Device
Semiconductor zener diodes have replaced the obsolete glow discharge zener diodes - ion gas-discharge electric vacuum devices. For the manufacture of zener diodes, silicon or germanium crystals (tablets) with n-type conductivity are used, into which impurities are added by an alloy or diffuse alloy method. To obtain an electron-hole pn junction, acceptor impurities, mainly aluminum, are used. The crystals are enclosed in housings made of polymer materials, metal or glass.
Silicon alloy zener diodes D815 (A-I) are produced in a sealed metal case, which is the positive electrode. Such elements have a wide operating temperature range – from -60°C to +100°C. Silicon alloy two-anode stabilizing diodes KS175A, KS182A, KS191A, KS210B, KS213B are produced in a plastic case. Silicon alloy temperature-compensated parts KS211 (B-D), used as reference voltage sources, have a plastic housing.
SMD zener diodes, that is, miniature components intended for surface mounting, are manufactured mainly in glass and plastic cases. Such elements can be produced with two or three terminals. In the latter case, the third pin is a “dummy”, does not carry any semantic load and is intended only for reliable fixation of the part on the printed circuit board.
Operating principle
The zener diode was discovered by the American physicist Clarence Melvin Zener, after whom it was named. Electrical breakdown of a pn junction can be caused by a tunnel breakdown (in this case, the breakdown is called Zener breakdown), avalanche breakdown, breakdown as a result of thermal instability, which occurs due to destructive self-heating by leakage currents.
And engineers design these elements in such a way that the occurrence of tunneling and/or avalanche breakdown occurs long before they are likely to experience thermal breakdown.
The magnitude of the breakdown voltage depends on the concentration of impurities and the method of doping of the pn junction. The greater the concentration of impurities and the higher their gradient in the junction, the lower the reverse voltage at which a breakdown occurs.
- A tunnel (Zener) breakdown
appears in a semiconductor in cases where the electric field strength in the pn zone is 106 V/cm. Such high voltage can only occur in highly doped diodes. At breakdown voltages in the range of 4.5...6.7 V, the tunnel and avalanche effects coexist, but at a breakdown voltage less than 4.5 V, only the tunnel effect remains. - In zener diodes with low doping levels or smaller gradients of dopant additives there is only an avalanche breakdown mechanism
, which appears at a breakdown voltage of approximately 4.5 V. And at voltages above 7.2 V, only the avalanche effect remains, and the tunnel effect disappears completely.
As mentioned earlier, when connected directly, the zener diode behaves in the same way as a regular diode when connected directly - it passes current. The differences between them arise when connecting in reverse.
A conventional diode, when connected in reverse, blocks the current, and the zener diode, when the reverse voltage reaches a value called the stabilization voltage, begins to pass current in the opposite direction. This is explained by the fact that when a voltage exceeding U nom is applied to the zener diode. device, a process called breakdown occurs in the semiconductor. The breakdown can be tunnel, avalanche, or thermal. As a result of the breakdown, the current flowing through the zener diode increases to a maximum value limited by the resistor. After reaching the breakdown voltage, the current remains approximately constant over a wide range of reverse voltages. The point at which voltage triggers current can be very precisely set during the manufacturing process by doping. Therefore, each element is assigned a certain breakdown (stabilization) voltage.
The zener diode is used only in “reverse bias” mode, that is, its anode is connected to the “-” power supply. The ability of a zener diode to trigger reverse current when the breakdown voltage is reached is used to regulate and stabilize the voltage when the supply voltage or connected load changes. The use of a zener diode makes it possible to provide a constant output voltage for the connected consumer when the power supply voltage drops or the consumer current changes.
Operating principle of stabilization diodes
Despite the fact that the SMD is similar to a diode, it is essentially a different element of the electrical circuit. Of course, it can serve as a rectifier, but is usually used to stabilize the voltage. This element is capable of maintaining a constant voltage in a DC circuit. This principle of operation is used in power supply of various radio equipment.
Zener diode and diode
Externally, SMD is very similar to a standard semiconductor. The similarity remains in the design features. But when designating such a radio element, unlike a diode, the letter G is placed on the diagram. If you do not delve into mathematical calculations and physical phenomena, then the operating principle of smd will be quite clear.
Note! When turning on such an SMD diode, you must observe reverse polarity. This means that the connection is made with the anode to the minus.
Passing through this element, a small voltage in the circuit provokes a strong current. As the reverse voltage increases, the current also increases, only in this case its growth will be observed weakly. When you reach the mark, it can be anything. It all depends on the type of device. When the mark is reached, a “breakdown” occurs. After the “breakdown” has occurred, a large reverse current begins to flow through the smd. It is at this moment that the operation of this element begins until its permissible limit is exceeded.
