1. Zener Diode as a Voltage Regulator
By Dr. Vaibhav Jain Associate Professor, Dept. of Physics, D.A.V (PG) College, Bulandshahr, U.P., India.

2. A voltage regulator supplies constant voltage to a load.
Voltage Regulator - Zener Diode
A voltage regulator supplies constant voltage to a load.

3. The breakdown voltage of a Zener diode is nearly constant over a wide range of reverse-bias currents.
This make the Zener diode useful in a voltage regulator, or a constant-voltage reference circuit.
3. The remainder of VPS drops across Ri
2. The load resistor sees a constant voltage regardless of the current
1. The zener diode holds the voltage constant regardless of the current

4. Example
A Zener diode is connected in a voltage regulator circuit. It is given that VPS = 20V, the Zener voltage, VZ = 10V, Ri = 222  and PZ(max) = 400 mW.
Determine the values of IL, IZ and II if RL = 380 .
Determine the value of RL that will establish PZ(max) = 400 mW in the diode.

5. For proper function the circuit must satisfied the following conditions.
The power dissipation in the Zener diode is less than the rated value
When the power supply is a minimum, VPS(min), there must be minimum current in the zener diode IZ(min), hence the load current is a maximum, IL(max),
When the power supply is a maximum, VPS(max), the current in the diode is a maximum, IZ(max), hence the load current is a minimum, IL(min)
AND
Or, we can write

6. For general thumb of rule for design this circuit is, so from the last Equation
Maximum power dispassion in the Zener diode is
EXAMPLE  
Consider voltage regulator is used to power the cell phone at 2.5 V from the lithium ion battery, which voltage may vary between 3 and 3.6 V. The current in the phone will vary 0 (off) to 100 mA(when talking). Calculate the value of Ri and the Zener diode power dissipation
simple Zener diode voltage regulator circuit

7. Solution:
The stabilized voltage VL = 2.5 V, so the Zener diode voltage must be VZ = 2.5 V. The maximum Zener diode current is
The maximum power dispassion in the Zener diode is
The value of the current limiting resistance is

8. Thank You