Zener Diode Series Resistor Calculator

Zener Diode Voltage Regulator Circuit

A Zener diode is a component that takes an unregulated DC voltage source to produce a regulated output. A series resistor “Rs” is typically required to produce a voltage drop and the absence of this resistor would result in damage to the diode.

In this circuit diagram, the unregulated input voltage is Vs, and the regulated output voltage is “Vz” produced across the diode. The series resistor “Rs” provides a voltage drop and its value calculated from the formula shown. Due to the simplicity of the circuit, it is a cost-effective way to produce a reference voltage.

These diodes operate in reverse conduction due to the Zener effect, described by the breakdown region in the diode characteristics curve.

It is able to regulate the voltage across it by passing a limited amount of current through it in reverse conduction. It is a very simple and interesting phenomenon worth reading about.

The reverse voltageVz”, and the reverse currentIz” are the main parameters to consider when choosing a suitable diode for regulation purposes. These parameters are also called “Zener current”, and “Zener voltage” in literature.

Resistor Calculator Formula

To calculate the value of the series resistor, you only need Vz and Iz, found from the table below, and the input voltage Vs, which you would know.

Therefore, as you can see, this article makes it far easier to calculate the resistance value using commonly available parameters.

Calculator

Vz (V)


Iz (A)


Vs (V)



Rs:



Power Dissipation P

Power rating of diode (watts)

The maximum power dissipation P of the diode is given by the formula P = Iz × Vz. Hence, the diode you choose must have a power dissipation rating above this value.

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

Choosing Vz, and Iz

The reverse conduction voltage -- engineered to occur at desired values -- enables this type of diode to regulate at specific voltages.

The chart below shows a typical range of reverse conduction voltages Vz and their associated reverse currents Iz expected from a common garden-variety BZXC family of diodes.

Vz (V)Iz (mA)
2.7360
3.0330
3.3300
3.6290
3.9280
4.3250
4.7215
5.1200
5.6190
6.2170
6.8155
7.5140
8.2130
9.1120
10105
1197
1288
1379
1571
1666
1862
2056
2252
2447
2741
3036
3333
3630
3928
4326
4723
5121
5619
6216
6815
7514
8212
9110
1009.4

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