Skip to content
Study Hall
Lesson 3 of 5Hardware, Software and Operating Systems
Unit 2 · Lesson 3 of 5

Building a System and Connecting Peripherals

Power supplies, ports, expansion cards, and the order of operations for assembling a computer without destroying it.

4 min read8 practice questionsAsk AI about this →

Key things to remember

  1. 1Drain residual power by unplugging and holding the power button for five seconds before opening a case.
  2. 2A chip can be destroyed by a static charge of about 30 V — far below the ~3000 V you can feel.
  3. 3PSU rails: +12 V (CPU and GPU, the main load), +5 V and +3.3 V for logic and drives.
  4. 4Size a PSU with roughly 30% headroom above the measured system draw.
  5. 5USB-C is a connector shape, not a speed standard.
  6. 6Hot-swappable: USB, Ethernet, SATA. Never hot-swap: RAM, CPU, PCIe cards.
  7. 7Motherboard standoffs prevent the board shorting against the case.

Assembling a computer is mostly about sequence and static. The parts only fit one way — connectors are keyed so a reversed plug will not seat — but the order you do things in decides whether the job takes twenty minutes or two hours, and one careless moment can put a charge through a component that kills it without leaving a mark.

You can destroy a chip with a charge you cannot feel.The numbersYou feel a shock at about 3 000 V.You hear a snap at about 5 000 V.A CMOS chip can die at 30 V.The habitsAntistatic wrist strap, clipped to chassis ground.Hold cards by the edges, never the contacts.Keep parts in the silver antistatic bag until fitted.Before you open a case1. Shut down, unplug the mains cord, then hold the power button for five seconds to drain the PSU.2. Never open a power supply or a CRT monitor — capacitors inside stay charged at lethal voltages.3. Work on a hard, non-carpeted surface, and keep drinks off the bench.
Fig 1The charge that destroys a chip is well below the one you can feel.
Do this before opening any case
Shut down, unplug the mains cord, then hold the power button for five seconds to drain the power supply's capacitors. Wear a grounded antistatic strap, or at minimum touch the bare metal chassis before handling components. Never open a power supply or a CRT monitor: the capacitors inside hold lethal voltage long after the unit is unplugged.

The power supply

The PSU converts mains AC into the DC rails the computer needs: +12 V (by far the biggest load, feeding the CPU and graphics card), +5 V and +3.3 V for logic and drives. Two numbers matter when choosing one. Wattage must exceed the total draw of the system with headroom — around 30% is a sensible margin, because a supply running near its limit runs hot and ages fast. Efficiency (the 80 PLUS ratings) tells you how much of the mains power reaches the components instead of becoming heat.

ConnectorGoes toNote
24-pin ATXMotherboardThe main power connector; often splits 20+4
8-pin EPS (4+4)CPU power, near the socketEasy to forget — the system will not POST without it
6/8-pin PCIeGraphics cardA card may need two; never use a Molex adapter on a high-draw card
SATA powerDrivesFlat L-shaped connector
Front-panel headerCase power switch, LEDs, USBTiny individual pins; check the manual for polarity

Ports and what they carry

Identify a port by its shape, then by what it carries.USB-Akeyboards, drives — up to 10 Gb/s on USB 3USB-Creversible; data, video and up to 100 W powerHDMIvideo + audio to a monitor or TVDisplayPortvideo, higher refresh rates than HDMIRJ-45wired Ethernet3.5 mmanalogue audioRule of thumb: if it carries video it also carries audio, except the old VGA connector, which carries neither digitally.
Fig 2Identify a port by shape, then by what it carries.
  • USB — the universal peripheral bus. USB 2.0 is 480 Mb/s, USB 3.0 is 5 Gb/s, USB 3.2 Gen 2 is 10 Gb/s. USB-C is a connector shape, not a speed — a USB-C port may be anything from USB 2.0 to Thunderbolt.
  • HDMI / DisplayPort — digital video plus audio. DisplayPort generally supports higher refresh rates and multiple monitors from one port.
  • RJ-45 — wired Ethernet, 1 Gb/s on almost all current boards.
  • 3.5 mm — analog audio, the only analog signal left on most machines.
  • Thunderbolt — runs PCIe and DisplayPort over a USB-C connector, which is how external GPUs and high-speed docks work.
Hot-swappable or not
USB, Ethernet and SATA (with AHCI enabled) can be connected while the system is running. RAM, CPU, PCIe cards and the CPU cooler absolutely cannot — connecting or removing those with power applied destroys components.

