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Jan 11 2019

The Working Principle of the Zener Diode and the Judgment of the Positive And Negative Poles

Warm hints: This article contains about 5000 words and reading time is about 20 min.


Introduction

The Zener diode is a surface contact type crystal diode made of silicon material, referred to as a Zener tube. This diode is a semiconductor device that has a very high resistance until the critical reverse breakdown voltage. When the voltage regulator is in reverse breakdown, it is almost constant in a certain current range (or within a certain power loss range), and the terminal voltage is almost constant, which shows the voltage regulation characteristics, and thus is widely used in a regulated power supply and a limiting circuit. Among them. The Zener diode is binned according to the breakdown voltage. Because of this characteristic, the Zener diode is mainly used as a voltage regulator or voltage reference component. Zener diodes can be connected in series for use at higher voltages, and more stable voltages can be obtained by series connection, called bidirectional regulators.

Article Core

Zener Diode

Purpose

Introduce the working principle of the Zener diode and the judgment of the positive and negative poles.

Application

Semiconductor industry

Keywords

Zener Diode

Catalog

Introduction


Working Principle of Zener Diode


Zener Diode Characteristics


Zener Diode Circuit Diagram


 

 

Zener Diode Positive And Negative Judgment

1. Zener Diode Positive and Negative

2. Zener Diode Positive And Negative Judgment

3. Identification Method of Positive and Negative Poles of Zener Diode

4. Zener Diode Positive And Negative Connection

Precautions for Using Zener Diodes


Zener Diode And Its Application


Main Parameters of Zener Diode


Zener Diode Replacement Device


Zener Diode Replacement Device



Working Principle of Zener Diode

To understand how a Zener diode works, just look at the reverse characteristics of the diode. The basic characteristic of all crystal diodes is unidirectional conduction. That is to say, the forward pressurization is turned on, and the reverse pressurization is not possible. There is a condition here that the reverse pressurization does not exceed the reverse withstand voltage of the tube. So what is the result after exceeding the withstand voltage? A simple answer is that the tube burned. But this is not the whole answer. The test found that as long as the reverse current value is limited (for example, a resistor is connected in series between the tube and the power source), the tube will not burn out although it is broken down. Moreover, it was found that after the reverse breakdown of the tube, the current changed from large to small, and the voltage dropped only slightly, and the voltage dropped sharply with the decrease of the current until it dropped to a certain current value. It is this characteristic that makes use of the Zener diode. The key to using a Zener diode is to design its current value.

What is  Zener diode and its  working principle?

The characteristic of a Zener diode is that after breakdown, the voltage across it is substantially constant. 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.

Generally, the triode is forward-conducting and reverse-cut; 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 tube exhibits a breakdown state, a large current is passed, but it is not damaged, and This phenomenon is very reproducible; conversely, as long as the tube is in a breakdown state, although the electricity flowing through the tube varies greatly, the voltage across the tube changes very little to stabilize the voltage. This special diode is called a Zener. The type of the Zener tube has 2CW, 2DW and other series, and its circuit symbol is shown in Figure 1.

Zener Diode Symbol

The voltage regulation characteristics of the Zener diode can be clearly expressed by the volt-ampere characteristic curve shown in Figure 2.

the volt-ampere characteristic curve of Zener diode

The Zener diode works by using the voltage regulation characteristic of the reverse hit multi-zone. 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 voltage regulator tube is fixed in the range of the mouth. For example, the regulation value of 2CW11 is 3.2 volts to 4.5 volts, where the regulation value of one tube may be 3.5 volts, and the other tube may be 4, 2 volts.

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

The stability of the Zener diode can be expressed by its dynamic resistance r:

r=ΔU(voltage variation) / ΔI(current variation)

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.

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

The stability of the Zener tube can be affected by temperature. When the temperature changes, its stable voltage also changes. The temperature coefficient of the stable voltage is commonly used to indicate this performance. For example, the stable voltage of the 2CW19 type regulator tube Uw = 12 volts, temperature The coefficient is 0.095% ° C, indicating that the steady voltage rises by 11.4 millivolts for every 1 ° C increase in temperature. In order to improve the stability of the circuit, appropriate temperature compensation measures are often used. When the stability performance is very high, temperature-compensated voltage regulators such as 2DW7A, 2DW7W, 2DW7C, etc. are required.


