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Jul 26 2019

Zener Diodes Tutorial: What is the Principle of Zener Diode?


This electronics video tutorial provides a basic introduction into zener diodes which is used as voltage regulators in DC circuits.


Ⅰ Introduction

1.3 Electronic symbol

1.6 The effect of positive and negative serie

3.1 Zener diode characteristic

Ⅳ Silicon Zener Diode

1.1 Terminolog

1.4 Volt-ampere characteristic

Ⅱ Voltage Regulation Principle

3.2 Grid voltage fluctuation and load variation

Ⅴ Example Analysis

1.2 Calculation of Voltage Regulation Resistance

1.5 Typical voltage regulation circuit diagram

Ⅲ Zener Diode Application Circuit

3.3 Major parameters

Ⅵ  Application Attentions

1.1 Terminology

Zener diode is a active device. It uses a pn junction reverse breakdown state to make a result that current can be varied in a wide range and the voltage is basically constant, that is, a diode has voltage regulation effect. This diode is a semiconductor device that has a very high resistance until having critical reverse breakdown voltage. At this critical breakdown point, the reverse resistance is reduced to a small value, in this low resistance region, the current increases and the voltage remains constant. So that the Zener is used primarily as a voltage regulator or voltage reference component.

When the reverse voltage of the Zener diode reaches a certain value, the reverse current suddenly increases, and the Zener diode enters the breakdown region, but it does not damage, but works at a normal state, which is the biggest difference from the ordinary diode.

After it enters this operating state, even if the reverse current varies over a wide range, the reverse voltage across the Zener diode can remain substantially unchanged. On the other hand, if the reverse current continues to increase to a certain value, the Zener diode will be completely broken down and damaged.

Therefore, when the Zener diode is used, it must be connected in series with a current limiting resistor. Otherwise, its power consumption exceeds the specified value, which may cause device damage.

1.2 Calculation of voltage regulation resistance

The performance of the Zener diode regulator circuit is related to the dynamic resistance in a  breakdown state, and is also related to the resistance value of the voltage regulator resistor R. The smaller the dynamic resistance of the Zener diode, the larger the voltage regulator R and the better the voltage regulation performance.

The dynamic resistance of the Zener diode varies with the operating current. The larger the operating current, the smaller the dynamic resistance. Therefore, in order to make a good voltage regulation effect, the working current should be selected properly. The operating current is larger that the dynamic resistance can be reduced effectively, but not to exceed the maximum allowable current (or maximum power dissipation) of the diode. And the operating current and maximum allowable current for various types of diodes can be found in the manual.

The stability of the Zener tube is also affected by temperature. When the temperature changes, its stable voltage also changes, which is usually expressed by the temperature coefficient of the stable voltage.

a. When the input voltage is the smallest and the load current is maximum, the current flowing through the Zener diode is minimum. At this time, IZ should not be less than IZmin, thereby calculating the maximum value of the voltage stabilizing resistor, and the actually selected voltage stabilizing resistance should be less than the maximum value, which is

b. When the input voltage is the largest and the load current is the smallest, the current flowing through the Zener diode is the largest. At this time, IZ should not exceed IZmax, thereby calculating the minimum value of the voltage stabilizing resistance. which is


1.3 Electronic symbol

Zener electronic symbol

1.4 Volt-ampere characteristic

Zener volt-ampere characteristic curve

Fig. 1 Volt-ampere Characteristic Circuit

The volt-ampere characteristics of a Zener diode are similar to those of a normal diode, except that

(1) steep reverse breakdown curve

(2) working at reverse breakdown state

Commonly used voltage regulation values: 3.3 V, 3.6 V, 3.9 V, 4.7 V, 5.1 V, 5.6 V, 6.2 V, 15 V, 27 V, 30 V, 75 V

1.5 Typical voltage regulation circuit diagram

Typical regulation circuit

Fig. 2 Typical Regulation Circuit

1.6 The effect of positive and negative series

1. In the power amplifier circuit, the gate G and the source S of the power tube always series with a Zener diode, which protects the G-S by limiting the voltage and prevents the insulation layer between the G-S from being breakdown by too high voltage.

