• 65 questions

CompTIA A+ Power Supply Practice Test

Input voltage, the 3.3 V, 5 V and 12 V rails, connectors, wattage and efficiency: objective 3.6 of Core 1.

Which mains frequency range must a desktop power supply accept?

Pick an answer, then press Check answer. Then you see Correct or Not quite, with the reason. Keys: A to D pick, Enter checks.

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Unofficial practice questions, not affiliated with CompTIA. Bank checked October 12, 2026.

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All 65 questions

Input: 110-120 VAC vs. 220-240 VAC 9

  • A desktop power supply has no voltage switch, yet it runs on both 120 V and 230 V outlets. What is that kind of input called? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), AC Input
  • A PC with a manual input voltage switch is shipped to the United Kingdom. Which setting does the switch need there? Wikipedia, Mains electricity by country, table of voltages by country
  • A power supply with a manual input switch is set for 115 V and plugged into a 230 V outlet. What is the usual result? Wikipedia, Power supply unit (computer), Input voltage switch
  • During a brownout the mains voltage sags below the supply's minimum input. What must be true of the power supply? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), Input Under Voltage
  • The power comes back after an outage. What must a desktop power supply do? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), AC Input
  • What kind of AC inlet must a desktop power supply have? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), AC Connector
  • Which mains frequency range must a desktop power supply accept? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), AC Input
  • Which nominal voltages stand for the two input ranges when a desktop power supply is tested? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), AC Input
  • Which two AC input ranges must a desktop power supply cover at full output? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), AC Input
Show all 65 questions (56 more)

Output: 3.3 V, 5 V and 12 V 14

  • A multimeter reads 4.60 V on the +5 V rail. Is that within the allowed range? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), DC Voltage Regulation
  • In a power supply with more than one 12 V rail, which rail feeds the processor? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), Processor Configurations
  • The motherboard pulls the PS_ON# signal low. What does the power supply do? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), PS_ON#
  • On the main power connector, what does a yellow wire carry? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), Main Power Connector
  • What color is a +5 V wire on the main power connector? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), Main Power Connector
  • What color is the +3.3 V wire on the main power connector? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), Main Power Connector
  • What does the PWR_OK signal tell the motherboard? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), PWR_OK
  • What is the lowest reading on a +12 V rail that is still within regulation? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), DC Voltage Regulation
  • Which of these is a job of the +5VSB standby output? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), +5VSB
  • Which output is a desktop power supply allowed to leave out? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), DC Voltage Regulation
  • Which power supply output stays on whenever AC power is present, even while the PC is shut down? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), +5VSB
  • Which two voltages does the 4-pin peripheral (Molex) connector supply? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), Peripheral Connectors
  • Which voltage did version 3.2 of the SATA specification drop from the SATA power connector? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), Serial ATA Connectors
  • Within which range must the +3.3 V rail stay? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), DC Voltage Regulation

20+4 pin and other connectors 12

  • A graphics card has one 2x4 (8-pin) auxiliary power socket. How much sustained power is that connector defined for? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), PCIe AIC Auxiliary Power Connectors
  • A power supply has a 20+4 pin motherboard connector. What are the two parts for? Wikipedia, Power supply unit (computer), Connectors
  • Can a 2x3, a 2x4 and a 12V-2x6 PCIe power plug stand in for one another? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), Modular Power Supply Connectors
  • Can a power supply with a 24-pin main connector run a motherboard that has a 20-pin socket? Wikipedia, Power supply unit (computer), Connectors
  • How many pins does the motherboard's main power connector have today? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), Main Power Connector
  • How many pins does the standard SATA power connector have? Wikipedia, SATA, SATA power connectors
  • How much sustained power is a 2x3 (6-pin) PCIe auxiliary power connector defined for? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), PCIe AIC Auxiliary Power Connectors
  • What are the 12 V CPU power connectors often called? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), Modular Power Supply Connectors
  • Which connector should makers leave off a new desktop power supply? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), Floppy Drive Connector
  • Which dedicated 12 V connectors feed the CPU on a motherboard? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), Modular Power Supply Connectors
  • Which PCIe auxiliary power connector is defined for up to 600 W? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), PCIe AIC Auxiliary Power Connectors
  • Which wire on the main power connector carries the signal that switches the supply on? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), Main Power Connector

Modular and redundant supplies 9

  • A rack server must keep running when one power supply fails. What does it need? Wikipedia, Power supply unit (computer), Redundant
  • A technician reuses the detachable cables from a different model of modular power supply. What is the risk? Wikipedia, Power supply unit (computer), Modular power supplies
  • How are the two units of a redundant power supply usually fed? Wikipedia, Power supply unit (computer), Redundant
  • How may the two units of a redundant power supply divide the work? Wikipedia, Power supply unit (computer), Redundant
  • On which end is the pin assignment of a modular power cable standardized? Wikipedia, Power supply unit (computer), Modular power supplies
  • One of two slot-mounted redundant supplies in a rack server fails. How can it be replaced? Wikipedia, Power supply unit (computer), Redundant
  • What does leaving unused cables off a modular power supply improve inside the case? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), Modular Power Supply Connectors
  • What is a semi-modular power supply? Wikipedia, Power supply unit (computer), Modular power supplies
  • What sets a modular power supply apart? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), Modular Power Supply Connectors

