Adjustable stabilizers LM317 and LM337. Features of application. Integral stabilizer LM317 Application of lm317t

A high-quality power supply with adjustable output voltage is the dream of every beginning 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 one. 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.

The LM317 is more suitable than ever for the design of simple, regulated sources and electronics with a variety of output characteristics, both variable output voltage and fixed voltage output. electric shock loads.

To facilitate the calculation of the required output parameters, there is a specialized LM317 calculator, which can be downloaded from the link at the end of the article along with the LM317 datasheet.

Technical characteristics of the stabilizer LM317:

• Providing output voltage from 1.2 to 37 V.
• Load current up to 1.5 A.
• Availability of protection against possible short circuit.
• Reliable protection of the microcircuit from overheating.
• Output voltage error 0.1%.

This inexpensive integrated circuit is available in TO-220, ISOWATT220, TO-3, and also D2PAK packages.

Purpose of the microcircuit pins:

Online calculator LM317

Below is an online calculator for calculating a voltage stabilizer based on LM317. In the first case, based on the required output voltage and the resistance of resistor R1, resistor R2 is calculated. In the second case, knowing the resistances of both resistors (R1 and R2), you can calculate the voltage at the output of the stabilizer.

For a calculator for calculating the current stabilizer on LM317, see.

Examples of application of the LM317 stabilizer (connection circuits)

Current stabilizer

The current stabilizer can be used in circuits of various battery chargers or regulated power supplies. The standard charger circuit is shown below.

This connection circuit uses a direct current charging method. As can be seen from the diagram, the charge current depends on the resistance of resistor R1. The value of this resistance ranges from 0.8 Ohm to 120 Ohm, which corresponds to a charging current from 10 mA to 1.56 A:

5 Volt power supply with electronic switching

Below is a diagram of a 15 volt power supply with soft start. The required smoothness of switching on the stabilizer is set by the capacitance of capacitor C2:

Switching circuit with adjustable output voltage

LM317 regulated power supply circuit

List of circuit elements:

• Stabilizer LM317
• T1 - transistor KT819G
• Tr1 - power transformer
• F1 - fuse 0.5A 250V
• Br1 - diode bridge
• D1 - diode 1N5400
  
• LED1 - LED of any color
  
• C1 - electrolytic capacitor 3300 uF*43V
  
• C2 - ceramic capacitor 0.1 uF
  
• C3 - electrolytic capacitor 1 µF * 43V
  
• R1 - resistance 18K
  
• R2 - resistance 220 Ohm
  
• R3 - resistance 0.1 Ohm*2W
  
• P1 - construction resistance 4.7K

Pinout of the microcircuit and transistor

The case was taken from the computer's power supply. The front panel is made of PCB, it is advisable to install a voltmeter on this panel. I haven't installed it because I haven't found a suitable one yet. I also installed clamps for the output wires on the front panel.

I left the input socket to power the power supply itself. A printed circuit board made for surface-mounted mounting of a transistor and a stabilizer chip. They were secured to a common radiator through a rubber gasket. The radiator was solid (you can see it in the photo). It needs to be taken as large as possible - for good cooling. Still, 3 amperes is a lot!

If the circuit requires a stabilizer for some non-standard voltage, then an excellent solution is to use the popular integrated stabilizer LM317T with the following characteristics:

• capable of operating in the output voltage range from 1.2 to 37 V;
• output current can reach 1.5 A;
• maximum power dissipation 20 W;
• built-in current limitation for short circuit protection;
• built-in overheating protection.

Description

For the LM317T microcircuit, the minimum connection circuit assumes the presence of two resistors, the resistance values ​​of which determine the output voltage, an input and output capacitor.

The stabilizer has two important parameters: the reference voltage (Vref) and the current flowing from the adjustment pin (Iadj).
The value of the reference voltage can vary from instance to instance from 1.2 to 1.3 V, and on average is 1.25 V. The reference voltage is the voltage that the stabilizer chip strives to maintain across resistor R1. Thus, if resistor R2 is closed, then the output of the circuit will be 1.25 V, and the greater the voltage drop across R2, the greater the output voltage will be. It turns out that 1.25 V on R1 adds up with the drop on R2 and forms the output voltage.

The first time I calculated the divider for the microcircuit using the formula from the LM317T datasheet, I was given a current of 1 mA, and then for a very long time I wondered why the voltage and real voltage were different. And since then I have been asking R1 and counting according to the formula:
R2=R1*((Uout/Uop)-1).
I test in real conditions and clarify the values ​​of resistances R1 and R2.
Let's see what they should be for the widespread voltages of 5 and 12 V.

But I would advise using the LM317T in the case of typical voltages, only when you urgently need to do something on your knees, and a more suitable microcircuit like 7805 or 7812 is not at hand.

And here is the pinout location of the LM317T:

1. Adjusting
2. Day off
3. Input

By the way, the domestic analogue of LM317 - KR142EN12A - has exactly the same connection circuit.

It’s easy to make an adjustable power supply on this chip: replace the constant R2 with a variable one, add a network transformer and a diode bridge.

You can also make a soft start circuit on LM317: add a capacitor and a current amplifier on a bipolar pnp transistor.

The connection circuit for digital control of the output voltage is also not complicated. We calculate R2 for the maximum required voltage and add chains of a resistor and transistor in parallel. Turning on the transistor will add, in parallel to the conductivity of the main resistor, the conductivity of the additional one. And the output voltage will decrease.

The current stabilizer circuit is even simpler than the voltage stabilizer, since only one resistor is needed. Iout = Uop/R1.
For example, in this way we get a current stabilizer for LEDs from lm317t:

• for single-watt LEDs I = 350 mA, R1 = 3.6 Ohm, power of at least 0.5 W.
• for three-watt LEDs I = 1 A, R1 = 1.2 Ohm, power of at least 1.2 W.

It’s easy to make a charger for 12 V batteries based on the stabilizer, that’s what the datasheet offers us. Rs can be used to set the current limit, while R1 and R2 determine the voltage limit.

If the circuit needs to stabilize voltages at currents of more than 1.5 A, then you can still use the LM317T, but in conjunction with a powerful bipolar transistor of the pnp structure.
If we need to build a bipolar adjustable voltage stabilizer, then an analogue of the LM317T will help us, but working in the negative arm of the stabilizer - LM337T.

But this chip also has limitations. It is not a low-dropout regulator; on the contrary, it only starts to work well when the difference between the output and output voltage exceeds 7 V.

If the current does not exceed 100mA, then it is better to use low-dropout ICs LP2950 and LP2951.

Powerful analogs of LM317T - LM350 and LM338

If the output current of 1.5 A is not enough, then you can use:

• LM350AT, LM350T - 3 A and 25 W (TO-220 package)
• LM350K - 3 A and 30 W (TO-3 package)
• LM338T, LM338K - 5 A

The manufacturers of these stabilizers, in addition to increasing the output current, promise a reduced control input current to 50 μA and improved accuracy of the reference voltage.
But the switching circuits are suitable for LM317.