Adjustable zener diode. TL431 voltage reference

TL 431 is a programmable shunt voltage regulator. Although this integrated circuit began to be produced in the late 70s, it still does not lose its position in the market and is popular among radio amateurs and large manufacturers of electrical equipment. The board of this programmable stabilizer contains a photoresistor, a resistance measurement sensor and a thermistor. TL 431 are widely used in a wide variety of electrical appliances, household and industrial equipment. Most often, this integrated zener diode can be found in power supplies for computers, televisions, printers and chargers for lithium-ion phone batteries.

TL 431 integrated zener diode

Key Features of the TL 431 Programmable Voltage Reference

• ​ Rated operating voltage at the output is from 2.5 to 36 V;
• Output current up to 100 mA;
• Power 0.2 Watt;
• Operating temperature range for TL 431C from 0° to 70°;
• The operating temperature range for TL 431A is from -40° to +85°.

The accuracy of the TL 431 integrated circuit is indicated by the sixth letter in the designation:

• Accuracy without a letter – 2%;
• Letter A – 1%;
• Letter B – 0.5%.

Its widespread use is due to its low price, universal form factor, reliability, and good resistance to aggressive environmental factors. But it should also be noted the accuracy of this voltage regulator. This allowed him to occupy a niche in microelectronics devices.

The main purpose of the TL 431 is to stabilize the reference voltage in the circuit. Provided that the voltage at the source input is below the rated reference voltage, the transistor in the programmable module will be closed and the current passing between the cathode and anode will not exceed 1 mA. If the output voltage exceeds the programmed level, the transistor will be open and electric current will be able to freely pass from the cathode to the anode.

Wiring diagram TL 431

Depending on the operating voltage of the device, the connection circuit will consist of a single-stage converter and expander (for 2.48 V devices) or a small capacity modulator (for 3.3 V devices). And also to reduce the risk of a short circuit, a fuse is installed in the circuit, usually behind the zener diode. The physical connection is influenced by the form factor of the device in which the TL 431 circuit will be located, and environmental conditions (mainly temperature).

Stabilizer based on TL 431

The simplest stabilizer based on the TL 431 is a parametric stabilizer. To do this, you need to include two resistors R 1, R 2 in the circuit through which you can set the output voltage for TL 431 using the formula: U out = Vref (1 + R 1/ R 2). As can be seen from the formula here, the output voltage will be directly proportional to the ratio of R 1 to R 2. The integrated circuit will keep the voltage at 2.5 V. For resistor R 1, the output value is calculated as follows: R 1 = R 2 (U out / Vref - 1).

This regulator circuit is typically used in fixed or variable voltage power supplies. Such voltage stabilizers on the TL 431 can be found in printers, plotters, and industrial power supplies. If it is necessary to calculate the voltage for fixed power supplies, then we use the formula Vo = (1 + R 1/ R 2) Vref.

Timing relay

The precision characteristics of the TL 431 allow it to be used for other than its intended purpose. Due to the fact that the input current of this adjustable stabilizer is from 2 to 4 μA, a temporary relay can be assembled using this chip. The role of a timer in it will be played by R1, which will begin to gradually charge after opening the contacts S 1 C 1. When the voltage at the output of the stabilizer reaches 2.5 V, transistor DA1 will be open, current will begin to flow through the LEDs of the PC 817 optocoupler, and the open photoresistor will close the circuit.

Thermal stabilizer based on TL 431

The technical characteristics of TL 431 make it possible to create thermally stable current stabilizers based on it. In which resistor R2 acts as a feedback shunt, a value of 2.5 V is constantly maintained on it. As a result, the value of the load current will be calculated using the formula In = 2.5/R2.

Pinout and serviceability check of TL 431

The TL 431 form factor and its pinout will depend on the manufacturer. There are options in old TO-92 and new SOT-23 packages. Don’t forget about the domestic analogue: KR142EN19A is also widespread on the market. In most cases, the pinout is applied directly to the board. However, not all manufacturers do this, and in some cases you will have to look for information on pins in the data sheet of a particular device.

