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Q1. A small silicon transistor with the characteristics given in Worksheet Q3 has a maximum safe power dissipation of 18 mW and it is to be operated on a 12 volt d.c. supply.

(a) Plot the maximum power dissipation curve on the characteristics.

(b) Determine the minimum value of collector load resistance for the transistor if this dissipation is not to be exceeded.

(c) If the transistor is used in a common emitter configuration and is biased at a base current of 60 mA and an alternating signal of  40 mA is applied to the base,

determine EACH of the following:

(i) The r.m.s. voltage variation between collector and emitter;

(ii) The r.m.s. value of the variation in collector current.

Q5. A capacitor connected in series with a resistor is tested on 240 V 50 Hz and the current is found to be 3.6 A. When the frequency is raised to 100 Hz the current increases to 4.8 A.

Determine EACH of the following:

(a) The values of the resistor and the capacitor;

(b) The power factor of the circuit at 50 Hz;

(c) The value of an inductor which, when connected in series with the pair, will give the same current of 3.6 A at 50 Hz but with a lagging power factor equal to the value obtained in part (b).

Q2. A 440 V 400 kVA three- phase transformer is designed to operate at maximum efficiency at ¾ full load and 0.8 power factor. The iron losses total 8 kW.

Determine EACH of the following:

(a) The efficiency at ¾ full load and 0.8 p.f.

(b) The total losses at full load;

(c) The full load efficiency at 0.7 p.f.

Q4. An unbalanced three phase load is supplied from a 440 V 50Hz four wire supply. The current in the red line is 6 A lagging by 30°, the current in the yellow line is 5A in phase and the current in the blue line is 7 A leading by 15°. Determine EACH of the following:

(a) The current in the neutral line;

(b) The phase angle of the neutral current relative to the voltage between the red line and the neutral line;

(c) The total power dissipated by the circuit.