2. When the two diodes are connected in series in reverse, the circuit connected in parallel can provide overvoltage protection. When the circuit is in overvoltage, the diode first breaks through to make the short circuit.

The maximum function of the Zener diode is to stabilize the voltage. It is necessary to ensure that the current does not exceed the limit through the series current limiting resistor. If there is no current limiting resistor, it can only provide a single overvoltage protection, and it is easy to cause avalanche breakdown permanent failure, resulting in a short circuit. Generally, the power supply of the CPU can be connected in parallel with a Zener diode that is 20% higher than the working voltage of it. When the power supply causes a voltage that is too high, the Zener diode is reverse-conducted to protect the CPU from being burned. To continue working normally, it is only needed to check the power supply and replace the Zener diode.

It can be seen from the above that the Zener diode is in the reverse current breakdown, within a certain current range (or within a certain power loss range), the terminal voltage is almost constant, showing the voltage regulation characteristics. This sentence contains two meanings:

1) The voltage-stabilizing diode is to be reverse-connected to the circuit.

2) Zener diode must work within a certain range (up to a stable current state) to stabilize.


Ⅱ Voltage Regulation Principle

To understand how a Zener diode works, just look at the reverse characteristics of it. The basic characteristic of all crystal diodes is the unidirectional conduction. That is, adding forward voltage is turned on, and the reverse voltage is blocked. In addition, the adding reverse voltage does not exceed the reverse withstand voltage of the diode, otherwise the Zener diode will be burned. But this is not the final result. The test found that as long as the reverse current value is limited (for example, a resistor is placed in series between the diode and the power source), it will not burn out although it is broken down. Moreover, it was found that after the reverse breakdown of the diode, the current decreased sharply, and the voltage dropped only slightly. The voltage dropped sharply with the decrease of the current to a certain current value. It is this principle that makes use of the Zener diode. And the most important point to using a Zener diode is to design its current value.

The characteristic of a Zener diode is that after breakdown, the voltage across it remains essentially the same. Thus, when the voltage regulator is connected to the circuit, if the voltage of each point in the circuit fluctuates due to fluctuations in the power supply voltage or other reasons, the voltage across the load will remain substantially unchanged.


Ⅲ Zener Diode Application Circuit

3.1 Zener diode characteristics

Generally, the normal diodes are forward-conducting and reverse-cut. When the reverse voltage applied to the diode, if it exceeds the capability of the diode, the diode is broken down. However, there is a diode whose forward characteristic is the same as that of a normal diode, but the reverse characteristic is special: when the reverse voltage is applied to a certain degree, although the diode exhibits a breakdown state, a large current is passed, but it is not damaged, and this phenomenon is very reproducible. On the contrary, as long as the diode is in a breakdown state, although the electricity flowing through the tube varies greatly, the voltage across the diode changes very little to stabilize the voltage. This is Zener diode.

The types of Zener diodes are 2CW, 2DW, etc. The circuit symbols are shown below.

electronic symbol

The voltage regulation characteristics of the Zener diode can be clearly expressed by the volt-ampere characteristic curve shown in the figure below.


volt-ampere characteristic curve

Fig. 3 Volt-ampere Characteristic Curve

The Zener diode works by using the voltage regulation characteristic of the reverse breakdown. Therefore, the Zener diode is connected in reverse in the circuit. The reverse breakdown voltage of the Zener diode is called the stable voltage, and the stable voltage of different types of Zener diodes is also different. The voltage regulation value of a certain type of Zener diode is fixed in a certain range. For example, the 2CW11 regulation value is 3.2 volts to 4.5 volts, where one diode may have a voltage regulation of 3.5 V and the other may be 4.2 V.

In practical applications, if the Zener diode is not selected to meet the required voltage regulation requirement, a Zener with a lower regulation voltage can be selected. And then one or several silicon diode as "pillow pads" can be connected in series to increase the stability voltage to meet the required value. This is achieved by using a silicon diode with a forward voltage drop of 0.6 V to 0.7 V. Therefore, the diode must be connected in the forward direction of the circuit, which is different from the Zener diode.