Volt-ampere characteristics
The current-voltage characteristic of a zener diode, like a conventional diode, has two branches - forward and reverse. The forward branch is the operating mode for a traditional diode, and the reverse branch characterizes the operation of the zener diode. A zener diode is called a reference diode, and a voltage source that contains a zener diode in its circuit is called a reference diode.
On the working reverse branch of the reference diode, three main values of the reverse current are distinguished:
- Minimum
. When the current is less than the minimum value, the zener diode remains closed. - Optimal
. When the current changes within a wide range between points 1 and 3, the voltage value changes insignificantly. - Maximum
. When applying current above the maximum value, the reference diode will overheat and fail. The maximum current value is limited by the maximum permissible power dissipation, which is highly dependent on external temperature conditions.
A little theory
Stable salary, stable life, stable state.
The last one is not about Russia, of course :-). If you look in an explanatory dictionary, you can clearly understand what “stability” is. On the first lines, Yandex immediately gave me the designation of this word: stable - this means constant, stable, not changing. But most often this term is used in electronics and electrical engineering. In electronics, constant values of a parameter are very important. This can be current strength, voltage, signal frequency and other characteristics. Deviation of the signal from any given parameter can lead to incorrect operation of the electronic equipment and even to its breakdown. Therefore, in electronics it is very important that everything works stably and does not fail.
In electronics and electrical engineering, voltage is stabilized. The operation of electronic equipment depends on the voltage value. If it changes to a lesser extent, or even worse, to an increase, then the equipment in the first case may not work correctly, and in the second case it may even burst into flames.
In order to prevent voltage spikes and drops, various voltage stabilizers were invented. As you understand from the phrase, they are used to stabilize the “playing” voltage.
Areas of use
The main area of application of these elements is stabilization of direct voltage in low-power power supplies or in individual units with a power of no more than tens of watts. With the help of reference diodes, they ensure the normal operating mode of transistors, microcircuits, and microcontrollers.
In simple design stabilizers, the zener diode is both a reference voltage source and a regulator. In more complex designs, the zener diode serves only as a source of reference voltage, and an external power transistor is used for power regulation.
Temperature-compensated zener diodes and parts with a hidden structure are in demand as discrete and integrated reference voltage sources. To protect electrical equipment from overvoltages, pulse avalanche zener diodes have been developed. To protect the inputs of electrical devices and the gates of field-effect transistors, ordinary low-power zener diodes are installed in the circuit. Insulated gate field-effect transistors (IGFETs) are manufactured with a single chip on which are located: a protective zener diode and a power transistor.
Main characteristics
The passport of the stabilizing diode indicates the following parameters:
- Rated stabilization voltage Ust
. This setting is selected by the device manufacturer. - Operating current range
. Minimum current – the current value at which the stabilization process begins. Maximum current is the value above which the device is destroyed. - Maximum power dissipation
. In low-power elements this is the nameplate value. In the passports of high-power zener diodes, to calculate the cooling conditions, the manufacturer indicates: the maximum permissible temperature of the semiconductor and the coefficient of thermal resistance of the housing.
In addition to the parameters indicated in the passport, zener diodes are characterized by other quantities, including:
- Differential resistance
. This property determines the instability of the device in terms of supply voltage and load current. The first drawback is eliminated by powering the stabilizing diode from a direct current source, and the second by connecting a DC buffer amplifier with an emitter follower between the zener diode and the load. - Temperature coefficient of voltage
. In accordance with the standard, this value is equal to the ratio of the relative change in stabilization voltage to the absolute change in outside temperature. In non-heat-stabilized zener diodes, when heated from +25°C to +125°C, the stabilization voltage shifts by 5-10% from the original value. - Drift and noise
. These characteristics are not defined for conventional zener diodes. For precision devices they are very important properties. In conventional (non-precision) zener diodes, noise is created by: a large number of foreign impurities and crystal lattice defects in the region of the pn junction. Methods for reducing noise (if necessary): protective passivation with oxide or glass (impurities are directed deep into the crystal) or by moving the pn junction itself deep into the crystal. The second method is more radical. It is in demand in low noise diodes with a hidden structure.
Calculation of a parametric stabilizer.
The initial data for the calculation for calculating the simplest parametric voltage stabilizer are:
input voltage U0
;
output voltage U1
= Ust – stabilization voltage;
output current IH
= ist;
For example, take the following data: U0 = 12 V, U1 = 5 V, IH = 10 mA = 0.01 A.