TL 431 is an integrated circuit and consists of 10 transistors. Because of this, it is impossible to check it with a multimeter. To check the serviceability of the TL 431 chip, you need to use a test circuit. Of course, there is often no point in looking for a burnt-out element and it is easier to replace the entire circuit.

Calculation programs for TL 431

There are many sites on the Internet where you can download calculator programs to calculate voltage and current parameters. They can indicate the types of resistors, capacitors, microcircuits and other components of the circuit. TL 431 calculators are also available online, they are inferior in functionality to installed programs, but if you only need the input/output and maximum values ​​of the circuit, then they will cope with this task.

Chip TL431- This is an adjustable zener diode. Used as a reference voltage source in various power supply circuits.

TL431 Specifications

• output voltage: 2.5…36 volts;
• output impedance: 0.2 Ohm;
• forward current: 1…100 mA;
• error: 0.5%, 1%, 2%;

TL431 has three terminals: cathode, anode, input.

Analogs TL431

Domestic analogues of TL431 are:

• KR142EN19A
• K1156ER5T

Foreign analogues include:

• KA431AZ
• KIA431
• HA17431VP
• IR9431N
• AME431BxxxxBZ
• AS431A1D
• LM431BCM

TL431 connection diagrams

The TL431 zener diode microcircuit can be used not only in power circuits. Based on the TL431, you can design all kinds of light and sound signaling devices. With the help of such designs it is possible to control many different parameters. The most basic parameter is voltage control.

By converting some physical indicator into a voltage indicator using various sensors, it is possible to make a device that monitors, for example, temperature, humidity, liquid level in a container, degree of illumination, gas and liquid pressure. Below we present several circuits for connecting the controlled zener diode TL431.

This circuit is a current stabilizer. Resistor R2 acts as a shunt, on which a voltage of 2.5 volts is established due to feedback. As a result of this, we obtain a direct current at the output equal to I=2.5/R2.

Overvoltage indicator

The operation of this indicator is organized in such a way that when the potential at the control contact TL431 (pin 1) is less than 2.5 V, the zener diode TL431 is locked, only a small current passes through it, usually less than 0.4 mA. Since this current value is enough for the LED to light up, to avoid this, you just need to connect a 2...3 kOhm resistance in parallel with the LED.

If the potential supplied to the control pin exceeds 2.5 V, the TL431 chip will open and HL1 will start to light up. Resistance R3 creates the desired limitation of the current flowing through HL1 and the zener diode TL431. The maximum current passing through the zener diode TL431 is around 100 mA. But the LED's maximum allowable current is only 20 mA. Therefore, it is necessary to add a current-limiting resistor R3 to the LED circuit. Its resistance can be calculated using the formula:

R3 = (Upit. – Uh1 – Uda)/Ih1

where Upit. - supply voltage; Uh1 – voltage drop across the LED; Uda – voltage on open TL431 (about 2 V); Ih1 – required current for the LED (5...15mA). It is also necessary to remember that for the TL431 zener diode the maximum allowable voltage is 36 V.

The magnitude of the voltage Uz at which the alarm is triggered (the LED lights up) is determined by the divider across resistances R1 and R2. Its parameters can be calculated using the formula:

R2 = 2.5 x Rl/(Uз - 2.5)

If you need to accurately set the response level, then you need to install a trimming resistor with a higher resistance in place of resistance R2. After fine tuning is completed, this trimmer can be replaced with a permanent one.

Sometimes it is necessary to check several voltage values. In this case, you will need several similar signaling devices on the TL431 configured for their own voltage.

Checking the serviceability of TL431

Using the above circuit, you can check the TL431 by replacing R1 and R2 with one 100 kOhm variable resistor. If by rotating the variable resistor slider the LED lights up, then the TL431 is working.

Low voltage indicator

The difference between this circuit and the previous one is that the LED is connected differently. This connection is called inverse, since the LED lights up only when the TL431 chip is locked.

