A Zener diode is very often used in a lot of electronic circuits for instance in various power supplies, voltage detectors, etc.

It is a very helpful device in electronics and works in various ways. But do you think its good enough? You must understand its working before you can use it in any circuits.

In this article, we will learn about a Zener diode and its principle of working along with examples of its applications and usage.

Here is a brief explanation of why we must learn about them.

What is a Zener diode?

Zener diode is basically a two-pin device. It is a type of semiconductor with different properties as that of a general diode.

Take a look at the image below which shows a Zener Diode (Right) symbol in an electronic circuit. Also shown in the diagram is the shape of a real one which looks like a normal diode (left).

There are many sizes of a diode depending upon the wattage. The large size of a diode has more watts of power. The picture however is only half a watt (1W).

Photo credit: Zener by TeOhk

How a Zener diode works:

If you are a beginner, it’s natural to take a lot of time in understanding the working of a Zener diode. I might be speaking from my own experience and you will think otherwise after going through this article. Here you can also find a lot of images to help with your understanding.

Take a look at the block diagram given below.

Comparison of bias between Diode and Zener Diode

Diode and Zener Diodes both have different functions and basic bias.

• Left: Rectifier Diode requires a forward bias to work.
• Right: Zener Diode requires a reverse bias to work.

What is more to understand about them?

Internal virtual circuit:

Shown below is an internal virtual circuit:

The Zener Diode operates by using a breakdown voltage, which is often called Zener diode voltage by some people. It works during the Reverse bias.

During this breakdown, across the Zener Diode, the voltage drop will be constant. Keeping this principle in mind, we can use the Zener diode to maintain a constant voltage.

Now, you must take a look at the Graph showing Zener Diode with its properties.

The Zener Diode has the same property graph as a general diode but has a different breakdown voltage. In this diode, the breakdown voltage contains a higher voltage value.

For example, a 1N4001 Diode has a breakdown voltage of 50V, etc. but the Zener diode has a low voltage level depending on the properties of the Zener Diode.

In the lower range of the breakdown voltage level It will have a small amount of voltage and current, flows through it if you are considering a reverse bias as shown in the graph.

The leakage current in the Zener diode is very low so it does not affect the work of a Zener Diode. I hope you get the basic idea now.

Let’s see the example of circuit diagrams where we have used this phenomena. We use it as a voltage detector it is very easy yet very helpful.

How can a Zener Diode be compared to anything?

When I started off, I did not understand how a Zener diode works but when I took a look at the image below I had a very clear understanding of it. Hope you will also feel the same.

Imagine a Zener diode that looks like a punctured can. Look at the block diagram given below.

Zener diode that looks like a punctured can:

My experience says that a picture explains concepts much better than written text. Is it true?

Let me explain the diagram to you.

• Faucets are connected to the power supply.
• Water acts as a conductor to conduct electricity.
• The water level in the can is comparable to Zener voltage which will be at the same level as the hole drilled on the side of the can.

Here is the step by step process.

• When the tap is opened the water will start to flow from the tap to the can. The side of the can is punctured.
• When the water level reaches the drilled hole, the water will start to flow out from it.
• The water level is constant, not likely to be higher than the hole level. The Zener Diode operation works with the same principle.

While supplying the current through R1 to Zener diode via K, cathode and pin A, the Anode is grounded.

Let’s say it is Zener Diode No. 1N5225 or BZX55C3V0 (3V) 0.5 watts. It has a Zener Diode voltage equal to 3 volts. (See Table 1).

