Zener Diode Series Resistor Calculator

Zener Diode Voltage Regulator Circuit

A Zener diode is an electronic component utilised in DC voltage regulator circuits. A series resistor “R” is typically required to produce a voltage drop "VZ" across the load.  In this circuit diagram, the unregulated input voltage is VS, and the regulated output voltage across the load is “VL”, which is the same as that across the diode. The series resistor “R” provides a voltage drop and we calculate its value from the formula below.

Zener diodes operate in reverse bias mode due to the Zener effect that occurs near the breakdown region of the diode characteristics curve. This enables it to regulate the voltage across its junctions by allowing a limited amount of current in reverse conduction to flow. It is a very simple and interesting phenomenon worth reading.

The voltage and wattage ratings are the main parameters to consider when selecting a suitable diode for regulation purposes. A limitation in this type of circuit design is the available range of standard diode values.

Resistor Calculator Formula


If you need more information, then the theory section of this page explains the meaning of these variables.


Current (IL) requirement of the load:

Minimum Zener current (IZ) requirement of the diode:

Zener Values

Regulated voltage (VL) requirement across load:

Zener Values

Source Voltage (VS):

Results are shown below.

Total Current Through Resistor R


Voltage Drop Across Resistor R


Value of Resistor R


Ideally you should chose the closest lowest from the E24 Series, to ensure that the diode receives sufficient Zener current to operate.

Diode Power Dissipation

Power rating of diode (watts):

All the diodes in the BZX85C range have an absolute maximum dissipation rating of 1.3 watts.

Resistor Power Dissipation

Power rating of resistor (watts):

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