In this example a Zener diode / voltage reference diode circuit design is used to supply a regulated 5.1 Volt rail consuming 2 mA, from an input voltage supply of 12 volts.

The following easy steps can be used to calculate the resistor required:

1) Calculate the difference in voltage across the series resistor

12 volts−5.1 volts=6.9 volts12 volts-5.1 volts=6.9 volts

2) Determine the resistor current. Choose this to be 15 mA. This will allow sufficient margin above the minimum Zener diode current for some variation in the load current.

3) Check the Zener diode power dissipation. At a current of 15 mA and a voltage across the power dissipation is:

15mA*  5.1V=76.5mW

This is nicely within the maximum limit for the diode

4) Determine the current through the series resistor. This is 15 mA for the Zener diode plus 2 mA for the load, i.e. 17 mA.

5) Determine the value of the series resistor. Using Ohms law this can be calculated from the voltage drop across it and the total current through it:

6.9V/ 17mA=0.405kΩ

The nearest value is 390 ohms

6) Determine the wattage of the series resistor. This can be determined using the value for the current through the resistor and the voltage across it calculated earlier:

V I=6.9V x 17mA=117mW

The resistor needs to be able to dissipate this level of heat. A quarter watt resistor should be adequate for this.