Radio,Television And Electronic Paper 3, WASSCE (SC), 2021

Question 1

 

AIM: To construct and investigate the operation of a d.c. power supply for a radio receiver.

  1.       Connect the circuit as shown in Fig. 1.
  2.       Ask the supervisor to check the circuit connection.
  3.       Put ON the power switch.
  4.       Set the multimeters to the a.c. range, measure and record voltages V1and V2respectively.
  5. Put OFF the power switch.
  6.       Connect the two diodes, D1 and D2, to the circuit as shown in Fig. 2.

                   

  1.        Put ON the power switch.
  2.        Measure and record voltage V3.

  3.       Put OFF the power switch.
  4.       Set the multimeter to the d.c. voltage range and put ON the power switch.
  5.       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.