Zener Diode Characteristics

Zener diodes typically operate in a reverse breakdown state.

The forward characteristics of a Zener diode are similar to those of a normal diode.

The reverse characteristic is that when the reverse voltage is lower than the reverse breakdown voltage, the reverse resistance is large and the reverse leakage current is extremely small. However, when the reverse voltage is near the critical value of the reverse voltage, the reverse current suddenly increases, called breakdown, at which the reverse resistance suddenly drops to a small value.

Although the current varies over a wide range, the voltage across the diode is substantially stabilized near the breakdown voltage, thereby achieving a diode regulation function.

The figure below shows the volt-ampere characteristic curve of the Zener diode:

Zener Diode Characteristics


Zener Diode Circuit Diagram

(1) A simple voltage regulator circuit composed of a silicon Zener diode is shown in Fig. 4. The silicon voltage regulator DW is connected in parallel with the load Rfz, and R1 is a current limiting resistor.

silicon Zener diode Circuit

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 grows less, the current flowing through the Zener diode will increase 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, causing Usc to also drop, the current in the Zener diode decreases sharply, 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 is rapidly reduced, so that the voltage drop across R1 is reduced again, keeping the voltage drop across R1 substantially constant, which stabilizes the load voltage Usc.

silicon Zener diode Circuit


(2) The acquisition of the reference voltage signal, the most primitive circuit, that is, a simple voltage regulator circuit composed of a current limiting resistor and a Zener diode, as shown in the block diagram of FIG.6.

Figure 6 Simple R, D Zener Diode Circuit

Figure 6 Simple R, D Zener Diode Circuit


Figure 7 Zener diode circuit with current / power amplification

Figure 7 Zener diode circuit with current / power amplification

In the circuit of the figure, due to the maximum breakdown current of the Zener diode, the current output capability of the circuit is extremely poor. Therefore, in general applications, a voltage follower is added as shown in Figure 7 to increase the current/power. Output capability.

In the circuit of Fig. 6, correctly selecting the resistance value of the current limiting resistor R is a prerequisite for the normal operation of the voltage stabilizing circuit. When the load circuit is unloaded, the current flowing through the Zener diode Dz does not exceed its maximum withstand value and is damaged; at the maximum load, it is still necessary to ensure that the current flowing through Dz exceeds the minimum breakdown current value, and it is still at Break through the work area. From the safety of the Zener diode, as long as the current flowing through it does not exceed the maximum reverse withstand current of the Zener diode, there is no danger of damage to the circuit components. The normal working area of a Zener diode refers to its reverse breakdown state under certain current conditions. At this time, in the region of large current change, the value of the terminal voltage change is relatively small, and even negligible.


Zener Diode Positive And Negative Judgment

1. Zener Diode Positive and Negative

Zener diode, the English name Zener diode, also known as Zener diode. By using a reverse breakdown state of the pn junction, the current can be varied over a wide range and the voltage is substantially constant, and a diode that acts as a voltage regulator is fabricated. This diode is a semiconductor device that has a very high resistance until the 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. The Zener diode is binned according to the breakdown voltage because of this characteristic. The Zener diode is mainly used as a voltage regulator or voltage reference component. Zener diodes can be connected in series for use at higher voltages, resulting in higher regulated voltages in series.

Understand the Zener diode, let's take a detailed explanation of how the positive and negative poles of the Zener diode should be judged and the connection of the positive and negative terminals of the Zener diode.


2. Zener Diode Positive And Negative Judgment

The easiest way is to look at the logo on the exterior. The negative pole is usually printed with a black or silver ring, and the positive pole is not. See below:

Zener Diode


Zener Diode


3. Identification Method of Positive and Negative Poles of Zener Diode

(1). From the appearance, the positive end of the metal-packed Zener diode body is flat, and the negative end has a semi-circular shape.

