ISCET Power Supply Circuits and Voltage Regulation 1 — Questions and Answers
Question 1: What is the primary purpose of a filter capacitor placed after a rectifier in a DC power supply?
- To increase the ripple frequency
- To smooth the pulsating DC output by reducing ripple (Correct answer)
- To step up the DC output voltage
- To limit the peak output current
Correct answer: To smooth the pulsating DC output by reducing ripple
A filter capacitor charges to the peak voltage and slowly discharges between peaks, reducing the ripple and smoothing the pulsating DC into a more stable output.
Question 2: A half-wave rectifier is connected to a 120V RMS transformer secondary. What is the approximate average DC output voltage (ignoring diode drop)?
- 120V
- 54V (Correct answer)
- 84.8V
- 170V
Correct answer: 54V
For a half-wave rectifier, Vavg = 0.318 × Vpeak = 0.318 × (120 × 1.414) ≈ 0.318 × 169.7 ≈ 54V.
Question 3: In a full-wave bridge rectifier, how many diodes conduct simultaneously during each half-cycle of the AC input?
- One
- Two (Correct answer)
- Three
- Four
Correct answer: Two
In a bridge rectifier, current flows through two diodes in series during each half-cycle — one on the top rail and one on the bottom rail.
Question 4: The ripple factor (r) of a power supply filter is best described as:
- The ratio of peak output voltage to average output voltage
- The ratio of RMS ripple voltage to average DC output voltage (Correct answer)
- The percentage of input AC that appears at the output
- The ratio of output current to input current
Correct answer: The ratio of RMS ripple voltage to average DC output voltage
Ripple factor r = Vrms(ripple) / Vdc, and a lower value indicates a better-filtered, more stable DC output.
Question 5: A 7812 three-terminal linear voltage regulator produces which regulated output voltage?
- 5V
- 8V
- 12V (Correct answer)
- 15V
Correct answer: 12V
The 78XX series regulators output a voltage equal to the last two digits; the 7812 outputs a fixed +12V regulated output.
Question 6: What is the minimum PIV (Peak Inverse Voltage) rating required for diodes in a full-wave bridge rectifier with a 24V RMS transformer secondary?
- 16.97V
- 24V
- 33.94V (Correct answer)
- 48V
Correct answer: 33.94V
In a bridge rectifier, each diode must withstand a reverse voltage equal to Vpeak = 1.414 × 24V ≈ 33.94V.
Question 7: Which type of voltage regulator uses a zener diode connected in parallel with the load as its primary regulation element?
- Series pass regulator
- Switching regulator
- Shunt regulator (Correct answer)
- Buck-boost regulator
Correct answer: Shunt regulator
A shunt regulator places the regulation element (zener diode or transistor) in parallel with the load to clamp the output voltage at a fixed level.
ISCET Power Supply Circuits and Voltage Regulation 1
What is the primary purpose of a filter capacitor placed after a rectifier in a DC power supply?