Question 22.11: In Fig. 22-16, Vin can vary from 17.5 to 22.5 V. What is the...
In Fig. 22-16, V_{in} can vary from 17.5 to 22.5 V. What is the maximum zener current? What are the minimum and maximum regulated output voltages? If the regulated output voltage is 12.5 V, what is the load resistance where current limiting starts? What is the approximate shorted-load current?
The maximum zener current occurs when the input voltage is 22.5 V
I_z=\frac{22.5 \ V-4.7 \ V}{820 \ \Omega }=21.7 \ mA
The minimum regulated output voltage occurs when the wiper of the 1-kΩ potentiometer is all the way up. In this case, R_1 = 1750 \ Ω, R_2 = 750 \ Ω, and the output voltage is:
V_{out}=\frac{1750 \ Ω + 750 \ Ω}{1750 \ Ω }(4.7 \ V) = 6.71 \ V
The maximum regulated output voltage occurs when the wiper of the 1-kΩ potentiometer is all the way down. In this case, R_1 = 750 Ω, R_2 =1750 Ω, and the output voltage is:
V_{out}=\frac{750 \ Ω + 750 \ Ω}{750 \ Ω }(4.7 \ V) = 15.7\ V
Current limiting starts when the voltage across the current-limiting resistor is approximately 0.6 V. In this case, the load current is:
I_{L}=\frac{0.6 \ V}{3 \ Ω } = 200 \ mA
With an output voltage of 12.5 V, the load resistance where current limiting starts is approximately:
R_{L}=\frac{12.5 \ V}{200 \ mA } = 62.5 \ \Omega
With a short across the load terminals, the voltage across the current-sensing resistor is approximately 0.7 V and the shorted-load current is:
I_{SL}=\frac{0.7 \ V}{3 \ \Omega } = 233 \ mA