Assembly order that actually works

  1. Fit the CPU into the socket OUTSIDE the case, on the bench. Align the triangle marker; never force it — a correctly aligned CPU drops in under its own weight.
  2. Apply thermal paste (a pea-sized dot) and attach the cooler. Tighten the mounting screws in a diagonal cross pattern, a little at a time.
  3. Fit the RAM, pressing until both retention clips snap closed. Use the slots the manual specifies for dual channel — usually slots 2 and 4.
  4. Install the M.2 drive if you have one. It is far easier now than after the graphics card is in.
  5. Mount the I/O shield, then the motherboard on its standoffs. Standoffs matter: a board touching the case directly will short out.
  6. Install the PSU and route the 24-pin and 8-pin EPS cables before adding the graphics card, which will block access.
  7. Fit expansion cards into the top PCIe x16 slot, then connect SATA data and power to any drives.
  8. Connect the front-panel header last, checking polarity for the LEDs, then test-boot before closing the case.
If it does not POST
Work through the four usual suspects in order: the 8-pin CPU power cable (most commonly forgotten), RAM not fully seated (press until both clips click), the front-panel power switch wired to the wrong pins, and the monitor plugged into the motherboard's video output when a graphics card is installed.

Worked examples

Reveal one step at a time and try to beat it to the next line.

Example 1
A build draws 125 W for the CPU, 220 W for the graphics card, 30 W for drives and fans, and 25 W for the motherboard and RAM. Recommend a power supply wattage with sensible headroom.
Example 2
A newly assembled computer has fans spinning and case lights on, but there is no display and no beep. List the checks in the order you would actually do them, with a reason for each.

Where marks get lost

  • Forgetting the 8-pin EPS CPU power connector — the fans spin, so the machine looks alive while never posting.
  • Not pressing RAM until both clips click. Partially seated RAM produces no display and no beep.
  • Handling cards and modules by their gold contacts instead of their edges.
  • Mounting a motherboard without standoffs, or leaving an extra standoff under an area with no screw hole, which shorts the board.
  • Opening a power supply to "check" it. Its capacitors hold a lethal charge long after it is unplugged.
  • Applying far too much thermal paste. A pea-sized dot spreads correctly under cooler pressure; excess insulates instead of conducting.

Flashcards 9

How do you drain residual power before opening a case?
At roughly what voltage can ESD destroy a CMOS chip?
Which PSU rail carries the CPU and GPU load?
Which connector is most often forgotten in a new build?
+ 5 more

Practice

1
A new build has spinning fans and lit LEDs but no display and no beep. What should you check first?
2
A system draws 95 W (CPU), 180 W (GPU), 20 W (drives) and 30 W (board and fans). Applying 30% headroom, what PSU wattage is the minimum? Round to the nearest watt.
W
3
Why is a wrist strap clipped to the chassis rather than just working carefully?
4
What must be fitted between a motherboard and a metal case to prevent a short circuit?
5
Which of these can safely be connected while the computer is running?
6
Put these assembly steps in a sensible order.
  1. 1Attach the cooler with thermal paste
  2. 2Mount the board on standoffs and connect the 24-pin and 8-pin cables
  3. 3Install RAM in the slots the manual specifies
  4. 4Fit the CPU into the socket on the bench
7
A USB-C port on a laptop transfers a large file far slower than expected. What is the most likely explanation?
8
Match each connector to what it powers.
24-pin ATX
8-pin EPS
6/8-pin PCIe
SATA power

Spot a mistake on this page? Let us know