The voltage stabilizing performance of the Zener diode can be expressed by its dynamic resistance r:

simple regulation circuit.png

Fig. 4 simple regulation circuit

Obviously, for the same current variation ΔI, the smaller the voltage variation ΔU across the Zener diode, the smaller the dynamic resistance, and the better the performance of the Zener diode.

regulation circuit

Fig. 5 Regulation Circuit

3.2 Grid voltage fluctuation and load variation

For any regulation circuit, the voltage regulation characteristics should be investigated from two aspects:

a. grid voltage fluctuation

b. load variation

zener diode as voltage regulator

grid voltage fluctuation

When the grid voltage rises, the input voltage Ui of the voltage regulation circuit increases, and the output voltage Uo also increases proportionally. Since Uo=Uz, according to the volt-ampere characteristics of the Zener diode, the increase of Uz will make Idz Sharply increasing, so does the Ir, Ur will increase sharply with Ir at the same time, and the increase of Ur will definitely reduce the output voltage Uo. Therefore, as long as the parameters are chosen properly, the voltage increment on R can be approximately equal to the increment of Ui, so that Uo is essentially unchanged. A brief description is as follows:


When the grid voltage drops, the change in each value is opposite to the above process.

It can be seen that when the grid voltage changes, the voltage regulation circuit compensates the change of Ui through the change of the voltage on the current limiting resistor R, that is, ΔUr ≈ ΔUi, so that Uo does not change.

load variation

When the load resistance RL decreases, that is, the load current IL increases, Ir increases, Ur also increases, Uo inevitably decreases, and Uz decreases. According to the volt-ampere characteristics of the Zener tube, the drop of Uz causes the Idz to decrease sharply. As a result, Ir is drastically reduced. If the parameters are chosen properly, ΔIdz≈-ΔIL can be made so that Ir is substantially unchanged, so that Uo is substantially unchanged. A brief description is as follows:


Obviously, as long as ΔIz ≈ - ΔIL is made in the circuit, Ir can be made substantially unchanged, thereby ensuring that Uo is substantially unchanged.

In summary, in the voltage stabilizing circuit composed of the Zener diode, the current regulating function of the Zener diode is used to compensate by the voltage or current change of the current limiting resistor R to achieve the voltage stabilizing purpose. The current limiting resistor R not only limits the current in the Zener diode to a normal operation, but also cooperates with the Zener diode to achieve the purpose of voltage regulation. 

3.3 Major parameters

After understanding the voltage regulation principle of the Zener diode, you must understand its major parameters:

Vz — flat voltage: It refers to the stable voltage value generated by the two ends of the Zener diode when passing the rated current. This value varies slightly depending on the operating current and temperature. Due to the difference in manufacturing process, the voltage regulation values of the same type of Zener are not exactly the same.

Iz — steady current: It refers to the current value passing through the diode when the Zener diode generates a stable voltage. Below this value, although the Zener diode can regulate voltage, the voltage regulation effect will be worse; above this value, as long as the rated power loss is not exceeded, it is allowed, and the voltage regulation performance will be better, but more power is consumed.

Rz — dynamic resistance: It refers to the ratio of the voltage change across the diode to the change in current, and this ratio varies with the operating current. Generally, the larger the current, the smaller the dynamic resistance. For example, when the operating current of the 2CW7C regulator is 5mA, Rz is 18 Ω; when the operating current is 10mA, Rz is 8 Ω; when it is 20mA, Rz is 2 Ω, operating current will overpass 20mA .

Pz — rated power: It is determined by the allowable temperature rise of the chip, and its value is the product of the stable voltage Vz and the maximum allowable current Izm.

Ctv — voltage temperature coefficient: It is a parameter indicating that the stable voltage value is affected by temperature.

IR — reverse leakage current. It refers to the leakage current generated by the Zener diode at the specified reverse voltage.


 Silicon Zener Diode

The following figure is a simple voltage regulation circuit composed of silicon Zener diode: silicon voltage regulator DW and load Rfz are in parallel, and R1 is a current limiting resistor.

