US What a PSU does
A computer power supply, or PSU (power supply unit), turns the power coming into a computer into the steady low-voltage DC that its parts run on. In a desktop it takes 100 to 240 V AC from the mains and produces several DC rails: 12 V for the processor and graphics card, 5 V and 3.3 V for drives and logic, and a 5 V standby rail that keeps wake-up circuits alive when the computer is off.
Modern supplies are switch-mode designs. They rectify the mains, switch it at high frequency through a small transformer and regulate the result, which is far smaller and more efficient than the old linear designs. The motherboard then converts the 12 V rail down again, through its voltage regulators, to the core voltage the processor needs.
Not every computer has a PSU inside the case. Mini PCs often use an external adapter, and many industrial PCs take DC straight from a vehicle, battery or cabinet supply and convert it on the board.
Types of computer power supply
The right type follows the power available on site and how critical the system is.
| Type | Input | Output | Typical use |
|---|---|---|---|
| ATX or SFX internal PSU | 100 to 240 V AC mains | 12 V, 5 V, 3.3 V, 5 V standby | Desktops, towers, workstations |
| Redundant hot-swap modules | AC or DC, often two feeds | Mainly 12 V to the backplane | Rack servers, storage, network kit |
| External adapter with DC jack | AC mains | Single 12 V or 19 V DC | Mini PCs, thin clients, displays |
| Wide-range DC input on the board | DC such as 9 to 36 V | Converted on the motherboard | Industrial, vehicle and cabinet PCs |
| DC-DC power module | Vehicle or battery DC | Regulated 5 V and 12 V | PC/104 stacks, rugged systems |
ATX and its smaller SFX version are defined in Intel’s desktop power supply design guide. The latest ATX 3.x revisions add the 12V-2×6 connector for high-power graphics cards.
Redundant power supplies
A redundant power supply uses two or more PSU modules that share the load, so the system keeps running if one fails. The common arrangement is 1+1, where either module can carry the whole load alone, or N+1 in larger systems. Modules are hot-swap, so a failed one slides out and is replaced without shutting down.
For full protection, each module is fed from a different power source, often called A and B feeds. Redundancy covers a failed PSU or a lost feed, but not a site-wide outage; that needs a UPS. Our rack mount servers include 1U edge AI servers with redundant AC or redundant DC power.
DC power jacks and wide-range input
Small and industrial computers often take DC rather than mains. The way that power connects matters as much as the voltage, because a loose plug on a vibrating machine causes the same outage as a failed supply.
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DC barrel jack. The round socket on most mini PCs and adapters. Common plugs are 5.5 mm with a 2.1 or 2.5 mm centre pin, and locking versions stop the plug working loose.
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Terminal block. Screw or spring terminals, often a 2- or 3-pin pluggable block, used on DIN-rail and cabinet PCs wired to a 24 V supply.
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Wide-range DC input. Inputs such as 9 to 36 V DC run directly from 12 V and 24 V vehicle, battery and cabinet supplies, and ride out normal voltage swings.
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Ignition sensing. Vehicle PCs watch the ignition line and shut down cleanly after the engine stops. Our FAQ on how ignition control works covers the timing.
Choosing wattage, efficiency and protection
Add up the peak power of the processor, graphics card, drives and expansion cards, then add headroom, commonly 20 to 30 percent, so the supply runs in its efficient range and copes with transient spikes. GPU-heavy systems need the most care, because one card can draw several hundred watts.
Efficiency is usually shown by an 80 PLUS rating, from the base level through Bronze, Silver, Gold and Platinum to Titanium. A higher rating wastes less power as heat, which matters in sealed or fanless enclosures. Good supplies also include over-voltage, over-current, short-circuit and over-temperature protection.
In rugged systems, the supply also has to meet the environment. Our industrial power supplies include isolated DC-DC supplies built to MIL-STD-1275, MIL-STD-704 and MIL-STD-461, and PC/104 power supplies that stack with the computer. Where a clean shutdown matters, an industrial UPS with supercapacitor storage keeps the system running through short dropouts.
Quick answers on power supplies
Power supplies: common questions
What does PSU stand for?
Power supply unit. The PSU is the part of a computer that converts incoming power into the low-voltage DC the components use.
What happens when a PSU fails?
Usually the computer will not start, or shuts down under load. A failing supply may also cause random restarts. In a server with redundant supplies, the second module takes over and the system keeps running.
How many watts does a computer need?
It depends on the parts. An office PC or mini PC may draw under 100 W, while a workstation with a high-end GPU can need 850 W or more. Size the supply to the peak load plus headroom.
What is a switching power supply?
A supply that regulates by switching at high frequency. Switch-mode supplies chop the input at tens or hundreds of kilohertz through a small transformer, which makes them smaller and more efficient than linear supplies. Almost all computer PSUs work this way.
Is a UPS the same as a power supply?
No. A PSU converts power for the computer. A UPS (uninterruptible power supply) sits before it and keeps power flowing from a battery or supercapacitor when the input fails.
We specify the power input, supply and backup for every system we build, and fit and validate it as standard before the system ships. Tell us the power available on site and the load, and we will match the supply to it.