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Integral stabilizer lm317. Voltage stabilizer on LM317. Some features of working with the lm317 chip

The LM317 linear integrated stabilizer circuit with adjustable output voltage was developed by the author of the first monolithic three-terminal stabilizers, R. Widlar, almost 50 years ago. The microcircuit turned out to be so successful that it is currently produced without changes by all major manufacturers of electronic components and is used in a variety of devices in different connection options.

general information

The circuitry of the device provides higher parameters for instability of parameters, in comparison with stabilizers for a fixed voltage, and has almost all types of protection used for integrated circuits: limiting the output current, shutting down when overheating and exceeding the maximum operating parameters.

At the same time, a minimum number of external components is required for the LM317; the circuit uses built-in stabilization and protection.

The device is available in three versions -L.M.117/217/317, differing in maximum permissible operating temperature:

  • LM117: from -55 to 150 °C;
  • LM217: from -25 to 150 °C;
  • LM317: from 0 to 125 oC.

All types of stabilizers are produced in standard TO-3 housings, various modifications of TO-220, for surface mounting - D2PAK, SO-8. For low power devices, TO-92 is used.

The pinout for all three-pin products is the same, which makes them easier to replace. Depending on the housing used, additional symbols are added to the marking:

  • K – TO-3 (LM317K);
  • T – TO-220;
  • P – ISOWATT220 (plastic body);
  • D2T – D2PAK;
  • LZ – TO-92;
  • LM – SOIC8.

All standard sizes are used for LM317, LM117 is available only in TO-3 housing, LM217 in TO-3, D2PAK and TO-220. LM317LZ microcircuits in TO-92 packages are distinguished by reduced values ​​of maximum power and output current, up to 100 mA, with similar other properties. Sometimes the manufacturer uses its own markings, for example, LM317НV from Texas Instruments - high-voltage regulators in the range of 1.2-60 V, while the housing pinouts coincide with products from other companies. Unlike other microcircuits, the abbreviation LM (LM) is used by all manufacturers. Explanation of other possible designations is given in the technical description of the specific device.

Basic electrical parametersL.M.117/217/317

The characteristics of the regulators are determined by the difference between the input (Ui) and output voltage (Uo) 5 volts, load current 1.5 amperes and maximum power 20 watts:

  • Voltage instability – 0.01%;
  • Reference voltage (UREF) – 1.25 V;
  • Minimum load current – ​​3.5 mA;
  • The maximum output current is 2.2 A, with a difference between input and output voltages of no more than 15 V;
  • The maximum power dissipation is limited by the internal circuitry;
  • Input voltage ripple suppression – 80 dB.

It is important to note! At the maximum possible value of Uin – Uout = 40 volts, the permissible load current is reduced to 0.4 amperes. The maximum power dissipation is limited by the internal protection circuit; for TO-220 and TO-3 cases it is approximately 15 to 20 watts.

Applications of Adjustable Stabilizer

When designing electronic devices containing voltage stabilizers, it is more preferable to use a voltage regulator on the LM317, especially for critical equipment components. The use of such solutions requires the additional installation of two resistors, but provides better power parameters than traditional microcircuits with fixed stabilization voltages and has greater flexibility for different applications.

The output voltage is calculated using the formula:

UOUT = UREF (1+ R2/R1) + IADJ, where:

  • VREF = 1.25V, control output current;
  • IADJ is very small - about 100 µA and determines the voltage setting error, in most cases it is not taken into account.

The input capacitor (ceramic or tantalum 1 μF) is installed at a significant distance from the power supply filter capacitance microcircuit - more than 50 mm; the output capacitor is used to reduce the influence of transient processes at high frequencies; for many applications it is not necessary. The switching circuit uses only one adjustment element - a variable resistor; in practice, a multi-turn resistor is used or replaced with a constant of the required value. The control method allows you to implement a programmable source for several voltages, switchable by any available method: relay, transistor, etc. Ripple suppression can be improved by shunting the control pin with a capacitor of 5-15 μF.