Silicon Stablilvolt Diode Circuit

Fig. 6 Silicon Stablilvolt Diode Circuit

The silicon Zener diode circuit is regulated by the reverse breakdown characteristic of the Zener diode. Due to the steep reverse characteristic curve, a large current change will only cause a small voltage change.

silicon stablilvolt diode circuit (b)

Fig. 7 Silicon Stablilvolt Diode Circuit (b)

How is this circuit regulated? If the grid voltage rises, the output voltage Usr of the rectifier circuit also rises, causing the load voltage Usc to rise. Since the Zener diode DW is connected in parallel with the load Rfz, as long as the root has a little increase, the current flowing through the Zener diode increases sharply, so that I1 also increases, and the voltage drop across the current limiting resistor R1 increases, thereby offsetting the rise of Usr keeps the load voltage Usc substantially unchanged. Conversely, if the grid voltage drops, causing Usr to drop, so does the Usc, the current in the Zener diode is drastically reduced, causing I1 to decrease and the voltage drop across R1 to decrease, thereby offsetting the drop in Usr and maintaining the load. The voltage Usc is essentially unchanged.

If Usr is constant and the load current increases, the voltage drop across R1 increases, causing the load voltage Usc to drop. As soon as Usc drops a little, the current in the Zener diode decreases rapidly, reducing the voltage drop across R1, and keeping the voltage drop across R1 substantially constant, which stabilizes the load voltage Usc.

In summary, the Zener diode acts as an automatic adjustment of the current. The smaller the dynamic resistance of the Zener diode, the larger the current limiting resistance and the better the stability of the output voltage.


 Example Analysis 

When using Zener diodes, they can’t limit a potential to a ideal value based on your actual requirement. For example, the following figure:

regulation circuit diagram

Fig. 8 Regulation Circuit Diagram

After the front end acquires the signal, it is amplified by the operational amplifier and then input into the ADC of the microcontroller, and only the output circuit is seen:

regulation circuit diagram (part)

Fig. 9 Regulation Circuit Diagram (part)

Capacitor C17 is the sample-and-hold capacitor, and resistor R31 and Zener diode D9 form a voltage regulation circuit. If the output voltage is greater than 3.3V, it will be clamped to 3.3V by the Zener diode. However, this is not the case, such diode has its own characteristic curve. Refer to the BZT52C3V3 regulator on the Kynix Semiconductor for the replacement of the 1N4728 regulator in the circuit with the BZT52C3V3: 

Zener Breakdown Characteristics (a)

Fig. 10 Zener Breakdown Characteristics (a)

Zener Breakdown Characteristics (b)

Fig. 11 Zener Breakdown Characteristics (b)

Looking at the curve of C3V3, it can be seen that when the current of the Zener diode is 0, its voltage is about 1.8V, which means that when the resistance of the current limiting resistor R31 on the circuit is infinite, the current flowing through the Zener is almost zero and the output voltage is about 1.8V. When the resistance of the resistor R31 is small, the current flowing through the diode is very large regardless of the internal resistance of the front output, and the output voltage can reach between 3.5 V and 4.0 V. Obviously, in both cases, the Zener diodes do not play their duties very well.

When the input voltage is less than 3.3V, the output and the front-end input of the Zener diode remain the same. When the front-end input voltage is greater than 3.3V, the Zener diode outputs 3.3V. But the reality is that there is no such Zener diode.

Assume that the input voltage in the above schematic diagram is Uo, the voltage of the Zener diode is Ui, the resistance of R31 is R, and the current through the diode is i, a formula can be obtained:

i = (Uo - Ui) / R

Change the formula into:

i = (-1/R) * Ui + Uo/R

This equation is drawn on the characteristic curve of the Zener:

Zener Breakdown Characteristics (c)

Fig. 12 Zener Breakdown Characteristics (c)

The intercept of the equation is Uo/R, which is the current when the voltage regulator is shorted. The intersection of the equation and the X axis is Ui=Uo. The focus of this line and the C3V3 curve is the operating point of the Zener diode. But this equation has not been determined because the values of Uo and R are not fixed. We know that the front-end input voltage is dealing with by the op amp. The op amp's operating voltage is 5V, so the op amp's output voltage will not exceed 5V at most, so that we assume Uo's range is between 0 and 5V.

