
types of transistors – image credit: wikipedia. https://commons.wikimedia.org/wiki/File:Transistorer_(cropped).jpg

ciruit symbols of transistors



internal diagram of transistor can be imagined as connecting two diodes, this picture will help you when you test a transistor. Test it as you testing a diode

Using and handling transistors

A transistor may be damaged because of over voltage, wrong pin connection can damage it too. if you are building a circuit using transistors, make sure you have used all the pins or terminals correctly and used correct voltage as mentioned in its data sheet, have chosen a correct value resistor for its base pin.

The resistor in the base pin of a transistor must be chosen for the value of input voltage,for example if you choose a very low value resistor for the base pin of a transistor but the transistor needs a high value resistor in its base pin, then the transistor will be overheated and can be damaged in a few seconds. Here are some advice to help you while using a transistor

● Never exceed voltage in a transistor more than its recommended voltage.

● Always download data sheets of transistors and see the manufacturer’s notes and other parameters.

● Data sheets are available in internet for free, so just enter the part number of the transistor in your browser to find its data sheet.

● Never replace power transistors with low power transistors.

● The base pin of a transistor is its key to turn it on, so a correct value resistor must be used in base pin of a transistolace your hot soldering iron on the pins of a transistor for a long time when soldering, it can overheat the transistor and damage.

A transistor damages if a large voltage and current passed into it, however, that doesn’t mean “Never”, because some transistors can damage as it ages or because of some physical and connectivity issues, or too much heat while soldering it to the PCB.

Testing a transistor

Testing a transistor is quite easy if you have some basic skills of using a multimeter.

If you got a transistor and you don’t know whether it’s a PNP or NPN you can make a search in the google to find its details or download its data sheet. Then select your Digital multimeter’s diode test function or continuity testing(usually both functions are in same position)

PNP transistor testing using a DMM(DIGITAL MULTIMETER)

To test a pnp, place your meter’s black probe in the base of the transistor and place the red probe in the collector and emitter pins, you should get a reading about “700” (reading may vary for different transistors) in both pins, and place the probes in both collector and emitter then it shouldn’t read anything, and place the red probe in base and black probe in both Collector and emitter it shouldn’t read anything. If these results are what you get then the transistor which you test is good. If it’s all in opposite than mentioned above then the transistor is damaged!

Testing NPN using DMM

Place your multimeter’s red probe in the base pin of the transistor and place the black probe on collector and emitter, you should get a reading around “700”. next, place the red probe on collector and emitter, now you shouldn’t get any reading.

place the black probe on base and red probe on both emitter and collector, it will not read anything. If these results are what you get when testing an NPN transistor, then it is good, If not, the transistor is damaged.


Note: this page covers only a very little information about transistors, for advanced learning, these links will help you



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