CET CET Power Supplies and Electrical Safety 1 — Questions and Answers
Question 1: What is the primary purpose of a filter capacitor placed after the rectifier in a DC power supply?
- Reduce output ripple by storing charge between rectifier pulses (Correct answer)
- Increase output voltage
- Protect the rectifier diodes from reverse voltage
- Regulate the output against load changes
Correct answer: Reduce output ripple by storing charge between rectifier pulses
The filter capacitor charges to the peak voltage and slowly discharges into the load between pulses, smoothing the pulsating DC.
Question 2: At what frequency does the ripple voltage appear in a full-wave bridge rectifier powered by a 60 Hz AC supply?
- 120 Hz (Correct answer)
- 60 Hz
- 30 Hz
- 180 Hz
Correct answer: 120 Hz
A full-wave rectifier produces two pulses per AC cycle, so ripple frequency equals twice the supply frequency: 2 × 60 = 120 Hz.
Question 3: Which type of voltage regulator dissipates the difference between input and output voltage as heat?
- Linear regulator (Correct answer)
- Switching regulator (SMPS)
- Buck converter
- Boost converter
Correct answer: Linear regulator
Linear regulators pass current through a series pass transistor that drops the excess voltage as heat, making them less efficient than switchers.
Question 4: How does a zener diode function as a voltage regulator in a shunt regulator circuit?
- It maintains a nearly constant voltage across it by varying the current through it within its breakdown region (Correct answer)
- It blocks reverse current to stabilize voltage
- It amplifies the reference voltage
- It switches on and off to regulate voltage
Correct answer: It maintains a nearly constant voltage across it by varying the current through it within its breakdown region
Operating in reverse breakdown, the zener holds a fixed voltage across itself while absorbing excess current, keeping output voltage stable.
Question 5: In a power supply, what does a crowbar circuit do?
- Immediately short the output through a thyristor (SCR) if voltage exceeds a safe threshold, blowing the fuse (Correct answer)
- Regulate output voltage under heavy load
- Filter high-frequency switching noise
- Provide soft-start to limit inrush current
Correct answer: Immediately short the output through a thyristor (SCR) if voltage exceeds a safe threshold, blowing the fuse
A crowbar circuit fires an SCR to short the output when overvoltage is detected, causing the fuse to blow and protecting downstream equipment.
Question 6: Which component in a linear power supply converts the AC mains voltage to a lower AC voltage suitable for rectification?
- Power transformer (Correct answer)
- Bridge rectifier
- Voltage regulator IC
- Filter capacitor
Correct answer: Power transformer
The power transformer steps down the mains AC voltage to the required level while also providing isolation from the AC line.
CET CET Power Supplies and Electrical Safety 1
What is the primary purpose of a filter capacitor placed after the rectifier in a DC power supply?