Diodes of type 1N4002 are installed in the presence of an output filter with large capacitors, an output voltage of more than 25 volts and a shunt capacitance of more than 10 μF. The LM317 microcircuit is rarely used at extreme operating conditions; the average load current for many solutions does not exceed 1.5 A. Installation of the device on a radiator is necessary in any case; with an output current of more than 1 ampere, it is advisable to use a TO-3 or TO-220 housing with a metal contact platform LM317T.

For your information. You can increase the load capacity of the voltage stabilizer by using a powerful transistor as a regulating element for the output current.

The load current of the device is determined by the parameters of VT1; any n-p-n transistor with a collector current of 5-10 A is suitable: TIP120/132/140, BD911, KT819, etc. Parallel connection of two or three pieces is possible. Any medium-power silicon with the corresponding structure is used as VT2: BD138/140, KT814/816.

The features of such circuits should be taken into account: the permissible difference between the voltages at the input and output is formed from the voltage drops across the transistor, about 2 volts, and the microcircuit, for which the minimum value is 3 volts. For stable operation of the device, at least 8-10 volts is recommended.

The properties of the LM317 series microcircuits make it possible to stabilize the load current over a wide range with high accuracy.

Current fixation is ensured by connecting just one resistor, the value of which is calculated using the formula:

I = UREF/R + IADJ = 1.25/R, where UREF = 1.25 V (resistance R in ohms).

The circuit can be used to charge batteries with a stable current and power LEDs, for which constant current is important when the temperature changes. Also, the current stabilizer on LM317 can be supplemented with transistors, as in the case of voltage stabilization.

The domestic industry produces functional analogues of LM317 with similar parameters - KR142EN12A/B microcircuits with load currents of 1 and 1.5 amperes.

An output current of up to 5 amperes is provided by the LM338 stabilizer with similar other characteristics, which allows you to use all the advantages of an integrated device without external transistors. A complete analogue of the LM317 in all respects, except polarity, is the negative voltage regulator LM337; bipolar power supplies can be easily built on the basis of these two microcircuits.

Video

Quite often there is a need for a simple voltage stabilizer. This article provides a description and examples of the use of an inexpensive (prices for LM317) integrated voltage stabilizer LM317.

The list of tasks solved by this stabilizer is quite extensive - this includes powering various electronic circuits, radio devices, fans, motors and other devices from the mains or other voltage sources, such as a car battery. The most common circuits are voltage-regulated.

In practice, with the participation of LM317, you can build a voltage stabilizer for an arbitrary output voltage in the range of 3...38 volts.

Specifications:

  • Stabilizer output voltage: 1.2... 37 volts.
  • Load-bearing current up to 1.5 amperes.
  • Stabilization accuracy 0.1%.
  • There is internal protection against accidental short circuit.
  • Excellent protection of the integrated stabilizer from possible overheating.


Power dissipation and input voltage of the LM317 stabilizer

The voltage at the stabilizer input should not exceed 40 volts, and there is also one more condition - the minimum input voltage should exceed the desired output voltage by 2 volts.

The LM317 microcircuit in the TO-220 package is capable of stable operation at a maximum load current of up to 1.5 amperes. If you do not use a high-quality heat sink, this value will be lower. The power released by the microcircuit during its operation can be determined approximately by multiplying the output current and the difference between the input and output potential.

The maximum permissible power dissipation without a heat sink is approximately 1.5 W at an ambient temperature of 30 degrees Celsius or less. If good heat dissipation from the LM317 case is ensured (no more than 60 g), the power dissipation can be 20 watts.

When placing a microcircuit on a radiator, it is necessary to isolate the microcircuit body from the radiator, for example, with a mica gasket. It is also advisable to use heat-conducting paste for effective heat removal.

Selection of resistance for stabilizer LM317

For accurate operation of the microcircuit, the total value of resistances R1...R3 must create a current of approximately 8 mA at the required output voltage (Vo), that is:

R1 + R2 + R3 = Vo / 0.008

This value should be taken as ideal. In the process of selecting resistances, a slight deviation (8...10 mA) is allowed.

