How To Identify Circuit Board Components: Ways To Identify Active and Passive Components

Here’s an article that shows you how to identify circuit board components.

As a newbie PCB designer, you might get slightly confused when identifying components on old board design layouts.

But you might see a few familiar parts when looking at assembled circuit boards.

Regardless, you should know how to identify circuit board components because the design process might involve reverse engineering.

Part of your assignment might require studying a PCB layout diagram or an assembled board before creating a new, improved design.

Read on to learn more!

From The PCB Assembly

If you look at an assembled printed circuit board keenly, you will spot some information printed on select electronic components.

This information is typical on parts like axial/radial capacitors and integrated circuits.

A PCB with a chip on board (note the information printed on the chip)

And the data will usually include the following.

  • Manufacturer
  • Manufacturing Part Number (MPN)
  • Component value
  • Serial number

You can input this data into a search engine to learn more about the electrical component.

Parts like through-hole resistors have color coding that indicates the component value, which you can use to find replacements.

Generally, it is easier to identify through-hole components when repairing or replacing them because they are sizable.

But dealing with surface-mount technology is tricky because the pieces are tiny.

From the Bill of Materials

Some PCB assemblies have the component reference designators written on the silkscreen layer.

These designators simplify the component identification process because you only have to match this data with the Bill of Materials.

A circuit board with reference designators printed on the silkscreen layer

But for this identification process to work, the BOM must be complete.

A complete BPM should show the MPN (Manufacturing Part Number) or SPN (supplier part number).

These reference designators might be shortened versions of the component name of different letters.

Here are the designators for typical circuit components.

DesignatorElectronic Component Full Name
CCapacitor
DDiode
RResistor
LInductor
ATTAttenuator
LEDLight Emitting Diode
QTransistor
POTPotentiometer
TTransformer
ZD/ZZener Diode
XTransducer
XTALCrystal
YCrystal/Oscillator
VRVariable Resistor
TCThermocouple
U or ICIntegrated Circuit
TBTerminal Block
LSLoud Speaker
MOVMetal Oxide Varistor
FFuse
CBCircuit Breaker
BTBattery
BRBridge Rectifier
GOscillator
JJumper or Jack
PPlug
KRelay/Contractor
PSPower Supply
TPTest Point

From The Design Files

What happens if you don’t have a complete BOM? The best solution is to look for the PCB layout files until you spot the specific component.

With complete libraries, you can point a part to its supplier information, enabling straightforward identification.

You can also get the manufacturing part number using this method.

A PCB layout file

A PCB layout file

After retrieving this data, you can:

  • Search online
  • Go to the BOM to compare the data
  • Compare the part’s footprint with the data in the component datasheet

These three documentation methods are highly precise, and we recommend them to identify PCB components.

What if You Can’t Identify a Component From the Design Documentation?

Finding documentation to identify the components can be impossible when dealing with old PCB assemblies.

Or the old board might have the markings on its silkscreen layer scrubbed off.

In such a case, you can identify board components using either of these two methods.

Test the Component

Before testing components, you must first understand how they work.

And to understand how they work, you must know the component types, either mechanical or electrical.

Mechanical components usually have an aluminum construction, although some are bronze, copper, or steel-made.

But all operate mechanically, with their primary role being to support the PCB operations.

On the other hand, electronic components provide electrical functions and can be passive or active.

Passive Components

Passive electrical components have no control over the flow of electric current inside them.

Also, they do not directly transfer electronic signals.

Most have two leads (passive two-terminal components), and they include the following.

Capacitors

A capacitor stores electric charge and has a farad rating to denote its size.

Smaller capacitors can have mF (millifarad) or µF (microfarad) ratings.

You can test for this capacitance value if you can’t see the circuit section marked C (reference designator).

Electrolytic capacitors

Electrolytic capacitors

Resistors

Resistors are some of the most typical passive electrical PCB components, and their purpose is to restrict the flow of electrical current through them.

Also, they can limit voltage flow across their pins.

The best way to identify these components is by looking at the color coding (color bands) on their surface.