If the monitored voltage value exceeds the level determined by the divider Rl and R2, the TL431 chip opens and current flows through resistance R3 and pins 3-2 of the TL431 chip. At this moment, there is a voltage drop on the microcircuit of about 2V, and it is clearly not enough to light the LED. To completely prevent the LED from burning, 2 diodes are additionally included in its circuit.

At the moment when the value under study is less than the threshold determined by the divider Rl and R2, the TL431 microcircuit will close, and the potential at its output will be significantly higher than 2V, as a result of which the HL1 LED will light up.

Voltage change indicator

If you only need to monitor voltage changes, the device will look like this:

This circuit uses a two-color LED HL1. If the potential is below the threshold set by the divider R1 and R2, then the LED lights up in green, but if it is above the threshold value, then the LED lights up in red. If the LED does not light up at all, this means that the controlled voltage is at the level of the specified threshold (0.05...0.1V).

Working with TL431 sensors

If it is necessary to monitor changes in any physical process, then in this case the resistance R2 must be changed to a sensor characterized by a change in resistance due to external influence.

An example of such a module is given below. To summarize the operating principle, various sensors are shown in this diagram. For example, if you use it as a sensor, you will end up with a photo relay that responds to the degree of illumination. As long as the illumination is high, the resistance of the phototransistor is low.

As a result, the voltage at the control contact TL431 is below the specified level, which is why the LED does not light up. As illumination decreases, the resistance of the phototransistor increases. For this reason, the potential at the control contact of the zener diode TL431 increases. When the response threshold (2.5V) is exceeded, HL1 lights up.

This circuit can be used as a soil moisture sensor. In this case, instead of a phototransistor, you need to connect two stainless electrodes, which are stuck into the ground at a short distance from each other. After the soil dries, the resistance between the electrodes increases and this causes the TL431 chip to operate and the LED lights up.

If you use a thermistor as a sensor, you can make a thermostat from this circuit. The circuit's response level in all cases is set by resistor R1.

TL431 in circuit with sound indication

In addition to the above lighting devices, you can also make a sound indicator on the TL431 chip. A diagram of such a device is shown below.

This sound alarm can be used to monitor the water level in any container. The sensor consists of two stainless electrodes located at a distance of 2-3 mm from each other.

As soon as water touches the sensor, its resistance will decrease, and the TL431 chip will enter linear operating mode through resistances R1 and R2. In this regard, self-generation appears at the resonant frequency of the emitter and a sound signal will be heard.

Calculator for TL431

To make calculations easier, you can use a calculator:

(103.4 Kb, downloads: 21,594)
(702.6 Kb, downloads: 14,619)

I’ve already written quite a lot about LEDs, but now readers don’t know how to properly power them so that they don’t burn out ahead of schedule. Now I continue to rapidly expand the section of power supplies, voltage stabilizers and current converters.

The top ten popular electronic components include the adjustable stabilizer TL431 and its brother PWM controller TL494. In power supplies it acts as a “programmable reference voltage source, the switching circuit is very simple. In switching power supplies based on TL431, feedback and reference voltage are implemented.

Check out the characteristics and datasheets of other ICs used for power supply.

• 1. Specifications
• 2. TL431 connection diagrams
• 3. Pinout TL431
• 4. Datasheet in Russian
• 5. Electrical characteristics graphs

Specifications

It has been widely used due to the superiority of its technical characteristics and the stability of parameters at different temperatures. The functionality is partially similar to the well-known one, only it operates at low current and is intended for adjustment. All features and typical switching circuits are indicated in the datasheet in Russian. The analogue of TL431 will be the domestic KR142EN19 and the imported K1156ER5, their parameters are very similar. I haven't seen any other analogues.

Main characteristics:

1. output current up to 100mA;
2. output voltage from 2.5 to 36V;
3. power 0.2W;
4. temperature range TL431C from 0° to 70°;
5. for TL431A from -40° to +85°;
6. price from 28 rubles for 1 piece.