Because the reference voltage of the AD part of the microcontroller system is 3.3V. If you hope the output voltage of the Zener diode not exceed 3.3V, it is necessary to keep the intersection of the above equation and characteristic curve to be no more than 3.3V, assuming that the intersection point voltage is 3.3V. At this time, the current through the Zener diode is 5mA , when our equation just passes this point:

Zener Breakdown Characteristics (d)

Fig. 13 Zener Breakdown Characteristics (d)

The output voltage of the Zener diode is exactly 3.3V, and we call this point the reference point. If the intersection of the equation and the curve is below the reference point, the output voltage of the Zener diode is less than 3.3V. If the intersection of the equation and the curve is above the reference point, the output voltage of the Zener diode is greater than 3.3V, which will affect the microcontroller and even burns out.

Zener Breakdown Characteristics (e)

Fig. 14 Zener Breakdown Characteristics (e)

The output voltage is higher than 3.3V is abnormal. In normal state, the voltage transmitted by the op amp is less than or equal to 3.3V, and we need the output voltage Uo of the op amp and the output voltage of the Zener diode to be less than 3.3V, that is Uo=Ui. When the input voltage of the op amp is less than or equal to 3.3V, the intersection of the equation and the X axis is Ui≤3.3V. At this time, the intersection of the equation and the curve is always smaller than the reference point, because the equation cannot be vertical. The Ui at the intersection is less than 3.3V, which means that our op amp output is 3.3V, and the output voltage of the Zener diode is less than 3.3V. This causes signal distortion, that is, the input signal and the output signal are inconsistent. It is absolutely not allowed in the system because the different voltage indicates the change of the corresponding measured value.

So what should we do if meet this problem? We have just discovered that the intersection of the characteristic curve and the X-axis is not Ui=0 but Ui=1.8V. At this time, when the voltage transmitted by our op amp is less than 1.8V, the values of Uo and Ui are the same. In other words, no signal distortion occurs:

Zener Breakdown Characteristics (f)

Fig. 15 Zener Breakdown Characteristics (f)

It can be seen that the intersection of the equation and the curve is always on the X axis, which is Ui = Uo. But the range is reduced, from 0 to 3.3V to 1.8V, AD detection accuracy is reduced, for the stability of the system, a Zener diode is necessary. Of course, if you select a Zener tube with a better characteristic curve (more expensive). At this time, the intersection of the characteristic curve of the Zener and the X axis may be 2.0V or more.

We can observe the characteristic curve to see the characteristic curve C3V9 of the 3.9V Zener tube. The Ui at the intersection with the X axis is about 3V. When the current of the Zener tube is about 1mA, the Ui is about 3.3V, there a 3.9V voltage regulator can be used for voltage regulation. The equation is as follows:

Zener Breakdown Characteristics (g)

Fig. 16 Zener Breakdown Characteristics (g)

Under normal circumstances, the output voltage of the op amp is in the range of 3.3V, and the intersection of the equation and the curve is on the X-axis, referring to the red line below. When the value exceeds 3.3V, in order to ensure that the intersection of the equation and the curve is below the reference point, we need to reduce the slope of the equation so that the intersection of the equation and the curve satisfies Ui ≤ 3.3V, and the slope of the equation is (-1/R). To reduce the slope is to increase the R value, that is, we can use the 3.9V regulator tube to increase the resistance of R31. Roughly we can see that when Ui=3.3V, i is about 1mA, we bring this point into the equation:

1mA = -3.3/R + Uo/R

When Uo takes the maximum value of 5V, R=1700 ohms is calculated. That is to say, when R is greater than or equal to 1700 ohms and Uo is less than or equal to 5V, the intersection of the equation and the curve is always smaller than the reference point. At the same time, our undistorted voltage range is 0 to 3V, which is much larger than 0 to 1.8V when using a 3.3V Zener diode.


