The figure belows shows a circuit which uses zener diode to

The figure belows shows a circuit which uses zener diode to regulate the input voltage at the output. Assume the zener voltage of the diode is 4.7 volts and its forward bias voltage is 0.7 volts. What would the voltage at V2 be when V1 = 6V if R2 has the following values: l kohms 10k ohms 100k ohms What would the voltage at V2 be when V1 = -6V and if R2 has the following values: 1 k ohms 10 k ohms 100k ohms

Solution

a) For V1=6 V

i) R2=1k ohm assume that zener diode is not there and find V2=V1*1/(1+10)=0.5455 V it is less than Vz i.e 4.7 V

Zener is not in break down region so V2=0.5455 V

ii)

R2=10k ohm assume that zener diode is not there and find V2=V1*10/(10+10)=3 V it is less than Vz i.e 4.7 V

Zener is not in break down region   so V2=3 V

iii)

R2=100k ohm assume that zener diode is not there and find V2=V1*100/(100+10)=5.45 V it is greater than Vz i.e 4.7 V

Now Zener is in break down region   so V2=Vz=4.7 V

----------------------------------------------------------

b) For V1=-6 V

i) R2=1k ohm assume that zener diode is not there and find V2=V1*1/(1+10)=-0.5455 V it is less than Vf(forward bias voltage) i.e -0.7 V

so V2=-0.5455 V

ii)

R2=10k ohm assume that zener diode is not there and find V2=V1*10/(10+10)=-3 V it is greater than Vf(forward bias voltage) i.e -0.7 V so V2=-0.7 V forward Conduction mode

iii)

R2=100k ohm assume that zener diode is not there and find V2=V1*100/(100+10)=-5.45 V it is greater than Vf(forward bias voltage) i.e -0.7 V so

V2=-0.7 forward Conduction mode