Hardware · Topic 10 of 13
Power Supplies
Choose and install a power supply unit by wattage, connector types, modular cables, efficiency rating, input voltage and redundancy, with safe handling.
- 5 min
- A PSU converts AC to DC for computer parts.
- Pick a wattage with headroom above the total load.
- The 24 pin connector powers the board, 4 or 8 pin powers the CPU and PCIe 6+2 powers graphics cards.
What it is
The power supply unit (PSU) converts alternating current (AC) from the wall outlet into the lower direct current (DC) voltages that computer parts need. Every part in the case draws power from it.
Choosing a PSU means checking that it has enough wattage, the right connectors and the right form factor for the case.
How it works
Wattage and sizing
Wattage is the power a PSU can supply. Add up the needs of the central processing unit (CPU), graphics card, drives, fans and other parts, then pick a PSU with some spare capacity. Headroom of roughly a quarter above the estimated load is a common guide, since a PSU works best below its limit and parts may be added later.
A graphics card often draws the most power in a gaming or workstation PC. The card maker's recommended PSU size is a good starting point.
Form factors
A PSU must fit the case.
Advanced Technology eXtended (ATX) is the standard size for desktop towers. A small form factor type (SFX) is used in compact cases. Check the case's PSU size before buying.
Connectors
| Connector | Powers |
|---|---|
| 24 pin (20+4) main | The motherboard; the 4 pin part detaches so it fits older 20 pin boards |
| 4 pin or 8 pin (4+4) CPU | The CPU on the motherboard |
| Peripheral Component Interconnect Express (PCIe) 6 pin or 8 pin (6+2) | High power graphics cards |
| Serial Advanced Technology Attachment (SATA) power | SATA drives and optical drives |
| Molex (4 pin) | Older drives and some fans or accessories |
Connectors are keyed so that they only go in one way. Do not force them. The CPU power connector is not the same as the PCIe connector, even if they look alike.
Cables to the case front panel, the graphics card and drives come from separate leads on the PSU.
Output voltages
A PSU turns mains alternating current into three main direct current (DC) rails.
| Rail | Wire colour | Typical use |
|---|---|---|
| 12 V | Yellow | CPU, graphics card, fans and drive motors |
| 5 V | Red | Logic circuits and drive electronics |
| 3.3 V | Orange | Logic circuits on the board and cards |
Black wires are ground. A PSU tester reads the rails when you plug in the main connector and switch on.
With a multimeter, set it to DC volts and probe the connector pins while the PSU is running. Each rail should stay within about 5 percent of its rating.
Modular types
| Type | Description |
|---|---|
| Non modular | All cables are fixed to the PSU; extra ones fill the case |
| Semi modular | Main cables are fixed, others are removable |
| Fully modular | Every cable is detachable, so you use only what you need |
Modular units improve airflow and cable tidiness.
Efficiency
The 80 PLUS programme certifies how efficiently a PSU converts power. Above the basic 80 PLUS level, the ratings from lowest to highest are Bronze, Silver, Gold, Platinum and Titanium. A more efficient PSU wastes less energy as heat, so it costs less to run and may be quieter.
Input voltage
Some PSUs have a switch for 115 volt or 230 volt input, used for different countries. Many modern units sense the input automatically.
Setting a PSU to 115 volts and plugging it into 230 volt mains can destroy it. Check the switch before connecting power.
Redundant power
Servers and some other systems use two PSUs so that the system keeps running if one fails. Such units are often hot swappable and connected to separate power circuits.
Safe installation
- Power off the computer and unplug the PSU.
- Fit the PSU in the case with the fan facing the correct way, as the case design requires, and screw it in.
- Connect the main 24 pin and the CPU power cable first.
- Add power for the graphics card and the drives.
- Tidy cables so they do not block fans.
- Check the voltage switch if there is one, then connect power and test.
Never open a PSU. Its internal capacitors can hold a dangerous charge for some time after unplugging, and the unit has no user serviceable parts.
Laptops and power adapters
A laptop uses an external adapter that supplies DC power. Use the adapter specified for the model, or a trusted compatible one. The wrong voltage or connector polarity can damage the laptop.
What to remember for the exam
- A PSU converts AC to DC for computer parts.
- Pick a wattage with headroom above the total load.
- The 24 pin connector powers the board, 4 or 8 pin powers the CPU and PCIe 6+2 powers graphics cards.
- SATA power is for SATA drives; Molex is older.
- Modular PSUs have removable cables; this helps airflow.
- 80 PLUS ratings range from Bronze to Titanium.
- A 115V/230V switch must match the local mains.
- Never open a PSU case, even when unplugged.
Common mistakes
Buying too small a PSU. A weak supply can lead to random shutdowns under load.
Confusing CPU and PCIe power connectors. They look alike but are keyed differently, and mixing them can cause damage.
Wrong voltage setting. A mis set switch can destroy the PSU.
Opening the PSU to repair it. Capacitors can keep a charge. Replace the unit instead.
Trusting wattage numbers alone. A cheap unit may not deliver its label rating, so choose a trusted brand.
Check yourself
What does the 24 pin connector power?
The motherboard.
What does a modular PSU let you do?
Attach only the cables you need, which reduces clutter and improves airflow.
Why should you check the 115V/230V switch before powering a PSU?
A wrong setting can damage the unit or fail to power the system.
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