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What is a PCB Transistor? Understanding Circuit Board Transistors

PCB transistor

A PCB transistor regulates the flow of electricity between components, enabling a circuit board to perform its function. If you’re looking to understand it more, continue reading below. Here’s a brief look at what we will be covering:

  • PCB transistor meaning and construction
  • The different types of the device
  • Its function on a circuit board
  • How to read and test one
  • How to replace it on a circuit board

What is a PCB Transistor?

A PCB transistor is a semiconductor device that regulates the flow of electricity on a printed circuit board. It typically switches or amplifies current, serving various functions that range from audio enhancement to signal detection and voltage stabilization.

The transistors are available in various shapes and sizes. They usually have three leads or pins. One lead is the base, while the others are named emitter and collector. Sometimes, these devices have metal parts attached for enhanced heat dissipation.

Inside a transistor device
Inside a transistor device
Recourse: https://www.youtube.com/watch?v=YbB_FjMDJUE

What is a PCB Transistor Made of?

At its heart are three layers of semiconductor materials called P (positive) or N (negative) types. These layers form three zones: base, emitter, and collector, with their arrangement producing PNP or NPN structures.

The collector can pick up and conduct input current, but only if permitted by the base. On the other hand, the emitter can send current through the circuit, but only if the base allows the emitter.

In other words, the base acts as the control terminal or pin. A small voltage applied to it opens the flow of electrons, allowing a much larger current between the emitter and collector.

Various types of transistor packages
Various types of transistor packages
Resource: https://youtu.be/rf-GIy7yjt8?si=t2jeyrXGUHubxV45

Types of PCB Transistors

There are two main types of circuit board transistors: bipolar junction and field-effect type. Each has a specific property that would make it useful on a circuit board or not.

Bipolar Junction Transistor

A BJT derives its name from using holes and electrons to conduct electricity and the presence of the PN junction. It typically has positive and negative semiconductors arranged into NPN and PNP layers. The bipolar transistor is the most common type and best used to enhance or amplify current.

Field Effect Transistor

The field effect type has three distinct regions: source, gate, and drain. The gate controls the voltage experienced at the drain. They’re relatively inexpensive available as JFET (junction field effect) and MOSFET (metal-oxide-semiconductor field-effect) variants.

A PCB with transistor and other components
A PCB with transistor and other components
Resource: https://www.youtube.com/watch?v=PJ5Cm1d2Wgc

Why Do We Use Transistors on a PCB

The modern-day transistor is tiny and usually contained in an IC. However, there are times when their discrete form is necessary, such as when an application requires a simple switching or current amplification device.

So, what does a transistor do in a circuit? When used on a printed circuit board, the device performs various functions, including the following:

  • Switching electric currents to on or off
  • Signal detection and rectification
  • Signal and current or voltage amplification
  • Voltage stabilization


The transistors are built to different specifications to perform the above roles. Examples include low and high-power and low and high-frequency types.

NPN and PNP-type transistor symbols
NPN and PNP-type transistor symbols
Resource: https://www.youtube.com/watch?AcxDiesy-nI

How to Read a PCB Transistor

Before testing a circuit board transistor, you must know how to read it first. The device usually has markings on the circuit board for easy identification.

To read them, look for two arrows with a line connecting them. The line extends to the other side of the arrow triangles. The arrows will usually point toward each other or in opposite directions.

Another line extends from the line connecting the arrows to the side. Rotate the board so the line from the side points to the left.

In this position, there are two ways of knowing if the top and bottom arrows are PNP or NPN emitters and collectors: by observing the top and bottom arrows/triangles.

  • Top Arrow – if pointing up, you’re looking at the collector of an NPN transistor; a PNP type if it points down.
  • Bottom Arrow – if it points up, it indicates the emitter of the PNP transistor; an NPN type if pointing down.
  • Left Line – this line usually points to the base pin. This line is absent in some PCBs, and the transistor has a metal housing. In such cases, the metal is the base.
How to test a transistor
How to test a transistor
Resource: https://www.youtube.com/watch?v=fOvxwlJADkY

How to Test a Transistor on a Circuit Board

PCB transistors serve essential functions and must operate flawlessly. But like other circuit components, they can fail and stop working. You want to know when that occurs. Here, we show you how to test one using a digital multimeter.

  • Switch off power to the circuit board by disconnecting it from the supply or removing the battery.
  • Dissipate the capacitors by touching their leads with metal. This action ensures the circuit contains no power whatsoever.
  • Identify the base, emitter, and collector leads using the earlier method.
  • Set the multimeter to diode test mode (or set it to read Ohms if using the analog type)
  • Connect the black probe to the base and use the other probe to touch the emitter. Note the reading.
  • Touch the red probe to the collector. The reading should be similar to when you touched the emitter with the other testing probe.
  • Now, reverse the multimeter probes. Touch the base with the red probe and use the black probe to touch the emitter and collector. Again, note the readings.
  • If the earlier reading was high but turned low when you reversed the probes (or vice versa), the device is working well.
PCB transistor replacement
PCB transistor replacement
Resource: https://www.youtube.com/watch?v=Q9za3fSMsJg

How to Replace a Transistor on a Circuit Board

A faulty transistor affects the functionality of your PCB. Here are the steps to replace one if faulty, including best practices, so you do not damage other board parts.

You’ll need a soldering iron, solder wire, and de-soldering wick or vacuum. It would help if you also had a pair of tweezers to hold the device during removal and installation.

PCB Transistor Removal

  • Ensure your workspace is well-ventilated.
  • Turn on the soldering iron and bring its temperature to about 200°C.
  • Place the de-soldering wick/copper braid on the device terminals and press the soldering iron against it. The copper’s function is to absorb the de-soldering heat.
  • The solder will melt after a few seconds. Remove the copper braid and cut off its used section.
  • Repeat the de-soldering steps on the other terminals.
  • Pull out the transistor from the PCB using a pair of tweezers.

How to Solder Transistor to PCB

  • Ensure you have the correct replacement package and pin-out (same material, polarity, frequency range, voltage range, etc.).
  • Clean the installation surface to remove contaminants and the remains of de-soldering.
  • Pre-tin the leads by melting solder on their surfaces and wicking it away with the soldering braid. Coat the soldering iron with solder, too.
  • Mount the transistor on the PCB. Solder it by touching each terminal with the soldering iron and solder wire.
  • Finally, clean the installation surface using the solder pump or vacuum.

Conclusion

The PCB transistor is a semiconductor device that amplifies or switches current to regulate, enhance, or control electrical functions. Your PCB needs it to function correctly. This device serves valuable roles that include current regulation and enhancement. Knowing how it works or how to read and test it is essential to ensure your circuit board works correctly.

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