Design of Zener regulator circuit:

A zenere regulator circuit is shown in fig. 6. The varying load current is represented by a variable load resistance R_L.

The zener will work in the breakdown region only if the Thevenin voltage across zener is more than VZ .  If zener is operating in breakdown region, the current through RS is given by

Fig. 6

and load current   

I_s= I_z + I_L

The circuit is designed such that the diode always operates in the breakdown region and the voltage V_Z across it remains fairly constant even though the current I_Z through it vary considerably.

If the load I_L should increase, the current I_Z should decrease by the same percentage in order to maintain load current constant I_s. This keeps the voltage drop across R_s constant and hence the output voltage.

If the input voltage should increase, the zener diode passes a larger current, that extra voltage is dropped across the resistance R_s. If input voltage falls, the current I_Z falls such that V_Z is constant.

In the practical application the source voltage, v_s, varies and the load current also varies. The design challenge is to choose a value of R_s which permits the diode to maintain a relatively constant output voltage, even when the input source voltage varies and the load current also varies.

We now analyze the circuit to determine the proper choice of R_s. For the circuit shown in figure,

    (E-1)

           (E-2)

The variable quantities in Equation (E-2) are v_Z and i_L. In order to assure that the diode remains in the constant voltage (breakdown) region, we examine the two extremes of input/output conditions, as follows:

• The current through the diode, i_Z, is a minimum (I_{Z min}) when the load current, i_L is maximum (I_{L max}) and the source voltage, v_s is minimum (V_{s min}).
• The current through the diode, i_Z, is a maximum (I_{Z max}) when the load current, i_L, is minmum (i_{L min}) and the source voltage v_sis minimum(V_{s max}).

When these characteristics of the two extremes are inserted into Equation (E-1),

we find                    (E-3)

  (E-4)

In a practical problem, we know the range of input voltages, the range of output load currents, and the desired Zener voltage. Equation (E-4) thus represents one equation in two unknowns, the maximum and minimum Zener current. A second equation is found from the characteristic of zener. To avoid the non-constant portion of the characteristic curve, we use an accepted rule of thumb that the minimum Zener current should be 0.1 times the maximum (i.e., 10%), that is,

   (E-5)

Solving the equations E-4 and E-5, we get,

      (E-6)

Now that we can solve for the maximum Zener current, the value of R_s, is calculated from Equation (E-3).

Zener diodes are manufactured with breakdown voltages V_Z in the range of a few volts to a few hundred volts. The manufacturer specifies the maximum power the diode can dissipate. For example, a 1W, 10 V zener can operate safely at currents up to 100mA.

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