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## Theory OF OPERATION :

Fig.1 Zener voltage regulator

Zeners are used to create a constant voltage inside a circuit somewhere, simply by providing them with a (roughly constant) current derived from a higher voltage within the circuit this is due the V-I curve of the zener diode. But it is important to know how the resulting zener voltage will change with applied current; this is a measure of its “regulation” against changes in the driving current provided to it. Included in the specifications of a zener will be its dynamic resistance, given at a certain current.

Because V-I curve of the diode varies this means that the zener resistance is dynamic we can apply the definition of dynamic resistance:

\large R_{dyn}= \frac{\Delta V}{\Delta I}
(1)

Then :

\large \Delta V = R_{dyn}*\Delta I
(2)

According to the ohms law we have :

\large I = \frac{V_{in}-V_{out}}{R}
(3)

But because the voltage and current are not constant we get :

\large \Delta I = \frac{\Delta V_{in} - \Delta V_{out}}{R}
(4)

Setting equation (2) and (4) we have :

\large \Delta V_{out} = R_{dyn}*\Delta I = \frac{R_{dyn}}{R}*(\Delta V_{in} - \Delta V_{out})
(5)

And finally :

\large \Delta V_{out} = \frac{R_{dyn}}{R}*\Delta V_{in}
(6)

As you can see from the formula (6) the output voltage can very depeding on the dynamic resitance of the zener diode but it will be mentained around of zner breakdown current. You can check the dynamic resitance of the zener diode in the datasheet.

## PRACTICAL EXAMPLE :

If you don’t see the example below than you should follow this steps:

– In your browser allow Java SE 7.

– Lower you java security settings (Go to Control Panel >> Java >> Security and set the security level to medium) .

– Edit Site List (Go to Control Panel >> Java >> Security and click on Edit Site List… and add eagerlearning.org in the list).

Let’s take a look at a zener voltage regulator and check out which is the outputted voltage.

Sorry, you need a Java-enabled browser to see the simulation.

If you want to change resistor, zener diode or DC voltage value than you should double click on the element and insert the desired value. First diagram is showing the V-I characteristic of the Zener Diode. The second diagram is basically the same, but you can also see the power dissipated and other characteristics.

Last updated on Mon , 03 2014