Question

(a) Draw I-V characteristics of a Zener diode. 

(b) Explain with the help of a circuit diagram, the use of a Zener diode as a voltage-regulator. 

(c) A photodiode is operated under reverse bias although in the forward bias the current is known to be more than the current in the reverse bias. Explain giving reason.

Answer 1

(a) I-V Characteristics of Zener diode:

(b) Zener diode as a Voltage Regulator 

The Zener diode makes its use as a voltage regulator due to the following property : When a Zener diode is operated in the breakdown region, the voltage across it remains practically constant for a large change in the current. A simple circuit of a voltage regulator using a Zener diode is shown in the Fig. The Zener diode is connected across load such that it is reverse biased. The series resistance R absorbs the output voltage fluctuations so as to maintain constant voltage across the load. The operation of the circuit may be explained as follows : 

Let  V_in bethe unregulated input dc voltage and V₀  be the output voltage across R_L  to be regulated and V_Z  be the Zener voltage of the diode. The value of the series resistance is so chosen that the diode operates in the breakdown region under the Zener voltage V_Z across it. Let I be the current drawn from supply, I_Z  the current through Zener diode and I_L  the current through load. Then obviously 

I = I_Z + I_L  or  I_Z = I - I_L 

If R_Z  is Zener diode resistance, then 

V₀ =V_Z = I_Z . R_Z  = I_LR_L 

Applying Kirchhoff’s law to the mesh containing resistance R, Zener diode and supply voltage V_in, we have

When the input voltage V_in is lower than the Zener voltage V_Z  of diode, there is no current conduction 

i.e., I_Z = 0. 

This implies V₀ =V_in. 

As input voltage V_in is increased so that it becomes equal to V_Z, the breakdown point is reached and the voltage across the diode V_Z =( V_in - RI ) becomes constant. A further increase of input voltage Vin  does not result in the corresponding increase in V₀  or V_Z  but merely increases the voltage drop across R. Thus in breakdown region, we have 

V₀ = V_Z = V_in - RI  …(ii) 

Fig. represents the plot of output voltage V₀  versus input voltage V_in. It is clear from the graph that the output voltage remains constant when the diode is in Zener region. It may be pointed out that for maintaining constant regulated output, the series resistance R for a given range of input voltage be so chosen that 

(i) the diode operates in Zener region and 

(ii) current should not exceed a certain value to cause burn out of diode. 

(c) A photodiode is used in reverse bias, although in forward bias current in more, then current in reverse bias because in reverse bias it is easier to observe change in current with change in light intensity.