Zener Diode Regulator Circuit
The Zener diode is similar to a generalpurpose diode. When a reverse voltage exceeds the diode breakdown voltage, current starts to flow through the diode. For a Zener diode, the current increases dramatically to the maximum circuit value (which is usually limited by a series resistor). The reverse saturation current remains fairly constant over a wide range of applied voltages. The "Zener voltage" is when the voltage across the Zener diode becomes stable. Because of this charecteristic, the Zener diode is often used as a voltage regulator in low power circuits.
Designing a voltage regulator using a Zener diode requires the following information:
 E_{S}(min)
The minimum unregulated voltage supplied to the circuit.
 E_{S}(max)
The maximum unregulated voltage supplied to the circuit.
 E_{Z}
The rated voltage of the Zener Diode.
 I_{L}(max)
The maximum current drawn by the load.
 I_{L}(min)
The minimum current drawn by the load (should normally be 0 VDC).
Knowing the above pieces of information, the calculations are fairly straight forward:
 The resistor (R_{S}) limits the power that needs to be absorbed by the Zener diode:
R_{S} = [ E_{S}(min)  E_{Z} ] / [ 1.1 * I_{L}(max) ]
 The power rating of R_{S} is calculated by the equation:
P_{R} = [ E_{S}(max)  E_{Z} ] * I_{L}(max)
 The power rating of the Zener diode is based on no current being delivered to a load:
P_{Z} = E_{Z} * [ { ( E_{S}(max)  E_{Z} ) / R_{S} }  I_{L}(min) ]
As an example, calculate the required values for the following circuit:
 E_{S} varies between 11 VDC and 13 VDC.
 E_{Z} is 5VDC.
 I_{L}(max) is 100 mA.
 I_{L}(min) is 4 mA, but assume it will be 0 mA.
The calculations are:
 R_{S} = [ E_{S}(min)  E_{Z} ] / [ 1.1 * I_{L}(max) ]
R_{S} = [ 11  5 ] / [ 1.1 * 0.100 ] = [ 6 / 0.110 ] = 54.5 ohms
 P_{R} = [ E_{S}(max)  E_{Z} ] * I_{L}(max)
P_{R} = [ 13  5 ] * [ 0.100 ] = [ 7 * 0.100 ] = 0.7 watts
 P_{Z} = E_{Z} * [ { ( E_{S}(max)  E_{Z} ) / R_{S} }  I_{L}(min) ]
P_{Z} = 5 * [ { ( 13  5 ) / 54.5 }  0 = 5 * [ { 7 / 54.5 }  0 ] = 5 * [ 0.128  0 ] = 5 * 0.128 = 0.642 watts
