Electronic Components on a Motherboard

25 min

A city on a board

Seen from above, a computer's main circuit board looks like a small city: wide "roads" of copper, tall "buildings" of capacitors and heat sinks, and "squares" of black chips. This board is the motherboard. Every component you have met in this unit is on it, many times over, and some of them in their millions. Recognising them helps you understand what each part of a computer does, and why it must be handled with care.

Definition

Motherboard: the main printed circuit board of a computer, which holds the processor, memory and other components and connects them to each other and to the power supply.

Printed circuit board (PCB): an insulating board with thin copper tracks that connect components soldered onto it.

Integrated circuit (IC, "chip"): a complete circuit of many transistors, resistors and capacitors made on one small piece of silicon.

Components you can recognise

Component on the motherboard	What it is	What it does there
Processor (CPU), in its socket	Integrated circuit of billions of transistors	Carries out the program's instructions; its transistors work as fast switches (lesson 3)
Memory (RAM) modules	Integrated circuits holding tiny capacitors and transistors	Store data and programs while the computer is running; lost when the power goes off
Chipset (often under a heat sink)	Integrated circuit	Directs data between the processor, storage, expansion slots and ports
Capacitors (cylinders and tiny blocks)	Passive	Smooth the supply voltage and absorb sudden changes in current (lesson 2)
Inductors (chokes) and power transistors	Passive and active	Form the voltage regulators that turn the supply's 12 V into the low voltage the processor needs
Resistors (tiny surface-mount blocks)	Passive	Limit currents and set voltages (lesson 1)
Diodes and LEDs	Semiconductor	Protect against reversed or excess voltage; LEDs show power and activity
Crystal oscillator (small metal can)	Quartz crystal	Vibrates at a precise frequency to give the timing signal, the computer's "heartbeat"
Coin cell (battery)	Cell	Keeps the clock and settings memory running when the computer is unplugged
BIOS or UEFI chip	Integrated circuit (memory)	Holds the start-up program that tests the hardware and loads the operating system
Slots, sockets and connectors	Connectors	Join expansion cards, drives, the power supply and outside devices

On modern boards almost all small components are surface-mount parts, soldered flat onto the copper pads. They are far smaller than the wired resistors of lesson 1, so their values are printed as codes or not marked at all. Older boards and power supplies use through-hole parts, whose wire legs pass through holes in the board.

Surface-mount components on a red circuit board: a square integrated circuit with rows of fine legs, surrounded by tiny rectangular resistors and capacitors labelled R and C on the board. Photo: Audriusa (uploader), CC BY-SA 3.0, via Wikimedia Commons

Worked example 1: the voltage regulator

A processor needs 1.2 V at a current of 50 A from a 12 V supply. (a) What power does it use? (b) If the regulator were 100% efficient, what current would it draw from the 12 V supply?

\text{(a)}\ P = VI = 1.2 \times 50 = 60\,\mathrm{W} \qquad \text{(b)}\ I = \frac{P}{V} = \frac{60}{12} = 5.0\,\mathrm{A}

A low voltage at a high current is why the regulator around the CPU socket needs large chokes, capacitors and power transistors, and why the processor needs a heat sink and fan. (Illustrative values.)

Worked example 2: the coin cell

The coin cell on a motherboard is rated 3.0 V and stores about 220 mA h of charge. The clock circuit draws 5.0 µA. Roughly how long could the cell last if the computer is never switched on?

t = \frac{220\,\mathrm{mA\,h}}{0.0050\,\mathrm{mA}} = 44\,000\,\mathrm{h} \approx 5\ \text{years}

(Illustrative values; real cells last somewhat less.)

Warning

Safety when handling circuit boards

Switch off and unplug a computer before opening it. Never open the power supply box itself: its large capacitors work at mains voltage and can stay charged after unplugging.
Static electricity from your body (S1 electrostatics) can destroy the tiny transistors in chips without any visible spark. Touch an earthed metal object, or wear an earthed wrist strap, before handling a board, and hold boards by their edges.
Old boards contain metals and other substances that pollute soil and water. Never burn electronic waste or throw it in an open dump; take it to a collection point for electronic waste.

Tip

Read the board like a map. Big square chip with a fan: processor. Long thin slots beside it: memory. Silver cylinders and black cubes crowded round the processor: its power supply. A coin cell: the clock. Edge connectors: the doors to the outside world.

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Practice

What is a motherboard? What is a printed circuit board?
Name three passive components and one active component found on a motherboard, and give the job of each.
Why do motherboards have so many capacitors near the processor?
What is the job of (a) the crystal oscillator, (b) the coin cell, (c) the BIOS or UEFI chip?
Explain the difference between surface-mount and through-hole components.
A processor uses 1.0 V at 65 A. Find its power, and the current it would draw from a 12 V supply through a regulator that is 100% efficient.
Why must you touch an earthed metal object before handling a motherboard?
A school wants to get rid of 20 old computers. Evaluate two choices: burying them in the school grounds, or taking them to an electronic-waste collection point.

Show answersHide answers

A motherboard is the main circuit board of a computer, holding and connecting the processor, memory and other components. A PCB is an insulating board with copper tracks that connect the components soldered onto it.
Passive (any three): resistors limit current; capacitors smooth the supply; inductors (chokes) store energy in the voltage regulator. Active: transistors switch, in the processor or in the regulator.
The processor's current changes very quickly; the capacitors store charge close by and supply or absorb these sudden changes, keeping its supply voltage steady.
(a) Gives a precise timing signal; (b) keeps the clock and settings running when the computer is unplugged; (c) holds the start-up program that tests the hardware and loads the operating system.
Surface-mount parts are tiny and soldered flat onto pads on the board's surface; through-hole parts have wire legs that pass through holes in the board and are soldered on the other side.
$P = 1.0 \times 65 = 65\,\mathrm{W}$ ; $I = \frac{65}{12} = 5.4\,\mathrm{A}$ .
To discharge any static charge on your body to earth, so it does not flow through the board's chips and damage their transistors.
Burying them lets metals and other harmful substances leak into soil and water, and wastes materials that could be recovered. A collection point can repair or recycle parts safely, so it is clearly the better choice, even if transport takes some effort.

Key points

The motherboard is a printed circuit board that holds and connects the processor, memory, chipset and other parts.
It uses every component in this unit: resistors, capacitors, inductors, diodes, LEDs and transistors, most of them in integrated circuits.
Capacitors and regulators keep the processor's low-voltage, high-current supply steady; a crystal provides timing; a coin cell keeps the clock running.
Unplug before opening, beware of static, and dispose of electronic waste responsibly.

1A city on a board

2Components you can recognise

A city on a board

Components you can recognise