The following diagram shows a Zener diode as a voltage regulator. The Zener diode is rated at Vz = 5 V and the desired current in load is 5 mA. The unregulated voltage source can supply up to 25 V. Considering the Zener diode can withstand four times the load current, the value of resistor Rs (shown in circuit) should be . [figure]
- 100
- 10
- 1000
- 4000
Answer
(3) 1000
Solution
Maximum current the Zener diode can withstand: IZ, = 4 5 mA = 20 mA
Rs must keep the current within this limit even in the worst case, i.e. at the highest input (25 V) when the whole current may pass through the Zener (load removed).
Voltage across Rs: Vin - Vz = 25 - 5 = 20 V
Rs = 20 V20 10-3 A = 1000
(With the load connected, the 20 mA divides into 5 mA through RL and 15 mA through the Zener, which is within its rating.)
Answer: (3)