Detailed characteristics and operating modes are indicated in the datasheet in Russian at the end of this page or can be downloaded

Example of use on a board

The stability of the parameters depends on the ambient temperature, it is very stable, there is little noise at the output and the voltage floats +/- 0.005V according to the datasheet. In addition to the household modification TL431C from 0° to 70°, a variant with a wider temperature range TL431A from -40° to 85° is available. The selected option depends on the purpose of the device. Analogues have completely different temperature parameters.

It is impossible to check the serviceability of the microcircuit with a multimeter, since it consists of 10 transistors. To do this, it is necessary to assemble a test switching circuit, by which you can determine the degree of serviceability; the element does not always fail completely, it may simply burn out.

TL431 connection diagrams

The operating characteristics of the stabilizer are set by two resistors. The options for using this microcircuit may be different, but it is most widespread in power supplies with adjustable and fixed voltage. Often used in current stabilizers in USB chargers, industrial power supplies, printers and other household appliances.

TL431 is found in almost any ATX power supply from a computer; you can borrow it from it. Power elements with radiators and diode bridges are also there.

This chip implements many charger circuits for lithium batteries. Radio constructors are produced for self-assembly with your own hands. The number of application options is very large; good schemes can be found on foreign sites.

Pinout TL431

As practice shows, the pinout of the TL431 can be different and depends on the manufacturer. The image shows the pinout from the Texas Instruments datasheet. If you remove it from some finished board, then the pinout of the legs can be seen on the board itself.

Datasheet in Russian

Many radio amateurs do not know English and technical terms very well. I have a fairly good command of the language of the intended enemy, but when developing it still bothers me constantly remembering the translation of electrical terms into Russian. The translation of the TL431 datasheet into Russian was done by our colleague, whom we thank.

TL431 is an integrated zener diode. In the circuit it plays the role of a reference voltage source. The presented element is used, as a rule, in power supplies. The device of a zener diode is quite simple. In total, the model uses three outputs. Depending on the modification, the housing can contain up to ten transistors. A distinctive feature of the TL431 is considered to be good thermal stability.

2.48 V connection circuit

The 2.48 V zener diode TL431 has a single-stage converter. On average, the operating current in the system reaches a level of 5.3 A. Resistors for signal transmission can be used with different voltage conductivities. The accuracy of stabilization in these devices fluctuates around 2%.

To increase the sensitivity of the zener diode, various modulators are used. As a rule, the dipole type is selected. On average, their capacitance is no more than 3 pF. However, in this case, much depends on the conductivity of the current. To reduce the risk of elements overheating, expanders are used. The zener diodes are connected through the cathode.

Turning on a 3.3 V device

For the TL431 zener diode, the 3.3V switching circuit involves the use of a single-stage converter. Resistors for pulse transmission are used of the selective type. The zener diode TL431 also has a 3.3 volt switching circuit with a small capacitance modulator. To reduce the risk, fuses are used. They are usually installed behind zener diodes.

To amplify the signal, you cannot do without filters. On average, the threshold voltage fluctuates around 5 W. The operating current of the system is no more than 3.5 A. As a rule, the stabilization accuracy does not exceed 3%. It is also important to note that the zener diode can be connected via a vector adapter. In this case, the transistor is selected as a resonant type. On average, the modulator capacitance should be 4.2 pF. Thyristors are used both phase and open type. To increase the conductivity of current, triggers are needed.

Today, these elements are equipped with amplifiers of different powers. On average, the threshold voltage in the system reaches 3.1 W. The operating current fluctuates around 3.5 A. It is also important to consider the output resistance. The presented parameter must be no more than 80 Ohms.

Connection to 14 V circuit

For the zener diode TL431, the 14V switching circuit involves the use of a scalar converter. On average, the threshold voltage is 3 W. As a rule, the operating current does not exceed 5 A. In this case, the permissible overload fluctuates around 4 Ah. Also, the zener diode TL431 has a 14V switching circuit with amplifiers of both single-pole and double-pole types. In order to improve conductivity, you cannot do without a tetrode. It can be used with one or two filters.

