Power is one of the easiest PC topics to mix up on the CompTIA A+ 220-1201 exam, because you’re dealing with two types of electricity at once. The wall provides AC power (alternating current), while the power supply unit (PSU) converts it into DC power (direct current) that your components can use.
Objective 3.6 focuses on three practical details you’ll see in both questions and real hardware. First, you need to tell the difference between common input ranges, 110 to 120 VAC versus 220 to 240 VAC, and know what that means for compatibility. Next, you’ll learn the main output rails, 3.3V, 5V, and 12V, and which parts of a PC rely on each.
Finally, you’ll connect it all to the motherboard through the 20+4-pin ATX connector, the primary power plug on most desktops. Power can injure you and damage equipment, so always unplug the system and discharge it before you touch internal cables.
AC input to a PC power supply: 110 to 120 VAC vs 220 to 240 VAC
When you look at a PSU, it helps to separate input from output in your mind. The wall provides AC power (either 110 to 120 VAC or 220 to 240 VAC in many regions). The PSU’s job is to convert that AC into the stable DC voltages your PC needs. If the PSU is compatible with the outlet (and set correctly, if it has a selector), your components still receive the same DC rails.
This is a common exam point because CompTIA A+ questions often test what changes at the wall, versus what stays the same inside the PC.
What changes when the wall voltage doubles (and what does not)
A desktop PC does not “run on” 120 V or 240 V internally. It runs on DC rails generated by the PSU (like 3.3 V, 5 V, and 12 V). So if you move the same computer from a 120 V region to a 240 V region, the motherboard and GPU still expect the same DC outputs. The only requirement is that the PSU can accept the new AC input.
What does change is how much current the PSU draws from the wall to make the same amount of power. Power is commonly described with this relationship: watts = volts x amps. Keep it simple: if the voltage goes up and the watts stay about the same, the amps go down.
A plain example: imagine a PC that is using about 240 watts while gaming.
- At 120 V, that is about 2 amps from the wall (240 W = 120 V x 2 A).
- At 240 V, that is about 1 amp from the wall (240 W = 240 V x 1 A).
The PC is still using about 240 watts either way. The PSU is still feeding the same DC rails to the parts. The difference is the current on the input side. Lower input current can reduce stress on wiring and can be helpful for high-power systems, but it does not make the computer “faster” or change the DC voltages your components receive.
How to spot a manual voltage selector and avoid the classic mistake
Most modern PSUs are auto-ranging, meaning they accept a wide input range without any switch. Some older or budget units have a manual voltage selector, often a small red switch labeled 115V/230V (or similar). It is usually on the back near the power socket.
If that switch is set wrong, you can get predictable failures:
- Set to 115 V but plugged into 230 V: the PSU may fail immediately. A fuse can blow, and the PSU can be damaged.
- Set to 230 V but plugged into 115 V: the PC often won’t power on (or it may start and shut off) because the PSU is not getting the input it expects.
Treat the selector like a “region setting” for the PSU. Before you press the power button, take 30 seconds and verify the basics. A short, technician-style check helps prevent the classic mistake:
- Check the PSU label for the accepted AC input (range or fixed value).
- Check the red switch (if present) and match it to the local voltage.
- Check the outlet (know if you’re on 120 V or 230 V service).
- Check the UPS or surge protector rating and output voltage (a travel adapter or the wrong UPS can cause problems).
On the exam and in the field, the safest habit is the same: verify the PSU input details before you assume a “no power” issue is a bad motherboard.
Reading the PSU label like a tech: input range, frequency, and amperage
The PSU label is your quick source of truth. On the input side, you’ll often see a line like:
AC input: 100 to 240 V, 50/60 Hz, 10 A
Each part has a specific meaning:
- 100 to 240 V: This is the allowed input voltage range. A PSU with this rating is usually auto-ranging and works in both 120 V and 230 V regions with the correct power cable (and no selector switch needed).
- 50/60 Hz: This is the AC frequency used by power grids. Many countries use 60 Hz (common in North America) while others use 50 Hz (common in much of Europe and other regions). The PSU lists 50/60 Hz to show it can operate correctly on either frequency.
- 10 A (example value): This is the maximum input current the PSU may draw under certain conditions. It is not the current sent to the motherboard. Think of it as an input-side limit for planning circuits, power strips, and UPS sizing.
A key exam detail is scope: the input section of the label describes what the PSU can accept from the wall (voltage, frequency, and max input current). It does not describe what it delivers to PC parts. Output rails (3.3 V, 5 V, 12 V) appear in a separate output table on the same label, and that table is the one tied to components and connectors.
The DC output rails you must know: 3.3V, 5V, and 12V
A PC power supply does not deliver one “computer voltage.” It outputs several DC rails, each built for a type of load. For the CompTIA A+ 220-1201 exam, the key rails to know are 12V, 5V, and 3.3V. Each one serves a different role, and each can fail in a way that produces clues.
A practical way to think about rails is to treat them like separate water lines in a building. They share the same source (the PSU), but each line feeds different fixtures. A weak 12V line can act like low water pressure to the whole building, while a missing 5V line can make only certain rooms unusable.
What the 12V rail powers in most modern PCs
In most modern desktops, 12V is the main workhorse rail. It carries the bulk of the PSU’s output power because today’s highest-draw parts either run on 12V directly or convert 12V into lower voltages on the board.
A clear example is the CPU. The CPU does not run at 12V, it runs at a much lower voltage. The motherboard’s voltage regulator modules (VRMs) take 12V (from the CPU power connectors and board traces) and step it down to what the processor needs. That conversion happens right near the CPU socket so it can respond fast to load changes.
GPUs follow a similar pattern. A graphics card can pull power from:
- The PCIe slot (which provides some power from the motherboard)
- One or more PCIe auxiliary power connectors from the PSU (6-pin, 8-pin, or 12VHPWR on newer cards)
In both cases, the heavy lifting is still based on 12V, with the card’s own regulators stepping it down for the GPU core and memory.
Beyond processors and graphics, 12V also feeds many “mechanical” or motor-like loads:
- Case fans and CPU cooler fans (common 12V fans, often speed-controlled by the motherboard)
- Pump motors in many liquid-cooling setups
- Drive motors, such as the spindle motor in a hard drive (the drive also needs logic power, which is often a different rail)
This is why high-end systems care about 12V capacity and stability. A PSU can have a high watt rating on paper, but what matters for performance builds is how much of that wattage is available on 12V, and how well it holds voltage under fast load swings (like when a GPU boosts). If 12V sags too far under load, the system may reset, shut down, or behave erratically.
Where 5V still matters and what can break when it is missing
Even though 12V does most of the heavy work, 5V is still active in many normal PC functions. It often supports “everyday” features that people notice first, such as USB power.
The most familiar 5V use case is USB. Standard USB ports provide 5V to power small devices and support charging. If a PSU has a weak or missing 5V output (or if a related motherboard power path fails), you can see symptoms that look like “USB problems” rather than “power problems.”
5V also supports parts of storage and motherboard behavior. Many drives use different voltages for different tasks. For example, drives may use one voltage for their logic electronics and another for motors. Modern SSDs do not have motors, but they still need stable power for their controller and flash management.