1. Based on the stabilization voltage, select a zener diode of type BZX85C5V1RL (Ust = 5.1 V, differential resistance rst = 10 Ohm).
2. Determine the required ballast resistance R1:
3. Determine the stabilization coefficient:
4. Determine the efficiency
Connection methods - serial and parallel
For imported parts in the accompanying documents, situations in which a serial or parallel connection is possible are not regulated. In the documentation for domestic reference diodes you can find two instructions:
- In devices of small and medium power, any number of single-series zener diodes can be connected in series or in parallel.
- In devices of medium and high power, any number of stabilizing diodes of a single series can be connected in series. For a parallel connection, calculations must be made. The total power dissipation of all parallel-connected zener diodes should not be higher than that of one part.
It is possible to connect reference diodes of different series in series if the operating currents of the created circuit do not exceed the rated stabilization currents for each series installed in the circuit.
In practice, to multiply the stabilization voltage, a series connection of two or three zener diodes is most often used. This measure is resorted to if it was not possible to obtain a part for the required voltage or it is necessary to create a high-voltage zener diode. When connected in series, the voltage of the individual elements is summed. This type of connection is mainly used in the assembly of high-voltage stabilizers.
Parallel connection of parts serves to increase current and power. However, in practice, this type of connection is rarely used, since different instances of reference diodes, even of the same type, do not have exactly the same stabilization voltages. Therefore, with a parallel connection, a discharge will occur only in the part with the lowest stabilization voltage, and breakdown will not occur in the rest. If a breakdown occurs, then some zener diodes in such a circuit will operate with underload, and others with overload.
To stabilize the alternating voltage, the zener diodes are connected in series and back-to-back. In the first half-cycle of an alternating current sinusoid, one element operates as a regular diode, and the second performs the functions of a zener diode. In the second half-cycle, the elements change functions. The output voltage shape is different from the input voltage. Its configuration resembles a trapezoid. This is due to the fact that a voltage exceeding the stabilization voltage will be cut off and the tops of the sine wave will be cut off. Series and counter connection of zener diodes can be used in a temperature-stabilized zener diode.
Conventionally graphic designation on diagrams
All devices have a graphic designation. This is necessary so as not to clutter the electrical circuit. The zener diode has its own conventional graphic designation, which is approved by the interstate standard of the Unified Standard of Design Documentation (ESKD).
The figure below shows how it is indicated on the diagram according to GOST 2.730-73; a zener diode is designated practically as a diode, since, in essence, it is one of its varieties.
For correct inclusion, you should distinguish between where there is a plus and where there is a minus. If you look at the above figure, then in it the plus (anode) is located on the left, and the minus (cathode) on the right. According to ESKD, the dimensions of UGO diodes should be 5/5 mm. This is illustrated in the figure below.
Composite Zener diodes
Composite zener diode is a device used in situations where currents and power greater than those allowed by technical conditions are required. In this case, a DC buffer amplifier is connected between the stabilizing diode and the load. In the circuit, the collector junction of the transistor is connected in parallel with the stabilizing diode, and the emitter junction is connected in series.
The circuit of a conventional composite zener diode is not intended for direct current use. But adding a diode bridge turns the compound zener diode into a dual-acting system that can operate with both forward and reverse current. Such zener diodes are also called double or two-anode. Zener diodes that can operate with voltage of only one polarity are called asymmetrical. And composite zener diodes, capable of operating in any direction of current, are called symmetrical.
Types of Zener diodes
On the modern electronics market there is a wide range of zener diodes adapted to specific application conditions.
Precision
These devices provide high output voltage stability. They are subject to additional requirements regarding the temporary instability of voltage and temperature coefficient of voltage. Precision devices include:
- Temperature compensated
. The temperature-compensated zener diode circuit includes a series-connected zener diode with a nominal voltage of 5.6 V (with a positive temperature coefficient) and a directly illuminated diode (with a negative coefficient). When these elements are connected in series, mutual compensation of temperature coefficients occurs. Instead of a diode, a second zener diode can be used in the circuit, connected in series and counter. - With hidden structure
. The breakdown current in a conventional zener diode is concentrated in the near-surface silicon layer, where the maximum amount of foreign impurities and crystal lattice defects is located. These design imperfections cause noise and unstable operation. In parts with a hidden structure, the breakdown current is “driven” inside the crystal by forming a deep island of p-type conductivity.
Fast acting
They are characterized by: a low value of the barrier capacitance, only tens of picofarads, and a short period of the transient process (nanoseconds). Such features allow the reference diode to limit and stabilize short-term voltage pulses.