A Series Zener Diodes

The A TL431 series is used for power supplies and inverters. How to check whether an element is connected correctly? In fact, this can be done using a tester. The threshold resistance indicator must be 80 ohms. The device is capable of operating through single-stage and vector type converters. In this case, resistors are used with a plate.

If we talk about the parameters, the circuit does not exceed 5 W. In this case, the operating current fluctuates around 3.4 A. To reduce the risk of transistor overheating, expanders are used. For A series models, they are only suitable for switching type. To increase the sensitivity of the device, powerful modulators are needed. On average, the output resistance parameter does not exceed 70 Ohms.

CLP series devices

The zener diode TL431 switching circuit has single-stage converters. The CLP model can be found both in inverters and in many household devices. The threshold voltage of the zener diode fluctuates around 3 W. The direct operating current is 3.5 A. The stabilization accuracy of the elements does not exceed 2.5%. Various types of modulators are used to adjust the output signal. In this case, triggers are selected with amplifiers.

ACLP series zener diodes

The zener diode TL431 switching circuit has vector or scalar converters. If we consider the first option, then the operating current level is no more than 4 A. In this case, the stabilization accuracy is approximately 4%. Triggers and thyristors are used to amplify the signal.

If we consider a connection diagram with a scalar converter, then modulators are used with a capacitance of about 6 pF. The transistors themselves are of the resonant type. Regular triggers are suitable to amplify the signal. It is also important to note that the sensitivity of the device fluctuates around 20 mV.

AC models

Cherry AC zener diodes TL431 are often used for dipole inverters. How to check the functionality of the connected element? This can be done using a regular tester. The output resistance parameter must be no more than 70 Ohms. It is also important to note that devices in this series are switched on through a vector converter.

In this case, scalar modifications are not suitable. This is largely due to the low threshold for current conduction. It is also important to note that the nominal voltage does not exceed 4 W. The operating current in the circuit is maintained at 2 A. To reduce heat losses, various thyristors are used. Today, expansion and phase modifications are produced.

Models with KT-26 body

In household electrical appliances, zener diodes TL431 are often found with the KT-26 housing. The switching circuit involves the use of dipole modulators. They are produced with different current conductivity. The maximum sensitivity parameter of the system fluctuates around 430 mV.

The output impedance reaches no more than 70 Ohms. Triggers in this case are used only with amplifiers. To reduce the risk of short circuits, open and closed type filters are used. The zener diode is directly connected through the cathode.

KT-47 body

TL431 (stabilizer) with a KT-47 housing can be found in power supplies of various powers. The element's connection circuit involves the use of vector converters. The modulator is suitable for circuits up to 4 pF. The direct output impedance of the devices is approximately 70 Ohms. To improve the conductivity of zener diodes, only beam-type tetrodes are used. As a rule, the stabilization accuracy does not exceed 2%.

For 5V power supplies

In 5 V power supplies, the TL431 is switched on through amplifiers with different current conductivities. The converters themselves are of the single-stage type. Also in some cases vector modifications are used. On average, the output impedance is about 90 ohms. The accuracy of stabilization in devices is 2%. Extenders for blocks are used of both switched and open types. Triggers can only be used with filters. Today they are produced with one and several elements.

Connection diagram for 10 V units

The circuit for connecting a zener diode to the power supply involves the use of a single-stage or vector converter. If we consider the first option, then the modulator is selected with a capacitance of 4 pF. In this case, the trigger is used only with amplifiers. Sometimes filters are used to increase the sensitivity of the zener diode. The threshold voltage of the circuit is on average 5.5 W. The operating current of the system fluctuates around 3.2 A.

