Radio,Television And Electronic Paper 3, WASSCE (SC), 2018

Question 1

 

AIM: To determine the characteristics of a step-down transformer in a Radio receiver.

  1. You are provided with a centre-tapped transformer.
  2. Hold the transformer in position as displayed in Fig. 1.

  1. Measure and record in Table 1 the resistance between terminals as shown in the Table.

                                                                  Table 1


Terminals

1 – 2

3 – 4

4 – 5

3 – 5

1 – 3

2 – 5

1 – 4

2 – 4

Resistance
(Ω)

 

 

 

 

 

 

 

 

  1. From the readings obtained in Table 1, deduce with reasons the terminals that make up the primary windings and the secondary windings.

(i)   Primary winding:
Terminals;
Reason(s).
(ii)  Secondary winding:
Terminals;
Reason(s).

  1. Name the terminals that show isolation between the transformer windings.
  2. Connect the radio power chord to the socket outlet (mains supply).
  3. At the radio end terminals of the power chord, measure and record the input voltage.
  4. Connect the power chord to the terminals 1 and 2 of the transformer in Fig.1 as shown in Fig.2.

 

  1. Close switch S.
  2. Measure and record the voltage across the following terminals:

(i)   Terminals 3 – 4;
(ii)  Terminals 4 – 5;
(iii)             Terminals 3 – 5.

  1. Open switch S.
  2. Comment on the values of voltages recorded in (j).
  3. Compare the results obtained in (g) and (j).
  4. Draw the circuit symbol of the transformer.
  5. Connect the diodes D1 and D2, and resistor RL to Fig.2 as shown in Fig.3.

  1. Close switch S.
  2. Measure and record the output voltages across terminals X and Y in a.c. and d.c. ranges respectively.

(i)   Vout (a.c.):
(ii)  Vout (d.c.).

  1. Comment on the results in (j) and (q).
  2. State one stage of a radio receiver where the transformer can be found.
  3. State one function of the transformer in the stage mentioned in (s).
  4. State two precautions taken in carrying out the experiment.

 

Observation

 

(c)                                                        Table 1


Terminals

1 – 2

3 – 4

4 – 5

3 – 5

1 – 3

2 – 5

1 – 4

2 – 4

Resistance (Ω)

1.5 –2.0k

12 – 13

12 – 13

14 – 15

∞/No reading

∞/No reading

∞/No reading

∞/No reading

             
(d)       (i)        Terminals : 1, 2                                                                                  
Reason(s): there is high resistance between them.                            
(ii)       Terminals: 3, 4 and 5                                                                         
Reason(s): there is low resistance between them.                                                     

 

(e) 1 – 3, 2 – 5, 1 – 4 and 2 – 4.                                                         
(g) 160 – 240 V.                                 
(j)         (i)       Terminals 3 – 4: 6.35V                                                                                  
(ii)       Terminals 4 – 5: 6.35V                                                                      
(iii)       Terminals 3 – 5: 12.70V                    
(l) The voltage across terminals 3 – 5 is equal to the sum of voltages across terminals 3 – 4
and terminal 4 – 5.  Voltage in (iii) = Voltage in (i) + Voltage in (ii). This shows that it is a centre-tapped transformer.                                                                                                           
(m) The voltage in (g) is higher/much higher than the voltages in (j)                                   
(n) Circuit symbol of the transformer

(o) Correct circuit connection                                                                        
(q)        (i)        Vout (a.c.) = 2.66V                                                      
(ii)       Vout (d.c.) = 5.09V                                                     
(r) The input voltage 6.35V has been rectified to 5.09V with 2.66V ripples             
(s) Power stage                                                                                                                      
(t) Step down the input voltage/isolate the circuit from the mains                                        
(u) Precautions taken in carrying out the experiment

  • Ensure tight connections.
  • Ensure that the switch is opened while connecting.
  • Avoid error due to parallax.
  • Ensure the voltmeter is switched to the appropriate range.
  • Avoid overcrowding the table with tools.
  • Tape all exposed wires.

Candidates were expected to measure accurately various quantities at different stages of a power supply unit and make deductions accordingly. Candidates were required to have a balance of theoretical principles with practical realities. For example, the transformer in the circuit only steps down voltage and not otherwise. Only a few candidates, according to the report, responded very well to Question 1.