  Application Attentions

1. Pay attention to the difference between the general diode and the Zener diode. A lot of general diodes, especially glass-encapsulated tubes, have similar color or shapes compared with Zener diodes. If you don't carefully distinguish them, you will use them incorrectly. The difference is: looking at the shape, a lot of Zener diodes are cylindrical, short and thick, and the general diode is slender; looking at the sign, the outer surface of the Zener diode is marked with voltage regulator value, such as 5V6, indicates that the voltage regulation value is 5.6V. Use a multimeter to measure voltage, according to the unidirectional conductivity, using the X1K block to judge the positive and negative polarity of the diode to be tested first, then using X10K block, black pen to be connected to the negative pole of diode and the red pen is connected to the positive pole of the diode. If the reverse resistance value is large, the possibility of a general diode is used. If the reverse resistance value becomes small, it is a Zener diode.

2. Note the difference between forward and reverse conduction of Zener diodes. When the Zener diode is used in forward conduction, it is basically the same as a normal diode, and the voltage at both ends after the forward conduction is basically constant, about 0.7V. In theory, the Zener diode can also be used in the forward direction, but its voltage regulation value will be lower than 1V, and the voltage regulation performance is poor. Generally, the forward conduction characteristic of the Zener diode is not used alone to stabilize, but with reverse breakdown characteristics to regulate. The reverse breakdown voltage value is the regulation value. Sometimes two Zener diodes are used in series, one uses its forward characteristic, and the other uses its reverse characteristic to regulate and make temperature compensate to improve the voltage regulation.

3. Pay attention to the effect of the current limiting resistor and the effect of the resistance. In a Zener diode voltage regulator circuit, a resistor R is generally connected in series. This resistor acts as a current limiter in the circuit and improves the voltage regulation effect. If the resistor is not applied, when R=0, it is easy to burn the Zener diode out, resulting in extremely poor voltage regulation effect. The larger the resistance of the current limiting resistor, the better the voltage regulation performance of the circuit, but the input and output voltage difference will be too large, and the power consumption will be more.

4. Pay attention to the pressure difference between the input and output. In normal use, the output voltage of the Zener diode voltage regulator circuit is equal to the voltage regulation value at both ends after reverse breakdown. If the voltage value input to the voltage regulator circuit is less than the voltage regulator voltage, the circuit will loss of voltage regulation, only when it is greater than the rated value, the voltage regulation effect will be made, and the larger the voltage difference, the larger the resistance value of the current limiting resistor should be, otherwise the voltage regulator tube will be damaged.

5. The Zener tubes can be used in series. After several series of voltage regulators are connected in series, a plurality of different voltage regulation values can be obtained, so that series connection is more common. The following example shows how to obtain the voltage regulation value after the two are used in series. If the voltage regulation value of a Zener diode is 5.6V, the other voltage regulation value is 3.6V, and the voltage of the Zener voltage regulator is 0.7V, there are four different voltage regulation values after the series connection.

6. Zener diodes are generally not used in parallel. After several Zener diodes are connected in parallel, the regulation value will be determined by the lowest one (including the voltage value after having forward conduction). Take two voltage regulators as an example to illustrate the calculation method of the voltage regulation value. There are four cases after the two parallel connections, and the voltage regulation value is only two. Zener diodes are not used in parallel unless some specified circumstances.

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    • Clayton Benignus on 2019-7-30 16:03:53

    Really helpful, but i have a question, that is, how Zener diod works At low voltage?

    • Andrew. H on 2019-8-10 10:21:01

    I really need to learn more about Zener diodes, your site has the key info I want. Thank you.

    • J. William on 2019-8-10 10:39:47

    hlo, you have the best learning sources to electronics and i would like to keep learning from you and again to keep sharing ideas, thank you.

    • Alphonse on 2019-8-10 11:15:19

    When stimulated by an external power source, the electrons freed from the silicon atoms by this stimulation are quickly replaced by the free electrons available from the doped Antimony atoms. Does that mean upon stimulation all electrons in the valence band of Antimony and Silicon detach themselves from the parent atom and are free to move? Can someone please explain this to me?

    • Angelo on 2019-8-10 11:25:22

    Good explanation.

    • Barnett on 2019-8-10 11:30:20

    How Zener diode works at low voltage?

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