(2). One end of the plastically sealed diode body with the color mark printed on it is the negative pole and the other end is the positive pole.

(3). For the Zener diode with unclear sign, you can also use the multimeter to judge its polarity. Turn the multimeter to the position of the measuring diode. Place the two pens on the ends of the diode. The buzzer is called, then the end of the red pen is positive, and the black pen is negative.


4. Zener Diode Positive And Negative Connection

Many friends have doubts about the positive and negative connections of Zener diodes, because some directions are opposite to the normal direction.

Zener Diode Positive And Negative Connection

We all know that the Zener diodes are all operating in the reverse hit-through state, so in general, the positive and negative poles of the Zener diode are reversed. And the Zener diode is boosted at the negative voltage through the current limiting resistor, and connected to the low voltage or ground at the positive pole, so the output is a positive voltage.


Precautions for Using Zener Diodes

When the Zener diode operates in the reverse breakdown state, the voltage across it is substantially constant. Using this property, it is often used in circuits to form a voltage regulator circuit. The voltage stabilizing circuit composed of the Zener diode has a small degree of stability and a small output current, but has the advantages of simplicity, economy, and practicality, and thus is widely used. In the actual circuit, to use the Zener diode, you should pay attention to the following problems:


1. We should 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 shapes to Zener diodes. If you don't carefully distinguish them, you will use them incorrectly. The difference is: look at the shape, a lot of Zener diodes are cylindrical, short and thick, and the general diode is slender if it is cylindrical; look at the logo, 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, according to the unidirectional conductivity, use the X1K block to judge the positive and negative polarity of the diode to be tested first, then use X10K block, black pen to connect the diode negative The red pen is connected to the positive pole of the diode. The measured resistance value is larger than that of the X1K gear. If the reverse resistance value is large, the possibility of a general diode is very large. If the reverse resistance value becomes small, then It is a Zener diode.


2. We should pay attention to the difference between forward and reverse use of Zener diodes. When the Zener diode is used in forward conduction, it is basically the same as the normal diode forward conduction. The voltage at both ends after the forward conduction is basically constant, and is about 0.7V. In theory, the Zener diode can also be used as a Zener diode in the forward direction, but its regulation value will be lower than 1V, and the voltage regulation performance is not good. Generally, the forward conduction characteristic of the Zener tube is not used alone to stabilize. Pressure, 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 both regulate and temperature compensate to improve the voltage regulation.


3. We should pay attention to the effect of the current limiting resistor and the effect of the magnitude of the resistance. In the Zener diode voltage regulator circuit, a resistor R is generally connected in series, as shown in Figure 1 or 2. The resistor acts as a current limiter in the circuit and improves the voltage regulation effect. If the resistor is not added, when R=0, it is easy to burn out the Zener tube, and the voltage regulation effect will be extremely poor. 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. We need to pay attention to the pressure difference between 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 of the Zener diode 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 relationship, there is a voltage regulation effect, and the larger the voltage difference, the larger the resistance of the current limiting resistor should be, otherwise the voltage regulator will be damaged.


5. The Zener can be used in series. After several series 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 voltage regulators 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 voltage regulator tube is 0.7V when it is conducting, then there are four different voltage regulation values after the series connection, as shown in the figure. (a) shows.


6. Zener tubes 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 forward conduction). Take the above two voltage regulators as an example to illustrate the calculation method of the voltage regulation value. There are four cases after two parallel connections, and the voltage regulation value is only two, as shown in Figure (b). Zener diodes are not used in parallel unless otherwise specified.

Precautions for Using Zener Diodes


Zener Diode And Its Application

The Zener diode is a surface contact type silicon diode made by a special process. The Zener diode operates in the reverse breakdown region, and within a certain current range (ΔIZ), the Zener diode is not damaged. Since the breakdown of the Zener diode is Zener breakdown, the Zener diode is also called a Zener diode. After the voltage regulator is added with a certain reverse voltage breakdown, the reverse current changes in a wide range, and the voltage across the tube remains basically the same, which is why the voltage regulator is regulated.