Wattage rating and efficiency 12

  • A +12 V rail is rated for 30 A. How many watts can that rail deliver? Wikipedia, Watt, Overview
  • A power supply draws 500 W and delivers 450 W. What becomes of the other 50 W? Wikipedia, Power supply unit (computer), Efficiency
  • A power supply draws 500 W from the wall and delivers 450 W to the PC. What is its efficiency? Wikipedia, Electrical efficiency, lead section
  • A power supply is 80% efficient and delivers 400 W to the PC. How much does it draw from the wall? Wikipedia, Electrical efficiency, lead section
  • At which load does Intel's design guide set its highest minimum efficiency? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), Overall Power Supply Efficiency Levels
  • On which rail does a modern ATX power supply deliver most of its rated capacity? Wikipedia, Power supply unit (computer), ATX12V standard
  • The parts of a build are calculated to draw about 400 W. Which supply size has 40% of headroom over that? Wikipedia, Power supply unit (computer), Power rating
  • What does the 80 PLUS specification require of a power supply? CLEAResult, What is 80 PLUS certification program?, program overview
  • What does the wattage figure on a power supply label state? Wikipedia, Power supply unit (computer), Power rating
  • Which 80 PLUS level comes next above Gold? CLEAResult, What is 80 PLUS certification program?, program overview
  • Which 80 PLUS level stands above Titanium? CLEAResult, What is 80 PLUS certification program?, program overview
  • Why can a power supply far larger than the PC needs waste electricity? Wikipedia, Power supply unit (computer), Efficiency

Power symptoms 9

  • A loose screw shorts the +5 V rail to ground. What must the power supply do? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), Short Circuit Protection (SCP)
  • A PC shuts down under heavy load and the fan in its power supply has stopped. Which protection most likely tripped? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), Over Temperature Protection (OTP)
  • A power supply is switched on at the bench with nothing connected, and it shuts itself off. What does that show? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), No-Load Situation
  • A short has been removed, but the power supply stays off. What brings it back if it does not recover by itself? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), Short Circuit Protection (SCP)
  • An output climbs far above its rated voltage. Which protection must latch the power supply off? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), Over Voltage Protection (OVP)
  • The fans spin but the PC never starts, and the PWR_OK line measures low. What does that point to? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), PWR_OK
  • In the original IBM POST beep codes, what does no beep at all point to? Wikipedia, Power-on self-test, Original IBM POST beep codes
  • Mains power drops out for a moment. For how long must a fully loaded power supply hold its outputs in regulation? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), Voltage Hold-Up Time
  • Why is the power cord unplugged before work inside a PC that is already shut down? Intel, ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (rev. 2.1a), PS_ON#

How this list was compiled: Own questions, written to the seven items of objective 3.6 (input voltage, output voltages, the 20+4 pin motherboard connector, redundant and modular power supplies, wattage rating, energy efficiency) and the power symptoms of objective 5.1 in CompTIA's A+ Core 1 (220-1201, V15) exam objectives, document version 4.0, linked from https://www.comptia.org/en-us/certifications/a/core-1-v15/ (read 2026-10-12). Facts were read on 2026-10-12 in Intel's ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide (revision 2.1a, public, at edc.intel.com), CLEAResult's 80 PLUS program page and Wikipedia's articles on the computer power supply unit, SATA, the power-on self-test, mains electricity by country, the watt and electrical efficiency. Each question names the text and section it was written from and links to it. Licence: Own work. Every question, wrong choice and explanation was written for this site from the public texts named in each question. Nothing is copied or reworded from a CompTIA exam, a released or sample question, or another practice product.

How it works

Sixty-five power supply questions for CompTIA A+ Core 1, unofficial and our own. CompTIA does not sponsor or endorse this page, and the real exam's questions are not on it.

Read the full guide

The seven items of objective 3.6

Objective 3.6 of Core 1 (220-1201, V15; CompTIA's objectives read October 12, 2026) asks you to install the appropriate power supply. It lists input of 110 to 120 VAC against 220 to 240 VAC, output of 3.3 V, 5 V and 12 V, the 20+4 pin motherboard connector, redundant and modular power supplies, wattage rating and energy efficiency.

Where the numbers come from

Voltages, wire colors and connector ratings are read from Intel's public ATX power supply design guide, and the efficiency levels from the 80 PLUS program page. Four sums on watts and efficiency are worked from the figures in the question. Nine questions cover power symptoms from objective 5.1.

  • The purple +5VSB wire is live whenever the cord is plugged in, even with the PC shut down.
  • +5 V may read 4.75 to 5.25 V and +12 V 11.20 to 12.60 V. Outside those limits, suspect the supply.
  • Watts are volts times amps: a 12 V rail rated for 30 A can deliver 360 W.

Questions and answers

What does a 20+4 pin power connector mean?

The main motherboard plug comes as two parts, 20 pins and 4 pins, which join to make the 24-pin connector of current boards. A supply with a 24-pin plug can also run a board that has a 20-pin socket.

What is the difference between 110-120 VAC and 220-240 VAC on a power supply?

They are the two mains ranges. A supply covers 90 to 135 VAC and 180 to 265 VAC, by a switch or by auto-ranging. A switch left at 115 V on a 230 V outlet usually damages the supply for good.

What wattage power supply do I need?

Add up what the parts draw, then leave headroom: supplies are sized around 40% above the calculated draw. A supply far larger than that is often less efficient at low load.

What is 80 PLUS?

A certification for power supplies that are at least 80% efficient at four rated-load levels (10%, 20%, 50% and 100%) and have a power factor of at least 0.9. Its levels run from Standard to Ruby.

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