Question 1
AIM: To determine the characteristics of a step-down transformer in a Radio receiver.
- You are provided with a centre-tapped transformer.
- Hold the transformer in position as displayed in Fig. 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 |
|
|
|
|
|
|
|
|
- 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).
- Name the terminals that show isolation between the transformer windings.
- Connect the radio power chord to the socket outlet (mains supply).
- At the radio end terminals of the power chord, measure and record the input voltage.
- Connect the power chord to the terminals 1 and 2 of the transformer in Fig.1 as shown in Fig.2.
- Close switch S.
- Measure and record the voltage across the following terminals:
(i) Terminals 3 – 4;
(ii) Terminals 4 – 5;
(iii) Terminals 3 – 5.
- Open switch S.
- Comment on the values of voltages recorded in (j).
- Compare the results obtained in (g) and (j).
- Draw the circuit symbol of the transformer.
- Connect the diodes D1 and D2, and resistor RL to Fig.2 as shown in Fig.3.
- Close switch S.
- 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.).
- Comment on the results in (j) and (q).
- State one stage of a radio receiver where the transformer can be found.
- State one function of the transformer in the stage mentioned in (s).
- 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.