Saturday, May 18, 2013

Zener Diode And Linear Voltage Regulation

Good Afternoon everyone!

This post will be about Zener Diode, a spacial purpose diode which is used widely for Linear Voltage Regulation. So, first I'm going to say something about Zener Diode then I will show you how they can be used in circuits.

First of All what is a Zener Diode?

A Zener diode is a diode which allows current to flow in the forward direction in the same manner as an ideal diode, but will also permit it to flow in the reverse direction when the voltage is above a certain value known as the breakdown voltage or "Zener knee voltage" or "Zener voltage" or "avalanche point". As I was saying it is a spacial type of diode which is designed to operate in its reverse breakdown region ( See the graph below).

The device was named after Clarence Zener, who discovered this electrical property. Many diodes described as "Zener" diodes rely instead on avalanche breakdown as the mechanism. Both types are used. Common applications include providing a reference voltage for voltage regulators, or to protect other semiconductor devices from momentary voltage pulses.

In simple word, Zener diode can keep a constant voltage across it, if all the parameters are correct. Many of our devices need constant voltage to run, for example a CMOS IC. This type of IC can not run with an unregulated voltage, that might destroy it, so we need constant voltage. A Zener diode can help us do so.

How does it work?

Zener diodes have a lower breakdown voltage which is called the Zener voltage, By contrast with the conventional device, a reverse-biased Zener diode will exhibit a controlled breakdown and allow the current to keep the voltage across the Zener diode close to the Zener breakdown voltage. For example, a diode with a Zener breakdown voltage of 6.4 V will exhibit a voltage drop of very nearly 6.4 V across a wide range of reverse currents. The zener diode is therefore ideal for applications such as the generation of a reference voltage or as a voltage stabilizer for low-current applications.

A Zener diode is heavily doped to reduce the reverse breakdown voltage. This causes a very thin depletion layer. So it has a sharp reverse breakdown voltage Vz. Reaching the Vz, two things happen,

i. The diode current increases rapidly (as you can see in the graph)
ii. The reverse voltage across the diode remains almost constant ( Constant Zener Voltage on the Graph)

So, Zener diode's working region is the region where the voltage across it stays almost same with different values of current. This permits Zener Diodes to be used as voltage regulator.

So, in the applications where constant voltage is needed we can use Zener Diodes.

Simple Diagram :

Zener Diodes keep a constant voltage across it which is very close to its Zener voltage. Zener voltage is labeled on the Zener diode. Using this simple circuit we can get constant voltage in the output from different input voltage levels.

In the figure above, we will just give different voltage levels as input, say we are using a source which has a voltage of 10V-15V. Now we need 5V constant for our application. So, we will just use a 5.6V Zener Diode. The Resistor will limit the current and keep it within Zener Diode's maximum current level. So, we are giving a variable input voltage and we will get a constant output voltage. 

But this is not a very good way to use it, this assembly burns too much power and generates a lot of heat as a result the output current becomes very low. To solve this problem Zener diodes are usually used with transistors.

A bit More Advanced & Practical Diagram:

If we add an emitter follower stage to a simple Zener based regulator forms a simple series voltage regulator. It improves the regulation of the circuit greatly and give us higher output current. It is called the series voltage regulator because the regulating element, here the transistor is in series with the load.

Here the Zener Diode creates a reference voltage that is very close to its Zener Voltage because full voltage of the unregulated input appears across the Zener Diode, the Resistor R limits the current and as such power. Transistor is the series control element.  For small applications like low current regulated voltage generator a D400 (NPN) transistor will be enough. For high current TIP series will be a good choice or any other powerful transistor.

Capacitors are added as smoothing element. Zener Diode and transistor can be used to Regulate both positive and negative voltage as shown in the diagram.

For 200mA use a 2N2222 or equivalent.
For 500mA use a D400 or equivalent.
For 1.5A use a BD135/136 or equivalent.
For 5A use a TIP 122 or equivalent.
For 15A use a 2N3055 or equivalent.

What happens here is

1. If the output voltage decreases , the increased base-emitter voltage causes transistor to conduct more,thereby raising the output voltage-maintaining the output constant

2. If the output voltage increases, the decreased base-emitter current causes the transistor to conduct less,thereby reducing the output voltage-maintaining the output constant.

How to find the proper Resistor Value: 

Let's assume that It is a 10V Zener Diode and maximum Zener Power is 500mW. Assume supply voltage to be around 15V.

Here transistor is the load.

Remember in this procedure, Zener diode has a voltage drop of 0.6V, so a 5.6V Zener Diode will actually give an output of 5V.

To find out the power dissipated by the transistor we can use this simple equation: 

Make sure to keep the power within its specified limit and also use a proper heat sink if necessary.

Zener Voltage will be labeled on the Zener Diode.
Zener Current can be found in the Zener Datasheet.
DC Current Gain hFE of the Transistor can be found in the Transistor Datasheet. 

Where it can be used?

This diagram can be used widely with many different circuits. It is very simple yet it works fine.

It can be used where regulated voltage is needed.

Like operating CMOS IC, Other Digital IC, Power supply for micro-controller and small hobby project circuits, Battery chargers and many more things !

You can easily modify this circuit. For example add a couple of indicators, change transistors or resistors to see what happens.

All the components used in this circuit diagram are very popular & can be easily found.

So, Happy Experimenting.

See how to convert AC to DC here.

See how a Zener Datasheet and a Transistor Datasheet looks like!

Read my other posts here!  Index page.