Zener tubes generally have two uses (the following IZ is the operating current, UZ is the nominal regulated voltage, UW is the actual operating voltage):

In normal operation, it is in the "on" state, IZ ≥ 0.1mA level, at this time the Zener tube acts as a voltage regulator, UW ≈ UZ. In normal operation, it is in the "off" state, that is, UW<UZ, IZ is on the order of 1μA (at least). When the UW "attempts" exceeds UZ, the Zener will be turned on, IZ will increase sharply, and in turn will prevent UW. Continue to rise, thus limiting or protecting.

In fact, the commonly used Zener tubes are mainly divided into two categories, one is the so-called "stabilizer tube" and the other is the TVS type device. The former is usually the first usage, the latter is usually the second usage. But it is not absolute, the two are only characteristic parameters. Ordinary Zener can also be used as a protection device, but the response speed is poor, and it is not suitable for occasions where it is necessary to suppress extremely high-speed pulse interference. TVS can also be used as a regulator, and of course not suitable.


Main Parameters of Zener Diode

(1). Stable voltage Vz: The stable voltage is the voltage value at both ends of the tube when the Zener diode is in normal operation. This value varies slightly with the operating current and temperature. It is the same type of Zener diode, and the stable voltage value also has a certain dispersion. For example, the stable voltage of the 2CW14 silicon Zener diode is 6 to 7.5V.


(2). Dissipated power PM: When the reverse current passes through the PN junction of the Zener diode, a certain power loss is generated, and the temperature of the PN junction will also rise. The dissipated power of the tube is determined based on the allowable PN junction operating temperature. Usually small power tubes are on the order of a few hundred milliwatts to a few watts.

Maximum Dissipated Power PZM: The maximum power loss of the Zener diode depends on the area of the PN junction and heat dissipation. When operating in reverse, the power loss of the PN junction is:

PZ = VZ * IZ, IZmax can be determined by PZM and VZ.


(3). Stable current IZ, minimum stable current IZmin, large stable current IZmax Stable current: reverse current when the working voltage is equal to the stable voltage; minimum stable current: the minimum reverse current required when the Zener diode operates at a stable voltage; Maximum steady current: The maximum reverse current allowed by the Zener diode.


(4). Dynamic resistance rZ: The concept is the same as the dynamic resistance of a general diode, except that the dynamic resistance of the Zener diode is obtained from its reverse characteristic. The smaller the rZ, the steeper the breakdown characteristics of the Zener tube.

Rz=△VZ/△IZ


(5). Stable voltage temperature coefficient: The change of temperature will

cause VZ to change, in the Zener tube,

When |VZ| >7 V, VZ has a positive temperature coefficient, and reverse breakdown is avalanche breakdown;

When |VZ|<4V, VZ has a negative temperature coefficient, and reverse breakdown is Zener breakdown;

When 4V<|VZ|<7V, the Zener tube can obtain a temperature coefficient close to zero.

Such a Zener diode can be used as a standard regulator.


Zener Diode Replacement Device

(1). Ordinary Diode (positive conduction voltage)

It is well known that the emitter junction of a common diode and a transistor has a threshold voltage. For a silicon device, the turn-on voltage is about 0.65 V. This voltage change is not significant as the on current changes. Therefore, any ordinary diode of silicon material can be used as a Zener diode of 0.6V, and its forward voltage drop is used as a voltage regulator value. Obviously, because the working current of the ordinary diode is often several times the working current of the Zener diode, after the replacement, the power output capability is enhanced and the voltage regulation area is widened. If you need a 2.5V Zener, you can take 3 ordinary diodes.

Purely diodes are used instead of Zener diodes. If the required voltage regulation value is high, multiple series connections are particularly inconvenient. This prompted me to look for other replacement devices.


(2). Light-emitting Diode (positive conduction voltage)

The forward voltage drop of the LED is about 1.7~2V, and one can reach the serial value of three diodes. If a 3.5V regulator is needed, the two series can be replaced. However, it should be noted that the operating current value of the LED is generally 10~20mA. Pay attention to the R value so that the current flowing through the LED is not more than 20mA.