Stabilizing diodes can be designed for stabilization voltages from several volts to several hundred volts. High-voltage zener diodes are installed on special coolers that can provide the necessary heat exchange and protect the element from overheating and subsequent destruction.
Adjustable Zener Diodes
When manufacturing stabilized power supplies, the necessary zener diode may be missing. In this case, an adjustable zener diode circuit is assembled.
The required voltage of the stabilizing diode is selected using resistor R1. To configure the circuit, a variable resistor with a nominal value of 10 kOhm is connected in place of resistor R1. After obtaining the required voltage value, determine the resulting resistance and install a resistor of the required value in a permanent place. For this circuit, you can use transistors KT342A, KT3102A.
Connection diagram
Let's consider the operation of a zener diode using the example of a parametric stabilizer circuit. This is a typical diagram. Here are the formulas for calculating the stabilizer.
Let's assume that there is 15 Volts, and the output needs to be 9 V. Using the voltage table in the reference book, we select the D810 zener diode. Let's calculate the current-limiting resistor R1 according to the figure below. It shows a current-limiting resistor and circuit diagram. The voltage regulation mode is marked on the current-voltage characteristic 1.2.
To ensure that the semiconductor does not fail, it is necessary to take into account the stabilization current and the load current. From the reference book we determine the stabilization current.
It is equal to 5 mA. The figure below shows part of the reference book.
We assume that the load current is 100 mA:
R1= (Uin-Ust)/(In+Ist)= (15-9)/(0.1+0.005)=57.14 Ohm.
If you need a powerful stabilizer, then you should assemble a circuit from a zener diode and a transistor.
If it is necessary to make a stabilizer for a low voltage of 0.2-1 V, a stabistor is used for this. It is a type of zener diode, but it operates in the forward branch of the current-voltage characteristic and is switched on in the forward direction, which is its unique feature.
In a similar way, you can make a power supply where the stabilizer is made of diodes. Like a stabistor, they are switched on in the forward direction. The required voltage is obtained by direct voltage drops across the diode; for silicon diodes it is in the range of 0.5-0.7V. In the absence of diodes, you can assemble a zener diode from a transistor.
The figure below shows a transistor circuit.
The industry also produces controlled zener diodes. Or, more precisely, this is a microcircuit - TL431. This is a universal microcircuit that allows you to adjust the voltage from 2.5 to 36 volts.
Adjustment is carried out by selecting a resistance divider. The diagram below shows a 5 volt stabilizer. The divider is assembled using resistors with a nominal value of 2.2 K.
A specialist should know how to check the performance of a zener diode with a multimeter. Let us immediately note that only a unidirectional element can be checked; dual (bidirectional) elements are not subject to such verification. If the Zener diode is working properly, then when the tester “tests” in one direction it will show an open circuit, and in the second there is minimal resistance. The faulty one rings in both directions.
Marking methods
On the body of the part there is an alphabetic or alphanumeric marking that characterizes the electrical properties and purpose of the device. There are two types of markings. Parts in a glass case are marked in the usual way. The stabilization voltage is written on the surface of the element using the letter V, which acts as a decimal point. The marking of four numbers and a letter at the end is less clear. It can only be decrypted using a datasheet.
Another way to designate stabilizing diodes is by color coding. The Japanese version is often used, which consists of two or three colored rings. If there are two rings, each of them represents a specific number. If the second ring is applied in double form, this means that a comma must be placed between the first and second digits.
How to distinguish a zener diode from a regular diode
Both of these elements have a similar designation in the diagram. In practice, you can distinguish a zener diode from a regular diode and even find out its rating, if it is no more than 35 V, using a multimeter attachment.
Multimeter attachment circuit
To implement a pulse-width modulation generator, a specialized MC34063 microcircuit is used. To ensure galvanic isolation between the power supply and the measuring part of the circuit, the voltage is controlled on the primary winding of the transformer. This allows you to make a rectifier on VD2. The output voltage stabilization point is set using resistor R3. The voltage on capacitor C4 is approximately 40 V. Current stabilizer A2 and the reference diode being tested constitute a parametric stabilizer, and a multimeter connected to the terminals of the circuit allows you to determine the voltage of the zener diode.
If the diode is connected in reverse polarity (anode to “-” and cathode to “+”), then the multimeter for a conventional diode will show 40 V, and for a zener diode - stabilization voltage.
To determine the performance of a zener diode with a known rating, a simple circuit is used, consisting of a power source and a current-limiting resistor of 300...500 Ohms. In this case, a multimeter is used to determine not the junction resistance, but the voltage. Switch on the elements as shown in the diagram and measure the voltage on the zener diode.
