CET Power Supply Design & Repair 2 — Questions and Answers
Question 1: Which component in a linear power supply is primarily responsible for reducing AC input voltage before rectification?
- Filter capacitor
- Power transformer (Correct answer)
- Zener diode
- Pass transistor
Correct answer: Power transformer
The power transformer steps down the AC mains voltage to a lower AC level suitable for the rectifier stage.
Question 2: In a switching power supply, what is the primary function of the PWM (Pulse Width Modulation) controller IC?
- Rectify the AC input
- Regulate output voltage by adjusting switch duty cycle (Correct answer)
- Filter output ripple
- Step down transformer turns ratio
Correct answer: Regulate output voltage by adjusting switch duty cycle
The PWM controller IC varies the duty cycle of the switching transistor to maintain a regulated DC output voltage.
Question 3: A technician measures excessive ripple on a power supply output. Which component is most likely failing?
- Rectifier diode
- Output filter capacitor (Correct answer)
- Input fuse
- Voltage reference IC
Correct answer: Output filter capacitor
Electrolytic output filter capacitors degrade over time and their increased ESR leads to excessive output ripple voltage.
Question 4: What does the term 'crowbar circuit' refer to in power supply design?
- A voltage boosting circuit
- An overvoltage protection circuit that shorts the output (Correct answer)
- A soft-start inrush limiter
- A bridge rectifier topology
Correct answer: An overvoltage protection circuit that shorts the output
A crowbar circuit places a short (via SCR) across the output when overvoltage is detected, blowing the fuse and protecting downstream loads.
Question 5: In a full-wave bridge rectifier with a 24 V AC RMS input, approximately what is the peak DC voltage before filtering?
- 24 V
- 17 V
- 33.9 V (Correct answer)
- 48 V
Correct answer: 33.9 V
Peak voltage equals RMS × √2, so 24 V × 1.414 ≈ 33.9 V; two diode drops reduce it slightly in practice.
Question 6: Which topology is most commonly used in ATX computer power supplies due to its efficiency at high power levels?
- Linear series-pass
- Half-wave rectifier
- Flyback converter
- Forward or half-bridge switching converter (Correct answer)
Correct answer: Forward or half-bridge switching converter
ATX power supplies use forward or half-bridge switching topologies because they achieve high efficiency (>80%) at power levels of 200 W and above.
Question 7: A power supply's output voltage is too high and does not respond to the trim pot adjustment. Which section should the technician inspect first?
- Input rectifier bridge
- Output filter capacitors
- Feedback/error amplifier network (Correct answer)
- Inrush current limiter
Correct answer: Feedback/error amplifier network
The feedback/error amplifier network (including the voltage divider and reference) controls output regulation, and a fault there will cause uncontrolled output voltage.
CET Power Supply Design & Repair 2
Which component in a linear power supply is primarily responsible for reducing AC input voltage before rectification?