The value of variable resistance R2 is directly related to the output voltage range. Typically, its resistance should be approximately 10...15% of the total resistance of the remaining resistors (R1 and R2), or you can select its resistance experimentally.

The location of the resistors on the board can be arbitrary, but for better stability it is advisable to place it away from the heatsink of the LM317 chip.

Circuit stabilization and protection

Capacitance C2 and diode D1 are optional. The diode protects the LM317 stabilizer from possible reverse voltage that appears in the designs of various electronic devices.

Capacitance C2 not only slightly reduces the response of the LM317 microcircuit to voltage changes, but also reduces the influence of electrical interference when the stabilizer board is placed near places with powerful electromagnetic radiation.

A high-quality power supply with adjustable output voltage is the dream of every novice radio amateur. In everyday life, such devices are used everywhere. For example, take any charger for a phone or laptop, power supply for a children's toy, game console, landline phone, and many other household appliances.

As for the circuit implementation, The design of the sources may be different:

  • with power transformers, a full-fledged diode bridge;
  • pulse converters of mains voltage with adjustable output voltage.

But in order for the source to be reliable and durable, it is better to choose a reliable element base for it. This is where difficulties begin to arise. For example, choosing domestically produced components as regulating, stabilizing components, the lower voltage threshold is limited to 5 V. But what if 1.5 V is required? In this case, it is better to use imported analogues. Moreover, they are more stable and practically do not heat up during operation. One of the most widely used is integral stabilizer lm317t.

Main characteristics, chip topology

The lm317 chip is universal. It can be used as a stabilizer with a constant output voltage and as an adjustable stabilizer with high efficiency. MS has high practical characteristics that make it possible to use it in various charger circuits or laboratory power supplies. At the same time, you don’t even have to worry about reliable operation under critical loads, because the microcircuit is equipped with internal short circuit protection.

This is a very good addition, because the maximum output current of the stabilizer on lm317 is no more than 1.5 A. But having protection will prevent you from accidentally burning it. To increase the stabilization current, it is necessary to use additional transistors. Thus, currents of up to 10 A or more can be regulated when using the appropriate components. But we’ll talk about this later, and in the table below we’ll present main characteristics of the component.

Microcircuit pinout

An integrated circuit was manufactured in a standard TO-220 package with a heat sink mounted on a radiator. As for the numbering of the pins, they are located according to GOST from left to right and have the following meaning:

Pin 2 is connected to a heatsink without an insulator, so in devices if the heatsink is in contact with the case, mica insulators must be used or any other heat-conducting material. This is an important point, because you can accidentally short-circuit the pins, and there will simply be nothing at the output of the microcircuit.

Analogs lm317

Sometimes it is not possible to find the specifically required microcircuit on the market, then you can use similar ones. Among the domestic components on lm317, there is an analogue that is quite powerful and productive. He is microcircuit KR142EN12A. But when using it, it is worth considering the fact that it is unable to provide a voltage less than 5 V at the output, so if this is important, you will again have to use an additional transistor or find exactly the required component.

As for the form factor, the KR has the same number of pins as the lm317 has. Therefore, you don’t even have to redo the circuit of the finished device in order to adjust the parameters of the voltage regulator or unchangeable stabilizer. When installing an integrated circuit It is recommended to install it on a radiator with good heat dissipation and cooling system. This is quite often observed in the manufacture of powerful LED lamps. But at rated load the device generates a little heat.

In addition to the domestic integrated circuit KR142EN12, more powerful imported analogues are produced, the output currents of which are 2-3 times higher. Such microcircuits include:

  • lm350at, lm350t - 3 A;
  • lm350k - 3 A, 30 W in another case;
  • lm338t, lm338k - 5 A.

Manufacturers of these components guarantee higher output voltage stability, low regulation current, increased power with the same minimum output voltage of no more than 1.3 V.

