A+ Core 1 3.6: Computer Power

In one line: the power supply converts wall AC into the DC voltages (mainly 3.3 V, 5 V, 12 V) the motherboard needs, sized by watts = volts × amps and by a 50%-of-capacity rule of thumb, with redundant hot-swappable units and 80 PLUS efficiency ratings for the infrastructure end — and always, always disconnect power before opening the case.

Companion to: Professor Messer, CompTIA A+ 220-1201 Core 1, objective 3.6 (Computer Power). The full version is the Computer Power class notes; the section overview is the Section 3 sheet.

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Safety — before anything else

• Always disconnect power before opening a computer. Two hazards: direct electrocution, and stored charge in capacitors that outlasts disconnection.
• Never connect yourself to a building’s electrical system, including the ground wire — it can become energised under fault conditions.
• Double-check disconnection; don’t assume from appearance.

AC vs DC

	AC	DC
What	Alternating Current — direction constantly reverses	Direct Current — one direction, constant voltage
Where	Wall outlet	What the motherboard needs
Job	Input to the PSU	Output from the PSU

The PSU’s whole job: convert AC → DC.

Amps, volts, watts

Unit	Measures	Hose analogy
Amp (A)	Electrons past a point per second	Volume of water flowing
Volt (V)	Electrical pressure	Water pressure
Watt (W)	Real power use	—

Watts = Volts × Amps. Example: 120 V × 0.5 A = 60 W.

Regional AC standards

Region	Voltage	Frequency
US / Canada	110–120 VAC	60 Hz
Europe	220–240 VAC	50 Hz

• Older PSUs: manual 120V/230V switch on the back — set it correctly before connecting. 120V-set PSU into a 230V source = overload, likely dramatic failure.
• Modern PSUs: auto-sense the input voltage, no switch needed.
• Unsure what an outlet provides? Test it with a multimeter.

DC output voltages

Voltage	Used for
+12 V	PCIe adapters, hard drives, higher-power components
+5 V	Motherboard components (more common on older boards)
+3.3 V	Newer boards — M.2 slots, RAM, onboard components
+5 VSB	Standby power — wake-on-LAN, front-panel power button
−12 V	Onboard LAN, some older PCI cards
−5 V	Obsolete — many modern PSUs don’t provide it

Exact voltages and amp ratings per rail: check the PSU manual or the label on the unit.

Motherboard connector

• 24-pin, delivering 3.3 V / 5 V / 12 V. Originally 20-pin.
• A 24-pin PSU connector works on an older 20-pin-only board — just leave the last 4 pins unconnected.
• Keyed — only fits one orientation.

Redundant power supplies

• Each unit supports 100% of the load alone.
• Normal operation: typically ~50% load each, sharing the work.
• Lose one → the other takes 100% automatically.
• Usually hot-swappable — replace while the system stays running.

Cabling styles

Style	Cables	Typical cost
Fixed	All permanently attached	Lower
Hybrid	Essentials fixed, extras modular	Middle
Fully modular	Everything detachable, connect only what you need	Higher

Sizing a PSU

1. Total up component requirements — CPU, storage, video card (often the big one), everything else.
2. Apply the 50% rule of thumb: your actual load should be ~50% of the PSU’s rated capacity.
3. Formula: rated capacity = actual load ÷ 0.5. (400 W load → 800 W PSU.)
4. Physical size doesn’t change with wattage within a form factor.
5. Buying far more than needed adds cost, not speed.

Efficiency: 80 PLUS

• Typical efficiency: 80–96%. Lost power becomes heat → more cooling load.
• Tiers, lowest to highest: 80 PLUS → Bronze → Silver → Gold → Platinum → Titanium.

🔐 Security notes

• Power safety failures are the single most severe hardware risk in this whole course — the disconnect-and-verify discipline is the actual control, not just caution.
• Redundant hot-swappable PSUs are the same “eliminate the single point of failure” principle as RAID and multi-channel memory, applied to the electrical layer.
• A misconfigured voltage switch is a self-inflicted, sometimes destructive, availability incident — internal and procedural, not adversarial, but real.
• Lower efficiency = more heat = more component degradation over time — an indirect but genuine reliability consideration.
• Anyone with physical case access can be injured by residual capacitor charge — control and train who’s authorised for hardware work, not just who can enter the building.

Practice drills

1. What must you check for before touching internal components, beyond unplugging the power cable?

Stored charge in capacitors — it can remain energised after disconnection.

2. A device draws 2 A at 120 V. Wattage?

240 W (120 × 2).

3. US/Canada vs Europe AC standards?

US/Canada: 110–120 VAC, 60 Hz. Europe: 220–240 VAC, 50 Hz.

4. What happens if a 120V-set PSU is plugged into 230V?

Overload — a dramatic, likely destructive failure. Always check/set the switch before connecting.

5. What is +5 VSB for?

Standby power — keeps the motherboard able to receive wake signals (wake-on-LAN, power button) while “off.”

6. Why is the 24-pin connector keyed?

So it can only be inserted one way round — prevents wrong-orientation connection.

7. Redundant PSUs at normal load — what's the split, and what happens on failure?

~50/50 split normally; the surviving unit takes 100% if one fails.

8. Component load calculates to 400 W. Target PSU rating using the 50% rule?

800 W (400 ÷ 0.5).

Key takeaways

• Always disconnect power and account for capacitor charge before opening a case — never touch a building’s ground wire.
• PSU converts AC (wall) to DC (motherboard): mainly 3.3 V / 5 V / 12 V, plus 5 VSB standby.
• Watts = volts × amps; size a PSU so real load is ~50% of rated capacity.
• 24-pin connector is keyed; redundant PSUs share load and are usually hot-swappable.
• 80 PLUS tiers run Bronze → Silver → Gold → Platinum → Titanium, lowest to highest efficiency.