Zener Diode as Voltage Regulator

The half-wave and full-wave rectifiers with filters are the simplest type of power supplies. These provide almost pure dc but have one deficiency. When load current is increased by decreasing resistance, the output voltage drops. This is because, when large current is drawn, the filter capacitor gets discharged more and its voltage across the load resistor reduces. Similarly, if the ac input changes, the dc output voltage also varies.

A supply with varying output voltage affects the performance of different devices being operated with it. To remove this deficiency, a Zener diode is used with simple power supplies which gives constant dc voltage. Such a circuit is called regulated power supply.

The Zener regulated voltage supply circuit consists of a Zener diode with breakdown voltage V_z. This will be equal to the stabilized output voltage V_O. A suitable series resistance R_s is included to control circuit current and dissipate excess voltage. The anode of Zener diode is connected to the negative terminal of input supply, and the cathode is connected in series with R_s to positive terminal of input supply, that is, the Zener is connected in reverse bias condition.

The load resistance is connected across the Zener diode. The Zener regulator will only operate if the input supply voltage to the regulator, V_i is greater than V_z. After breakdown, the voltage across it remains nearly constant and is independent of the current passing through it. The current I_s flowing passing through R_s is given by

I_s = (V_i –V_z)/R_s

This current divides in two parts: the Zener current I_z and load current I_L. Applying Kirchoff’s law,

I_s = I_z + I_L

I_z = I_s – I_L

For Zener diode to operate, some current I_Zmin should always flow through it. Therefore, the load current I_L should always be less than the main current I_s. Typical value of I_Zmin may range from 5 mA to 20 mA.

If load current is zero, the entire I_s will pass through Zener diode and output voltage V_O will be equal to V_z. When some load current is drawn, say I_L, the Zener current will decrease by the same amount but the output voltage will remain V_z.

Similarly, if the ac main voltage increases or decreases, the input voltage, V_i will increase or decrease accordingly. It will result in change of I_s. Due to change in I_s, the change in V_i will appear as a drop across the series resistance R_s. The Zener voltage V_z and hence V_O will remain unchanged. 

The power dissipation in Zener diode is given by

P_d = V_z × I_z

This dissipation should not exceed the maximum power dissipation rating recommended by the manufacturer for Zener diode.