TL431 ICsThe TL431 integral stabilizer is typically used in power supplies. But for him, you can still find a lot of areas of use. We describe some of these schemes in this article, and also describe useful and simple devices made with the help of the TL431 chip. But in this case, do not be afraid of the term "microcircuit", it has only three outputs, and it looks like a simple low-power TO90 transistor.


  • 1 What is a TL431 chip?
    • 1.1 Signaling devices and indicators
    • 1.2 Critical current detector
  • 2 Alarm Level

What is a TL431 chip?

It so happens that all electronics know the magic numbers TL431, analogue 494. What it is?

Texas Instrument Enterprise was at the origins of semiconductor development. They have always been in the first places in the production of electronic components, constantly being held in the top ten world leaders. The first integrated circuit was developed in 1958. employee of this firm Jack Kilby.

Today, TI manufactures a wide range of microcircuits, their name starts with the letters SN and TL. These are respectively logical and analog microcircuits, forever included in the history of TI, and still have widespread use.

Among the favorites in the list of "magic" chips you need, most likely, an integral stabilizer TL431. In the 3 output case of this microcircuit 10 transistors are installed, and the function performed by it is identical with a simple zener diode (Zenner diode).

But thanks to this complication, the microcircuit has an increased steepness of characteristics and higher thermal stability. Its main feature is that with the help of an external separator, the stabilization voltage can change the current in the range of 2.6... 32 Volts. In modern TL431 analogue of the lower threshold has 1.25 volts.

The TL431 analogue was developed by engineer Barney Holland when he was involved in copying another company's stabilizer circuit. In our country, they would say stripping, not copying. And Holand borrowed from the original circuit the source of the reference voltage, and already on this basis developed a separate stabilizing chip. Initially, it had the name TL430, and after certain modifications of the mill it was called TL431.

Since then, a lot of time has passed, but today there is not a single power supply for a computer, wherever it is installed. The circuit has also found use in almost all pulsed, weak power supplies. One of these sources today is in any home - it is charging for mobile phones. This longevity can only envy.

Holand also developed the equally well-known and demanded TL494 scheme until today. it dual frequency PWM controller, on the basis of which many types of power sources are made. Therefore, the figure 494 is also rightfully referred to as “magical.” But we proceed to the consideration of different products based on the TL431.

Signaling devices and indicators

Chip OverviewCircuit TL431 analog can be used not only in its intended purpose as zener diodes in power supplies. On the basis of this chip it is possible to create different sound signaling devices and lighting indicators. With these devices you can check many different parameters.

For starters, this ordinary electricity voltage. If, on the basis of sensors, we can represent some physical quantity as a voltage, then it is possible to create equipment that controls, for example:

  • humidity and temperature;
  • water level in the tank;
  • gas or liquid pressure;
  • illumination.

Critical current detector

Critical current detectorThe principle of operation of this detector is based on the fact that during the voltage on the control electrode of the Zener diode DA1 (output 1) less than 2.6 volts, the zener diode is closed, only a low current passes through it, usually not more than 0.20... 0.30 mA But this current is enough for a weak glow of the diode HL1. To prevent this from happening, you can connect a resistor in parallel with the diode approximately 1... 2 com.

If the voltage on the control electrode is more than 2.6 volts, then the zener diode will open and the diode HL1 will light up. The required voltage limit through the zener diode DA1 and the diode HL1 creates R3. The highest current of the Zener diode has 100 mA, while the same parameter for the HL1 diode is only 22 mA. It is from this condition that the resistor R3 can be calculated. More precisely, the resistance is calculated using the formula below.

R3 = (Upit - Uhl - Uda) / Ihl, where:

  • Uda - current on an open circuit (usually 2 Volt);
  • Uhl - direct current drop on the diode;
  • Upit - power supply current;
  • Ihl is the diode voltage (is in the range of 4... 12 mA).

You also need to remember that the highest voltage for the TL431 is only 36 volts. This parameter must not be exceeded.

Alarm Level

How to connectThe current on the control electrode, when the diode HL1 (Uz) is turned on, is set by the separator R1, R2. The characteristics of the separator are determined by the formula:

R2 = 2.5xR1 / (Uz - 2.5)

For the most accurate adjustment of the turn-on threshold, instead of the resistor R2, you can put a trimmer, with a factor of 1.5 times higher than it turned out. Then, when the tincture is done, it can be changed to a fixed resistor, its resistance should be equal to the resistance of the installed part of the trimmer.

How does the TL431 check the wiring diagram? In order to control several current levels, 3 of these alarms will be necessary, each of them being tuned to a specific voltage. In this way, you can make a whole line of the scale and indicators.

To power the display circuit, which consists of a resistor R3 and diode HL1, you can use a separate even unstabilized power supply. In this case, the controlled current is fed to the upper output of the resistor R1, which must be disconnected from the resistor R3. With this connection, the monitored current can be in the range from 3 to tens of volts.

The difference of this circuit from the previous one is that the diode is connected differently. This connection is called inverse, since the diode is turned on only if the circuit is closed. In the case when the monitored current exceeds the threshold set by the separator R1, R2, the circuit is open, and the current passes through the resistor R3 and the outputs of the 3-4 circuits.

In this diagram, the voltage drops to 2 volts, which is not enough to turn on the LED. So that the diode is guaranteed not to turn on, two diodes are installed in series with it.

If the monitored current is less than that specified by the separator R1, R2, the circuit closes, the current at its output will be significantly more than 2 Volts, because the HL1 diode turns on.

If you only need to check the change in current, then the indicator can be made according to the scheme.

Indicators and their typesThis indicator uses a 2-color diode HL1. If the monitored current exceeds the set value, the red diode turns on, and if the current is lower, then green. If the voltage is located near this threshold, both LEDs are extinguished, because the transmission position of the Zener diode has a certain slope.

If you need to track the change of some physical quantity, then R2 is replaced by a sensor that changes the resistance under the influence of the environment.

Conventionally, there are several sensors on the diagram at the same time. If it is a phototransistor, then there will be a photorelay. As long as there is enough light, the phototransistor is open, and its resistance is small. Because the current at the control output DA1 below thresholdAs a result, the diode does not shine.

As the light decreases, the resistance of the phototransistor increases, which leads to an increase in voltage at the control output of DA1. If this voltage is greater than the threshold (2.5 volts), then the zener diode opens and the diode lights up.

If you connect a thermistor, instead of a phototransistor, to the input of the chip, for example, a series of MMT, then the temperature indicator will come out: when the temperature decreases, the diode will turn on.

The trigger threshold in any case is set using a resistor R1.

In addition to the described light indicators, an audio indicator can be made on the basis of the TL431 analogue. To control the water, for example, in the bath, the sensor is connected to the circuit of two stainless steel plates, which are a couple of millimeters apart.

If water reaches the sensor, then its resistance decreases, and the chip with R1, R2 enters the linear mode. So, autogeneration occurs at the resonant frequency of HA1In this case a beep will occur.

Summing up, I would like to say that the main use of the TL434 chip is, of course, power supplies. But, as you can see, the capabilities of the chip only by this function are absolutely unlimited, and many devices can be assembled.