CET Power Supply Design & Repair 2

Question 1

Which component in a linear power supply is primarily responsible for reducing AC input voltage before rectification?

Accepted Answer

Power transformer

Answer

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?

Accepted Answer

Regulate output voltage by adjusting switch duty cycle

Answer

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?

Accepted Answer

Output filter capacitor

Answer

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?

Accepted Answer

An overvoltage protection circuit that shorts the output

Answer

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?

Accepted Answer

33.9 V

Answer

Peak voltage equals RMS × √2, so 24 V × 1.414 ≈ 33.9 V; two diode drops reduce it slightly in practice.

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