Zener Voltage Regulator

Zener Voltage Regulator

AU : May-08,12,13,14,15,16,17, Dec.-09,10,17,18

• The simplest shunt voltage regulator circuit uses a zener diode, to regulate the load voltage. The Fig. 1.24.1 shows the arrangement of zener diode in a regulator circuit.

• To understand the working of the circuit, let us revise the V-I characteristics of a zener diode, as shown in the Fig. 1.24.2.

• The zener diode is used in its reverse biased region. Under reverse biased condition, the current through the diode is very small of the order of few u.A, upto certain limit. When the sufficient reverse bias is applied, electrical breakdown of the zener diode occurs. The large current flows through the zener diode. Such a breakdown occurs at a voltage called zener voltage V_Z- Under this condition, whatever may be the current, the voltage across the zener is constant equal to V_Z. The large current due to breakdown is limited by connecting the resistance in the circuit.

• As the voltage across the zener diode remains constant equal to V_Z, it is connected across the load and hence the load voltage Vo is equal to the zener voltage V_Z . Thus zener diode acts as an ideal voltage source which maintains a constant load voltage, independent of the current.

1. Regulation with Varying Input Voltage

• The Fig. 1.24.3 shows a zener regulator under varying input voltage condition.

• It can be seen that the output is

V_o = V_z is constant.

I_L = V_o / R_L  V_z / R_L = Constant

And   I = I_Z + I_L

• Now if V_in  increases, then the total current I increases. But I_L is constant as V_z is constant. Hence the current I_z increases to keep I_L constant.

• But as long as I_Z is between I_Zmin and I_Zmax, the V_Z i.e. output voltage V_o is constant. Thus the changes in input voltage get compensated and output is maintained constant.

• Similarly if decreases, then current I decreases. But to keep IL constant, Iz decreases. As long as Iz is between IZmax and IZmin, the output voltage remains constant.

• Process flow chart for zener regulator under varying V_in is,

• The maximum power dissipation for the zener diode is fixed and given by,

P_D = V_Z I_Zmax

Ex. 1.24.1 Over what range of input voltage will the zener in a voltage regulator in circuit maintain 30 V across 2000 Ω load, assuming that series resistance R = 200 Ω and zener current rating is 25 mA ?

Sol : This circuit is shown in the Fig. 1.24.A

Thus the range of input voltage is 33.5 V to 38 V for which the circuit will work as regulator.

Ex. 1.24.2 Design a zener regulator for the following specifications

Output voltage, V_o = 5 V, Input voltage, = V_t = 12 ± 3, I_Zmin = 10 mA

Load current, I_L = 20 mA, Zener wattage, P_Z = 500 mW

Sol-: 

When R = Rmin, for      the current I will be at its maximum i.e. I_Z will be at its maximum.

Step 3 : To calculate Rmax, I must be minimum i.e. I_Z must be minimum i.e. 10 mA.

Thus R must be greater than 83.33 Ω and less than 133.33 Ω for proper regulating action.

2. Regulation with Varying Load

• The Fig. 1.24.5 shows a zener regulator under varying load condition and constant input voltage.

• The input voltage is constant while the load resistance R_L is variable. As V. is constant and V_o = V_z is constant, then for constant R the current I is constant.

I = V_in – V_z / R constant = I_L + I_z

• Now if R_L decreases so I_L increases, to keep I constant

I_z decreases. But as long as it is between I_Zmin and I_Zmax output voltage V_o will be constant. Similarly if RL increases so IL decreases, to keep I constant Iz increases. But as long as it is between I_Zmin and I_Zmax' outPut voltage V_o will be constant. 

• Process flow chart for zener regulator under varying load is,

Ex. 1.24.3 Determine the minimum and the maximum load currents for which the zener diode in Fig . 1.24.6 will maintain regulation. What is the minimum R_L that can be used? V_Z = 10 V, I_Zmin =  Assume r_Z = 0 Ω

Ex. 1.24.4 For the following circuit, find the maximum and minimum values of Zener diode current.

AU : Dec.-18, Marks 6

Sol. :

Thus minimum and maximum valves of zener diode current are 1 mA and 9 mA respectively.

Review Questions

1. Explain the working of a zener diode as a shunt regulator.

AU : Hay-08, 12, 13, 14, 15, 17, Dec.-O9, 10, 18, Harks 8

2. Explain the VI characteristics of Zener diode.

AU : Hay-16, Harks 8

3. In a shunt voltage regulator using zener diode, calculate minimum and maximum values of current limiting resistor (R).

Given : V_z = 8 V, Source voltage = 30 V, Load current = 0-50 mA, I_Zmin = 5 mA, P_Zmax = 1 W.

4. In a shunt regulator circuit using zener diode, the series resistance used is 1.2 kW. It provides 4.7 V to a load of 2.2 kW. If IZmin is 1 mA and IZmax is 20 mA, what is the range of the input voltage for the output voltage to remain constant ?

5. There is an application which needs the output voltage to be regulated. Choose an appropriate diode/device, that would ensure this operation with appropriate circuit, describe how it regulates voltage. Consider a specific example, design the circuit with appropriate values of components involved. State the important constraints that need to be considered.