Connection features

On the lm317t, the switching circuit is quite simple and consists of a minimum number of components. However, their number depends on the purpose of the device. If a voltage stabilizer is being manufactured, it will require the following parts:

Rs is a shunt resistance, which also acts as a ballast. Select a value of about 0.2 Ohm if you want to provide a maximum output current of up to 1.5 A.

The resistive divides with R1, R2, connected to the output and the housing, and the regulating voltage comes from the middle point, forming deep feedback. Due to this, a minimum ripple coefficient and high stability of the output voltage are achieved. Their resistance is selected based on the ratio 1:10: R1=240 Ohm, R2=2.4 kOhm. This is a typical voltage regulator circuit with an output voltage of 12 V.

If you need to design a current stabilizer, This will require even fewer components:

R1, which is a shunt. They set the output current, which should not exceed 1.5 A.

To correctly calculate the circuit of a particular device, always you can use the lm317 calculator. As for the calculation of Rs, it can be determined using the usual formula: Iout. = Uop/R1. On lm317, the LED current stabilizer is of quite high quality, which can be made of several types depending on the power of the LED:

  • to connect a single-watt LED with a current consumption of 350mA, you must use Rs = 3.6 Ohm. Its power is selected to be at least 0.5 W;
  • To power three-watt LEDs, you will need a resistor with a resistance of 1.2 Ohm, the current will be 1 A, and the dissipation power will be at least 1.2 W.

On lm317, the LED current stabilizer is quite reliable, but it is important to correctly calculate the shunt resistance and select its power. A calculator will help in this matter. Also, various powerful lamps and homemade spotlights are made using LEDs and based on this MS.

Building powerful regulated power supplies

The internal transistor lm317 is not powerful enough, to increase it you will have to use external additional transistors. In this case, components are selected without restrictions, because their control requires much lower currents, which the microcircuit is quite capable of providing.

The lm317 regulated power supply with an external transistor is not much different from the usual one. Instead of a constant R2, a variable resistor is installed, and the base of the transistor is connected to the input of the microcircuit through an additional limiting resistor that turns off the transistor. A bipolar switch with p-n-p conductivity is used as a controlled switch. In this design, the microcircuit operates with currents of about 10 mA.

When designing bipolar power supplies you will need to use the complementary pair of this chip, which is lm337. And to increase the output current, a transistor with n-p-n conductivity is used. In the reverse arm of the stabilizer, the components are connected in the same way as in the upper arm. The primary circuit is a transformer or a pulse unit, which depends on the quality of the circuit and its efficiency.

Some features of working with the lm317 chip

When designing power supplies with a low output voltage, at which the difference between the input and output values ​​does not exceed 7 V, it is better to use other, more sensitive microcircuits with an output current of up to 100 mA - LP2950 and LP2951. At low drop, lm317 is not able to provide the required stabilization coefficient, which can lead to unwanted pulsations during operation.

Other practical circuits on lm317

In addition to conventional stabilizers and voltage regulators based on this chip, there are also Can you make a digital voltage regulator?. To do this, you will need the microcircuit itself, a set of transistors and several resistors. By turning on the transistors and upon receipt of a digital code from a PC or other device, the resistance R2 changes, which also leads to a change in the circuit current within the voltage range from 1.25 to 1.3 V.

LM317 is a low cost IC Voltage regulator With built-in protection against output short circuit and overheating, an easy-to-assemble linear DC voltage regulator can be made on the LM317, which can be adjustable. Such microcircuits come in different packages, for example TO-220 or TO-92. If the hull is TO-92, then the last two letters of the name will be LZ i.e. so: LM317LZ, the pinouts of this microcircuit differ in different cases, so you need to be more careful, there are also such microcircuits in SMD packages. You can order LM317LZ in bulk in a small batch at the link: LM317LZ (10 pcs.), LM317T at the link: LM317T (10 pcs.). Consider the stabilizer circuit:

Figure 1 - DC voltage stabilizer on the LM317LZ chip


In addition to the microcircuit, this stabilizer contains 4 more parts; resistor R2 regulates the voltage at the output of the stabilizer. For ease of assembly, you can use the following diagram:

