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)
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