• BZX55C2V0 (2V)
• BZX55C2V2 (2.2V)
• BZX55C2V4 (2.4V)
• BZX55C2V7 (2.7V)
• BZX55C3V0 (3V)
• BZX55C3V3 (3.3V)
• BZX55C3V6 (3.6V)
• BZX55C3V9 (3.9V)
• BZX55C4V3 (4.3V)
• BZX55C4V7 (4.7V)
• BZX55C5V1 (5.1V)
• BZX55C5V6 (5.6V)
• BZX55C6V2 (6.2V)
• BZX55C6V8 (6.8V)
• BZX55C7V5 (7.5V)
• BZX55C8V2 (8.2V)
• BZX55C9V1 (9.1V)
• BZX55C10 (10V)
• BZX55C11 (11V)
• BZX55C12 (12V)
• BZX55C13 (13V)
• BZX55C15 (15V)
• BZX55C16 (16V)
• BZX55C18 (18V)
• BZX55C20 (20V)
• BZX55C22 (22V)
• BZX55C24 (24V)
• BZX55C27 (27V)
• BZX55C30 (30V)
• BZX55C33 (33V)
• BZX55C36 (36V)
• BZX55C39 (39V)
• BZX55C43 (43V)
• BZX55C47 (47V)

This shows that whether the power supply has a higher voltage than the Zener diode voltage, it will still be within a certain limit.

Basic Zener diode; forward bias and reverse bias:

The Zener diode always keeps the voltage drop across it at 3V. The remaining voltage drop will be across the resistor.

Not only that, go through the next circuit diagrams.

See an example of the basic Zener diode between a forward bias (A) and a reverse bias (B).

• Look at the circuit A.
When we enter a forward bias voltage the anode pin has more voltage than the cathode. The positive current enters the anode of Zener Diode (ZD1) via Resistor (R1).

The operation of a Zener diode is like a general diode. It will allow the current to flow through it and there is the voltage drop across of about 0.6V. The rest of the voltage drop is across the resistor.

When the voltage of the Zener diode is combined with the resistor, we get the voltage equal to the power supply.

• Look at the circuit B.
In contrast, when we enter a reverse bias the cathode pin has more voltage than the anode pin.

At this time, the Zener Diode will have different features than normal diodes.

The common diodes will not allow currents to flow through them.

But in this situation, the Zener diode will allow the current to pass through it only when the voltage across the reverse bias is greater than the Zener diode voltage.

In this situation, it works perfectly. Since the power supply is 6V and the voltage of the Zener diode is 3V and, the voltage across the Zener diode remains constant. It is the same breakdown voltage level as shown above.

This voltage level (Vz) can be changed by changing the number of Zener diodes as the manufacturer must have specified. There are many numbers and many sizes available as mentioned above.

What is more to learn? We will learn the uses of a Zener diode with the help of many examples of circuits shown below.

How to use a Zener diode:

Normally a Zener diode is used as the regulator circuit. There are many forms of that circuit explained as follows.

Simple current and constant voltage regulator

Look at the basic circuit below.
It is a low current regulator circuit that is controlled by the resistor R1 and the output voltage has a constant value equal to the zener diode voltage in any loads.

We can calculate the appropriate resistance R1 using the following formula:

R1 = (Vin – Vz)/ (IL + Iz)

It comes to understanding easily with practice. The current is Iz, while the load is connected. We usually set the current to 5mA. With this, we get a new formula.

R1 = (Vin -Vz)/ (IL + 5mA)

With this formula, we have chosen the resistance to be dependent upon only on the current flowing through the load. If we want to calculate the current for some real circuits we have to offset for IZ current making sure that there is no load as well.

If there is no continued loading, the current will flow through all Zener Diodes. It should allow the wattage of the Zener Diode to endure while it is no-load as well.

Want to see the real calculations to find R1?

Remember:

If we are choosing a Zener Diode we need to look at the watts that the Zener diode is able to tolerate.

It can be calculated as follows:
The wattage power lost in the Zener Diode (P) is equal to the Zener diode voltage (Vz) times the current passing through the Zener diode (Iz).

P = Vz x Iz

Note: Iz is obtained from the voltage across the resistor divided by the resistance of that resistor(R).

Do you get an idea of the calculations now?

Basic Higher current Zener and transistor regulator

Look at the circuit shown below. It is very much similar to the previous circuit but it can supply a higher current. The reason is that the transistor is a helper to increase the current up.

We connect it in series before output. Then use a Zener diode voltage as a bias voltage is used for the Transistor. The output voltage of this circuit is less than Zener diode to about 0.6V.