(3). The Emitter Junction of the Transistor (reverse breakdown voltage)

This is an ideal replacement for the Zener. One of the triode parameters is Vbeo, which is the value of the reverse breakdown voltage of the emitter junction. I remember that at that time, it was urgent to use a 6V voltage regulator in the repair, pick up a 3DG6 crystal triode, and after applying the current limiting resistor to the base, apply the reverse voltage of the emitter junction and measure the voltage value of 6V instead of the original. The regulator tube repairs the device. The most widely used is the 90xx series of transistors. If a 5V regulator is required, it can be replaced by the 9013 emitter junction. The emitter junction of any triode has a relatively stable breakdown voltage value. In fact, the transistor is also a Zener diode replacement device.

Figure 12 Zener diode substitute circuit

Figure 12 Zener diode substitute circuit


(4) Specific Type of Diode (reverse breakdown voltage)

In one troubleshooting, it is not necessary to use a 110V Zener diode. There is no ready-made regulator tube at hand. It is not realistic to use a series of other series. There is a high-frequency low-power rectifier diode 1N4148 on hand, a series connection of 100k resistors, adding DC500V, a test, the reverse breakdown voltage of IN4148, can not be exactly 110V. Ordinary rectifier diodes, such as the IN40xx series, have reverse breakdown voltage values of VRRM of 50V, 100V, 400V, and 600V. If applied in reverse, this is simply a series of Zener diodes.


The Essence of Series Zener Diodes And Shunt Zener Diodes

From the difference between the voltage regulator component Dz or the voltage regulator component VT and the load RL, it can be divided into a parallel (shunt) voltage regulator circuit and a series (voltage divider) voltage regulator circuit, two circuit forms, as shown in Figure 13:

Figure 13 series, parallel regulator diode circuit

There are several reasons for the Uo voltage change, such as temperature changes, but the most critical two, namely the change of input voltage and the change of load current, have the most significant effect.

If the load circuit is equivalent to the resistor RL, the internal resistance change of the Zener diode is equivalent to RDz (variable resistance), and it can be inferred that the voltage regulation process is like this:


(a) The circuit is a shunt regulator circuit, and Dz and RL are connected in parallel.

When RL is fixed, only when the input voltage changes (for example, rises), in order to maintain Uo, the resistance value of RDz changes to a small point, and the shunting ability of the supply current is enhanced to make Uo fall back and maintain the original value. Unchanged; 

When the input voltage is constant and the load resistance RL changes (for example, it becomes smaller), the increase of the load current causes Uo to have a tendency to fall. At this time, the resistance value of RDz increases, and the shunt decreases, causing Uo to rise to the original value.


(b) The circuit is a series regulator circuit, and VT and RL are in series relationship.

Briefly analyze the voltage regulation control process when the input voltage is constant and the load changes.

When the load current rises, that is, RL becomes small, it is obvious that the resistance value of the RVT becomes smaller in the same proportion, so that the original partial pressure value can be maintained. Similarly, when the RL becomes larger, the RVT also becomes proportionally larger. In order to maintain the constant Uo.

It can be seen that whether it is series voltage regulation or shunt voltage regulation, whether the input voltage changes or the load current changes, the voltage regulator component or the voltage regulator adjustment component can only maintain the output Uo without changing the resistance/current change. The regulation regulator element acts here as a variable resistor. In other words, the voltage regulator component or the voltage regulator component synchronously adjusts the change of the loop current to maintain the output voltage constant. It is a voltage regulation control realized by variable current adjustment.

The shortcoming of the linear regulator circuit is that the power consumption of the adjustment component itself is large, that is, there is a terminal voltage and at the same time, most or all of the load current flows. Sometimes, the power consumption of the trim component itself will exceed the power consumption of the load circuit, resulting in inefficiency. Due to this weakness, the emergence of a switching type DC-DC converter power supply has come, and the era of switching power supplies replacing linear power supplies is coming.


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