Zener Regulator Design

One approach to Zener regulator design is to consider the desired load current and the amount of ripple present in the available unregulated supply.

If the available power supply has

= v

,

or ripple factor = %

and a zener voltage

= V

Then to design for regulated output current of 0 to

= mA

requires a resistor value

=

This design with a load resistance

=

will give an output current

=

mA

Caution!! If this current is greater than the maximum design current you chose above, then it will not happen! You have chosen too small a load resistance. Also, the power values below will not be correct, because you are beyond the design range and will not be supplying enough voltage to the zener diode to allow it to give the zener voltage out.

and output power

= W

To do this, it will require input power

= W.

With this design approach, you must check the power ratings of your output resistor and Zener diode to make sure they are able to handle the power requirements.

The power rating of the load resistor must be at least

= W

and the power rating of the zener diode must be

= W

This design strategy is a practical, operational one: to design for a ripple-free output voltage up to a specified current maximum. Once done, you can choose load resistances which push the current up to this maximum and maintain regulation. However, it is easy to choose load resistances which push the current over the design maximum, and then the powers which this routine calculates will not be correct - they assume that the zener diode is operating at it's design voltage, and if you choose too small a load resistance, you drop the supply voltage at the zener below that voltage. The zener would then be acting like a voltage source, which of course it cannot do - it must have a supply voltage greater than the design voltage in order to regulate to that voltage.

Calculation note: You may substitute values for the zener voltage, the input voltage, the ripple voltage or ripple factor, and the load resistor. The other parameters will be calculated. Default values will be entered for unspecified parameters used in the calculation.

Caution about use of the ripple factor: the relationship between the ripple factor and the amplitude of the ripple factor uses some details about the nature of common filters, and the numerical value of the ripple factor will be accurate only for rectified 60 Hz inputs where the ripple is fairly small. See the development of the ripple expression.