The voltage of a Zener Diode will drop between base and emitter of transistor about 0.6 volts.

The maximum current that the circuit can supply depends on the capability of the transistor.

If the transistor has a high tolerance to the current it can supply very high current. On the other hand, if there is little resistance it will supply a lower current.

Parts lists

• Q1: 2N3053, 0.7A 40V NPN Transistor
• ZD1: 12V 0.5W Zener Diode
• C1: 10uF 16V Electrolytic Capacitor
• R1: 1.2K 0.25W 5% Resistor
• R2: 4.7K 0.25W 5% Resistor

Check out these related circuits, too:

Have you observed it? The output is 11.4V only but we want 12V. What do you do here?

The solution to this problem is to make the output voltage equal to the Zener diode voltage.

Look at the circuit diagram. Add Diode to offset B-E transistor voltage.

Add Diode to offset B-E transistor voltage

By bringing the Rectifier Diode to series with a Zener diode, the voltage across the diode will be just offset to the voltage across the pin B-E of the transistor.

The output voltage is therefore equal to the Zener Diode voltage.

Make 3V DC regulator using Zener Diode and transistor

It is much easier if we use suitable a Zener diode.

Look at the circuit shown below.

We have used the Zener Diode No. 1N5227 or BZX55C3V6. It has a Zener Diode voltage equal to 3.6V.

When the current flows through the base to the emitter, the voltage across the base and emitter will be of about 0.6V.

Therefore we need a reserve voltage of 0.6V. The output voltage is approximate 3V

For other devices, it has the same principle of DC power supply.

When the transformer reduces the voltage to 9V, it will pass to the rectifier diodes D1 and D2 (full-wave rectifier) so that it can be DC voltage.

Then C1 will make the DC current smoother. It passes Resistor R1 to the cathode of Zener Diode.

Next, C2 is a filter capacitor to keep the Zener voltage stable. C3 is a filter capacitor that reduces a ripple.

This circuit can give output 3V at 800mA max.

See parts list below

• Q1: 2SC1061, 4A 40V NPN transistor
• ZD1: 3.6V 0.5W Zener diode ,1N5227 or BZX55C3V6
• D1,D2: 1N4001, 1A 50V Diode
• R1: 5K 0.25W 5% Resistor
• C1,C3: 1,000uF 16V Electrolytic Capacitors
• C2: 1uF 16V Electrolytic Capacitor
• T1: 117V/230V AC primary to 9V-0-9V,1A secondary transformer

What is more to learn?

Voltage comparator Zener op-amp regulator

Which circuit is better?

In addition to this method, we also have a way to compare the output voltage with the Zener diode voltage. This is done by using an op-amp as a comparator. As shown in the figure below.

Voltage comparator Zener op-amp regulator circuit

When the power goes into the input, there is a voltage of 12V across the Zener diode. Therefore, pin 3 of the op-amp (CA3140) has the voltage equal to 12V as well.

When the power goes into the input, there is a voltage of 12V across the Zener diode. Therefore, pin 3 of op-amp has the voltage equal to 12V too.

It causes the output-pin 6 of an op-amp to have a positive voltage. The bias Q1 works and the current flow through pins C-E and R3.

If pin 2 and pin 3 have a higher voltage, then the voltage will come out pin 6. The bias Q1, has more current flowing through it until the voltage in pin 2 and 3 are equal.

We will see that this circuit has high stability than only one transistor used in a circuit.

Parts you will need

• IC1: CA3140, 4.5MHz, Bimos Operational Amplifier With MOSFET Input/Bipolar Output
• Q1: 2N3053, 0.7A 40V NPN Transistor
• ZD1: 12V 0.5W Zener diode
• C1: 10uF 25V Electrolytic capacitor
0.25W Resistors, tolerance: 5%
• R1, R3: 1.2K
• R2: 4.7K

Conclusion

We can see that the Zener Diode can be used in various circuits and has many advantages.

Here are a few related posts you might want to read:

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