The stabilization parameter, as a rule, does not exceed 3%. If we consider a circuit with a vector converter, then we cannot do without a transceiver. It can be used either open or chromatic. The modulator is installed with a capacitance of 5.2 pF. The expander is quite rare. In some cases, it can increase the sensitivity of the zener diode. However, it is important to consider that the thermal losses of the element increase significantly.

Diagram for 15 V blocks

The zener diode TL431 switching circuit through a 15 V block is carried out using a single-stage converter. In turn, the modulator is suitable with a capacitance of 5 pF. Resistors are used exclusively of the selective type. If we consider modifications with triggers, then the threshold voltage parameter does not exceed 3 W. The stabilization accuracy is around 3%. Filters for the system are suitable for both open and closed types.

It is also important to note that an expander may be installed in the circuit. Today, models are produced mainly of the switched type. For modifications with transceivers, the current conductivity does not exceed 4 microns. In this case, the sensitivity of the zener diode fluctuates around 30 mV. The output impedance reaches approximately 80 Ohms.

For car inverters

For the AC series zener diodes TL431 are often used. The connection circuit in this case involves the use of two-digit triodes. The filters themselves are used in the open type. If we consider circuits without an expander, the threshold voltage fluctuates around 10 W.

The direct operating current is 4 A. The system overload parameter is allowed at 3 mA. If we consider modifications with expanders, then in this case high-capacity modulators are installed. Resistors are used as standard selective type.

In some cases, amplifiers of different power are used. The threshold voltage parameter, as a rule, does not exceed 12 W. The output impedance of the system can range from 70 to 80 ohms. The stabilization accuracy rate is approximately 2%. The operating current of the systems is no more than 4.5 A. The zener diodes are directly connected through the cathode.

Let me make a reservation right away that this article is not a panacea. This may not work for some people.

First, I'll talk about the TL431 and what it does. TL431 is a controlled zener diode with which you can obtain a stabilized voltage within a wide range from 2.5 volts to 36 volts. Using this microcircuit, you can make a reference voltage source for power supplies, as well as for various measuring circuits.

Figure taken from ON Semiconductor datasheet

Below are two datasheet options for this chip.

1. ON Semiconductor datasheet https://www.onsemi.com/pub/Collateral/TL431-D.PDF
2. Texas Instruments datasheet http://www.ti.com/lit/ds/symlink/tl431.pdf

The pinout of this chip is best displayed in the ON Semiconductor datasheet

One small detail found in the Texas Instruments datasheet

In all the figures there is one inscription “top view”, this translates as “top view”; if you look at the datasheet inattentively, without knowing what this may mean, you can solder it incorrectly on the board.

I used the TL431 chip in one of my circuits, and it turned out to be faulty. After searching the forums, I found a way to test this microcircuit. And in some places I saw how this microcircuit is called using a multimeter, but, alas, this is not the case. I also first tried to check with a multimeter but immediately put this event aside. And I decided to try checking it using a universal component tester, which I had previously purchased on Aliexpress.

During the check I made a table. First I checked in dual-terminal mode (if the table shows two pins, you just need to combine both pins together).

Measurement results of the first specimen

Dimension 1 – REF; 2 - cathode.

Dimension 1 – anode; 2 - cathode.

Dimension 1 - REF, cathode; 2 – anode.

Dimension 1 – REF; 2 – cathode, anode.

Measurement 1 – REF, 2 – anode, 3 – cathode.

Measurement results of the second specimen.

There is a slight difference. Looking at the table you notice a certain pattern. For example, in line 4, this is actually the operating mode of the TL431 to produce 2.5 volts. But the most interesting thing is the measurement mode in the three-terminal mode. In one case it is defined as a transistor, and in the second case as a missing part. The most interesting thing is when the transistor is defined: the NPN structure transistor is defined, the REF pin is defined as the emitter, the anode as the base, and the cathode as the collector. Between the REF and the cathode there is a cathode diode, which is directed towards the cathode.

Based on this data, it is already possible to judge whether the microcircuit has been fixed or not, and also to determine the pinout.