Question 1
AIM: To construct and investigate the operation of a d.c. power supply for a radio receiver.
- Connect the circuit as shown in Fig. 1.
- Ask the supervisor to check the circuit connection.
- Put ON the power switch.
- Set the multimeters to the a.c. range, measure and record voltages V1and V2respectively.
- Put OFF the power switch.
- Connect the two diodes, D1 and D2, to the circuit as shown in Fig. 2.
- Put ON the power switch.
- Measure and record voltage V3.
- Put OFF the power switch.
- Set the multimeter to the d.c. voltage range and put ON the power switch.
- Measure and record voltage V3.
Observation
The expected responses were:
(v) Function of capacitor in the circuit in Fig. 3
- Filtering
- It removes the a.c. ripples and leave only pure d.c. current
(w) (i) Effect of open circuit capacitor on the audio output of radio receiver
- Humming
- No filtering
(ii) Effect of open circuit in the primary of the transformer on the audio output of
radio receiver
- Dead set
- No power supply
(x) Precautions that should be taken in carrying the experiment
- Ensure correct polarity of capacitor
- Ensure correct polarity of diodes
- Ensure insulation of primary winding leads
- Ensure tight connections
Candidates were expected to progressively construct a d.c. power supply unit, measure and make deductions based on readings taken. Thereafter, use the determined values to deduce the effect on the output of a radio receiver. Majority of the candidates that attempted the tasks in question 1 responded well to questions on readings but only a few of them could make appropriate deductions from readings taken.