The Zener diode is similar to a general-purpose 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