220-1101 Question 304
Single answerOutput 3.3V vs. 5V vs. 12VA technician is troubleshooting a desktop PC that powers on, but a newly installed SATA hard drive does not spin up and is not detected in BIOS. The power supply is known good, and the SATA data cable has already been replaced. Using a multimeter on the SATA power connector, the technician measures 0V on the 12V pin, while the 5V and 3.3V rails are present. Which of the following is the MOST likely result of this missing voltage?
- A
The drive will fail to power its motor, so it will not spin up or initialize properly
- B
The motherboard BIOS settings will be erased because the CMOS requires 12V from the SATA connector
- C
The system fans connected to motherboard headers will stop because they use 12V from the SATA power connector
- D
The USB ports will stop functioning because USB depends on the 12V rail for device power
Show answer and explanation
Correct answer: A
Explanation
PC power supplies provide multiple DC output rails, commonly including 3.3V, 5V, and 12V. In practical A+ troubleshooting, understanding which components use which rail helps isolate failures quickly. For SATA power, 3.5-inch hard drives typically rely on 12V for the motor and 5V for logic circuitry. If 12V is absent, the drive often will not spin up, making it unavailable to BIOS and the operating system. By contrast, USB power is 5V, and CMOS retention is handled by the motherboard battery rather than a SATA connector. This aligns with standard PC power distribution practices and ATX/SATA power conventions used in desktop systems.
- A. Correct.
Correct. Traditional 3.5-inch SATA hard drives commonly use the 12V rail to power the spindle motor and actuator, while the 5V rail powers onboard logic. If 12V is missing at the SATA power connector, the drive may not spin up at all, which matches the symptom of a drive that is neither heard nor detected. This is a practical troubleshooting point when verifying PSU output rails.
- B. Incorrect.
Incorrect. CMOS settings are maintained by the motherboard's CMOS/RTC battery, typically a 3V coin-cell battery such as a CR2032, not by 12V from a SATA power connector. A missing 12V rail on a drive power lead would not erase BIOS settings.
- C. Incorrect.
Incorrect. Fans connected to motherboard headers receive power through the motherboard, not from the SATA power connector attached to a specific drive. Although many fans do operate on 12V, that 12V is delivered through fan headers or other dedicated power leads, not through the hard drive's SATA power cable.
- D. Incorrect.
Incorrect. Standard USB ports provide 5V power, not 12V. A missing 12V rail on a SATA power connector would not directly affect USB power delivery. This option reflects a common misconception that all PC peripherals rely on the 12V rail.