Figure 2 - DC voltage stabilizer on the LM317LZ chip


All DC voltage stabilizers are divided into 2 types:
1) linear (as for example in our case, i.e. on LM317),
2) pulsed (with higher efficiency and for more powerful loads).
The operating principle of linear (not all) stabilizers can be understood from the figure:

Figure 3 - Operating principle of a linear stabilizer


From Figure 3 it is clear that such a stabilizer is a divider, the lower arm of which is the load and the upper arm is the microcircuit itself. The input voltage changes and the microcircuit changes its resistance so that the output voltage remains unchanged. Such stabilizers have low efficiency because some of the energy is lost on the chip. Switching stabilizers are also a divider, only their upper (or lower) arm can either have a very low resistance (open key) or very high (private key), alternating such states creates PWM with a high frequency and the voltage at the load is smoothed by a capacitor (and/ or the current is smoothed by a choke), thus creating high efficiency, but due to the high PWM frequency, switching stabilizers create electromagnetic interference. There are also linear stabilizers in which the element that carries out stabilization is placed parallel to the load - in such cases, this element is usually a zener diode and in order for stabilization to be carried out, current is supplied to this parallel connection from a current source, the current source is made by installing a resistor with a high resistance in series with the voltage source , if voltage is applied directly to such a stabilizer, there will be no stabilization and the zener diode will most likely burn out.

Vin (input voltage): 3-40 Volts
Vout (output voltage): 1.25-37 Volts
Output current: up to 1.5 Amps
Maximum power dissipation: 20 Watt
Formula to calculate output (Vout) voltage: Vout = 1.25 * (1 + R2/R1)
*Resistance in Ohms
*Voltage values ​​are obtained in Volts

This simple circuit allows you to rectify alternating voltage into direct voltage thanks to a diode bridge made of diodes VD1-VD4, and then use an accurate substring resistor of the SP-3 type to set the voltage you need within the limits of the integrated stabilizer chip.

I used old ones as rectifier diodes FR3002, which once upon a time fell out of an ancient computer from the year 1998. Despite their impressive size (DO-201AD housing), their characteristics (Ureverse: 100 Volts; Idirect: 3 Amps) are not impressive, but that’s enough for me. For them we even had to widen the holes in the board, their pins are too thick (1.3mm). If you slightly change the board in the layout, you can immediately solder a ready-made diode bridge.

A radiator to remove heat from the 317 chip is required; it’s even better to install a small fan. Also, at the junction of the TO-220 chip case substrate with the heatsink, drop a little thermal paste. The degree of heating will depend on how much power the chip dissipates, as well as on the load itself.

Microcircuit LM317T I did not install it directly on the board, but brought out three wires from it, with the help of which I connected this component with the others. This was done so that the legs would not become loose and, as a result, would not be broken, because this part will be attached to the heat dissipator.

To be able to use the full voltage of the microcircuit, that is, adjust from 1.25 and right up to 37 Volts, we set the substring resistor with a maximum resistance of 3432 kOhm (in the store the closest value is 3.3 kOhm). Recommended type of resistor R2: interlinear multi-turn (3296).

The LM317T stabilizer chip itself and others like it are produced by many, if not all, companies producing electronic components. Buy only from trusted sellers, because there are Chinese counterfeits, especially often the LM317HV microcircuit, which is designed for an input voltage of up to 57 Volts. You can identify a fake microcircuit by its iron backing; in a fake, it has a lot of scratches and an unpleasant gray color, as well as incorrect markings. It should also be said that the microcircuit has protection against short circuits and overheating, but don’t count on them too much.

Do not forget that this (LM317T) integrated stabilizer is capable of dissipating power with a radiator only up to 20 Watts. The advantages of this common microcircuit are its low price, limitation of internal short circuit current, internal thermal protection

The scarf can be drawn with high quality even with an ordinary parchment marker, and then etched in a solution of copper sulfate/ferric chloride...

Photo of the finished board.