Slowly increase the voltage of the power supply. At the value of the stabilization voltage, the voltage on the zener diode should stop growing. If this happens, it means the element is working. If, with a subsequent increase in the IP voltage, the diode does not begin to stabilize, it means that it is not working properly.
Types of 12 volt stabilizers
Depending on the design and method of maintaining a 12-volt voltage, there are two types of stabilizers:
- Pulse stabilizers, consisting of an integrator (battery, high-capacity electrolytic capacitor) and a switch (transistor). Maintaining the voltage in a given range of values occurs due to the cyclic process of accumulation and rapid release of charge by the integrator when the key is open. According to their design features and control method, such stabilizers are divided into key devices with a Schmitt trigger, equalizers with pulse width and pulse frequency modulation.
- Linear - voltage-stabilizing devices in which zener diodes or special microcircuits connected in series are used as a regulating device.
Pulse
Linear
The most common and popular among car enthusiasts are linear devices, characterized by ease of self-assembly, reliability and durability. The pulse type is used much less frequently due to the high cost of parts and the difficulties of independent production and repair.
Classic model
Classic stabilizers are a large class of devices assembled based on semiconductor parts such as bipolar transistors and zener diodes. Among them, the main function of maintaining the voltage at 12 V is performed by zener diodes - a type of diodes connected in reverse polarity (the plus of the power supply is connected to the cathode of such a semiconductor device, and the minus to the anode), operating in breakdown mode. The essence of how these semiconductor parts work is as follows:
- When the voltage of the power source connected to the zener diode is less than 12 V, it is in the closed position and does not participate in adjusting this characteristic of the electric current.
- When the threshold of 12 Volts is exceeded, the zener diode “opens” and maintains this value in the range specified by its characteristics.
If the voltage supplied to the zener diode exceeds that stated as the maximum by the manufacturer, the device very quickly fails due to the effect of thermal runaway.
In order for any zener diode model to serve as long as possible, it is recommended to specify the voltage range and current in which it should be operated according to its specifications. Depending on the connection, there are two versions of the classic stabilizer: linear - the adjusting elements are connected in series with the load; parallel – voltage stabilizing devices are located parallel to the powered devices.
Integral stabilizer
The devices are assembled using small-sized microcircuits capable of operating at an input voltage of up to 26-30 V, delivering a constant 12-volt current of up to 1 Ampere. A special feature of these radio components is the presence of 3 legs - “input”, “output” and “adjustment”. The latter is used to connect an adjustment resistor, which is used to adjust the microcircuit and prevent it from overloading.
More convenient and reliable equalizers assembled on the basis of stabilizing microcircuits are gradually replacing analogues assembled on discrete elements.
How to choose the right zener diode?
Zener diodes are low power stabilizers. Therefore, they must be selected so that the entire load current plus the minimum stabilization current can pass through them without overheating.
To correctly select a zener diode for an electrical circuit, you need to know the following parameters: minimum and maximum input voltage, output voltage, minimum and maximum load current. The stabilization voltage of the zener diode is equal to the output voltage. And it is best to calculate the maximum current that can pass through a zener diode in a particular circuit, and the power dissipation at the maximum current, using an online calculator.
How to make a 12V stabilizer
Simple, but at the same time quite effective, reliable and durable stabilizing devices can be made independently, using simple zener diodes and special small microcircuits such as LM317, LD1084, L7812, KREN (KR142EN8B).
Stabilizer on LM317
The assembly process of such a voltage-stabilizing device consists of the following steps:
- A 130-ohm resistance is soldered to the middle output contact of the microcircuit.
- A conductor is soldered to the input right contact, supplying an unstabilized voltage from the power source.
- The left adjustment contact is soldered to the second leg of the resistor installed at the output of the microcircuit.
The soldering process of such a stabilizer takes no more than 10 minutes and, taking into account the inexpensive microcircuit, does not require large investments. Using a similar device, LED lights and strips are powered.
Chip LD1084
The assembly of a device for stabilizing the voltage of an automobile on-board network using the LD1084 microcircuit is carried out as follows:
- A conductor with positive voltage from the diode bridge is soldered to the input contact of the microcircuit.
- The emitter of a bipolar transistor is soldered to the adjustment contact, the base of which, through two resistors with a nominal value of 1 kOhm, supplies the current of the low and high beam headlights.
- Two resistors are soldered to the output contact (one is a regular 120 Ohm, and the second is a trimmer, 4.7 kOhm) and a 10 µF electrolytic capacitor
To smooth out the current ripple, another electrolytic capacitor with a capacity of 10 μF is installed after the diode bridge.
