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Q9. An unstabilised d.c. supply voltage varies between 25 V and 35 V. A voltage stabiliser circuit comprising a 12 V zener diode and a series resistor R is connected across the unstabilised supply. The Zener has a slope resistance of 14 Ω and requires a minimum operating current of 1 mA. A 0-80mA variable load is to be supplied by the stabiliser circuit.

(a) When the supply voltage is minimum and the load current demand is a maximum, calculate EACH of the following:

(i) The maximum value for R to give a stable load voltage; (4)

(ii) The load voltage. (2)

(b) Using the value of R determined in Q9(a), calculate EACH of the following.

(i) The load voltage when the supply voltage and load current are both at maximum value; (4)

(ii) The Zener diode current when the supply voltage is minimum, and the load is switched off; (3)

(iii) The load voltage when the supply voltage is 30 V and the load current is 30 mA. (3)