These bands show resistance value and tolerance.

However, this identification can only work on through-hole technology resistors.

Resistor color coding

Resistor color coding

Surface-mount technology resistors are tiny and don’t have color bands.

So you might have to test the component to see if there is a voltage or electrical current drop.

Fuses

Fuses usually have a glass body with a visible fuse wire inside and metal caps on the glass tube ends.

So it is easier to identify them visually than test them electrically.

But a surface-mount fuse can have different physical characteristics.

So you might have to push some current through it.

Inductors

Inductors store electrical energy in a magnetic field as electrical current flows through them.

So you can test the component to see if it becomes magnetic.

Inductors mounted next to capacitors on a PCB

Inductors mounted next to capacitors on a PCB.

But if it has silkscreen markings, look for the reference designator “L” or inductor value given in:

  • H (Henry)
  • mH (millihenry)
  • µH (microhenry)
Connectors

As the name suggests, these pieces link, connect or integrate several components.

Usually, they help attach sizable components to printed circuit boards or two PCBs together.

In most cases, connectors integrate with jumpers, forming the pins on the ends.

And these connector pins can be small or large to suit different applications.

For instance, slim connectors are ideal for connecting flat cables, while large units can plug into them.

Batteries

Batteries store electrical energy, meaning you can measure the voltage across their terminals to identify the component. Alternatively, you can check the component’s shape.

It is usually either coin, cylindrical, or rectangular-shaped.

Also, some batteries have green or blue shrink wrapping. You can use this external characteristic to identify the component.

Active Components

Active electrical components directly control the flow of electrical signals in the circuit.

And like the passive type, they are available in through-hole and surface mount technologies.

The three primary active components include the following.

Integrated Circuits

Integrated circuits are usually the brains behind the circuit and are easy to spot because they are flat and broad.

They can include operational amplifiers, timer devices, electronic oscillators, or memory devices.

Integrated circuits on a circuit board

Integrated circuits on a circuit board

When identifying through-hole integrated circuits, consider the following.

  • These chips can come in dual or single-inline packages
  • They have a long body
  • The microchips contain multiple leads on one or both sides

But surface-mount integrated circuits can come in a single inline, dual inline, quad chip, or BGA (Ball Grid Array) package.

A BGA integrated circuit

A BGA integrated circuit

Diodes

Diodes are available in different types, but all regulate voltages one way.

So they are one-way current/voltage switches that can rectify AC to DC.

And the best way to identify them on circuit boards is visually.

Through-hole diodes are usually cylindrical with black, opaque glass shells and protruding leads.

But the shape can vary depending on the type, and the various types include:

Diodes mounted on a circuit board

Diodes mounted on a circuit board

But you can identify surface-mount diodes using their polarity marking.

LEDs are diodes but operate differently; they convert electrical energy into light.

Transistors

These electronic devices amplify, insulate, and rectify electrical signals in circuit boards and integrated circuits.

But they are best known as electronic switches because they can conduct or cut off electronic signals.

It is vital to note that transistors are tiny components.

And they can operate on extremely low voltages even without a filament current.

Also, they are available in several types, which include the following.

  • MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistors)
  • PNP Darlington Transistor
  • N-Channel MOSFET
  • PNP BJT (Bipolar Junction Transistor)
Different transistor types

Different transistor types

Identify the Component Function

It might not be wise to test some components (especially active ones) because the electrical current can damage them.

So try to identify the component’s function in the circuit.

You can do so by checking its pinout and the other components connected to its pins.

Also, you can check the component’s power level by analyzing the linked parts and board markings.

An up-close image of a PCB (note the component pinout)

An up-close image of a PCB (note the component pinout)

Wrap Up

As you can see, several ways exist of identifying circuit board components.

But the most accurate method is using the design documentation.

However, you can resort to manual identification if this information is unavailable.

This process can either be by electrical testing, identifying the component function, or visual checking.

That’s it for this article. We hope you found it insightful.

If you need help identifying these components, comment below. We’ll be in touch to assist.