Adjustable stabilizers LM317 and LM337. Application features
LM317 Regulated Power Supply Diagram
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 microfarad * 43V
- C2 - ceramic capacitor 0.1 microfarad
- C3 - electrolytic capacitor 1 microfarad * 43V
- R1 - resistance 18K
- R2 - resistance 220 Ohm
- R3 - resistance 0.1 Ohm * 2W
- P1 - building 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 textolite, it is desirable to install a voltmeter on this panel. I haven't installed it because I haven't found the right one yet. I also installed clips for output wires on the front panel.
The input outlet was left to power the PSU itself. A printed circuit board made for surface mounting of a transistor and a stabilizer microcircuit. I fixed them on a common radiator through a rubber gasket. The radiator took a solid one (you can see it in the photo). It should be taken as large as possible - for good cooling. Still, 3 amps is a lot!
If the circuit needs a stabilizer for some non-standard voltage, then the best solution is to use the popular integrated LM317T stabilizer with the following characteristics:
- capable of operating in the range of output voltages from 1.2 to 37 V;
- output current can reach 1.5A;
- maximum power dissipation 20 W;
- built-in current limit, for short circuit protection;
- built-in overheating protection.
In the LM317T microcircuit, the switching circuit in the minimum version assumes the presence of two resistors, the resistance values \u200b\u200bof which determine the output voltage, the input and output capacitors.
The regulator has two important parameters: the reference voltage (Vref) and the current flowing from the trim pin (Iadj).
The value of the reference voltage can vary from instance to instance from 1.2 to 1.3 V, and the average is 1.25 V. The reference voltage is the voltage that the stabilizer microcircuit seeks to maintain on resistor R1. Thus, if the 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. It turns out that 1.25 V on R1 adds up with a drop on R2 and forms an output voltage.
But I would advise using the LM317T in the case of typical voltages, only when you urgently need to do something on your knee, and there is no more suitable chip like 7805 or 7812 at hand.
And here is the pinout of the LM317T:
- adjusting
- Day off
- Input
By the way, the domestic analogue of LM317 - KR142EN12A has the same switching circuit.
It is easy to make an adjustable power supply on this microcircuit: instead of a constant R2, put a variable one, add a network transformer and a diode bridge.
On the LM317, you can also make a soft start circuit: add a capacitor and a current amplifier on a bipolar pnp transistor.
The switching circuit for digital control of the output voltage is also not complicated. We count R2 on the maximum required voltage and in parallel add chains of a resistor and a transistor. 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 voltage, since only one resistor is needed. Iout \u003d Uop / R1.
For example, in this way we get a current regulator for LEDs from lm317t:
- for single-wool LEDs I = 350 mA, R1 = 3.6 Ohm, power not less than 0.5 W.
- for three-watt LEDs I \u003d 1 A, R1 \u003d 1.2 Ohm, with a power of at least 1.2 W.
Based on the stabilizer, it is easy to make a charger for 12 V batteries, that's what the datasheet suggests to us. With Rs you can set the current limit, and R1 and R2 define 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 pnp-structure bipolar transistor.
If you need to build a bipolar adjustable voltage stabilizer, then the LM317T analog will help us, but it works 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 starts to work well only when the difference between the output and output voltage exceeds 7V.
If the current does not exceed 100mA, then it is better to use low-drop ICs LP2950 and LP2951.
Powerful analogues 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 - 5A
The manufacturers of these stabilizers, in addition to increasing the output current, promise a reduced current of the control input to 50 μA and improved accuracy of the reference voltage.
But the switching circuits are suitable from LM317.
Vin (input voltage): 3-40 Volts
Vout (output voltage): 1.25-37 Volts
Output current: up to 1.5 Amps
Maximum power dissipation: 20 Watts
Formula for calculating the 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 AC voltage to DC thanks to a diode bridge of VD1-VD4 diodes, and then set the voltage you need within the allowable stabilizer integrated circuit with an accurate SP-3 trimmer resistor.
I took the old ones as rectifier diodes FR3002, which once upon a time fell out of the oldest computer of the 98th year. With impressive dimensions (DO-201AD case), their characteristics (Ureverse: 100 Volts; Idirect: 3 Amperes) are not impressive, but this is enough for me. For them, I even had to expand the holes in the board, it was painful for their conclusions to be thick (1.3mm). If you change the board a little in the leyot, you can immediately solder the finished diode bridge.
A radiator to remove heat from the 317 chip is required, it is even better to install a small fan. Also, at the junction of the substrate of the TO-220 package of the microcircuit with the heatsink, drip a little thermal paste. The degree of heating will depend on how much power the chip dissipates, as well as on the load itself.
microchip LM317T I did not install directly on the board, but brought three wires from it, with the help of which I connected this component to the rest. This was done so that the legs would not loosen and, as a result, would not be broken, because this part will be attached to the heat dissipator.
A trimmer resistor for the possibility of using the full voltage of the microcircuit, that is, adjustments from 1.25 and up to 37 Volts, is set with a maximum resistance of 3432 kOhm (the closest value in the store is 3.3 kOhm.). Recommended resistor type R2: subscript multiturn (3296).
The LM317T stabilizer chip itself and the like are produced by many, if not all electronic component companies. Buy only from trusted sellers, because there are Chinese fakes, especially the LM317HV chip, which is rated for input voltage up to 57 Volts. You can identify a fake microcircuit by its iron substrate; in a fake one, it has many scratches and an unpleasant gray color, as well as incorrect markings. It must also be said that the microcircuit has protection against short circuits, as well as overheating, but do not count on them much.
Do not forget that this (LM317T) integral stabilizer is capable of dissipating power with a radiator only up to 20 watts. The advantages of this common microcircuit are its low price, internal short circuit current limitation, internal thermal protection.
A handkerchief 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.
Quite often there is a need for a simple voltage regulator. This article provides a description and examples of the use of an inexpensive (LM317 price) integrated voltage regulator LM317.
The list of tasks to be solved by this stabilizer is quite extensive - this is the power supply of 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 with voltage regulation.
In practice, with the participation of the LM317, it is possible to build a voltage regulator for an arbitrary output voltage in the range of 3 ... 38 volts.
Specifications:
- Stabilizer output voltage: 1.2 ... 37 volts.
- Withstanding current up to 1.5 amps.
- Stabilization accuracy 0.1%.
- There is an internal protection against accidental short circuit.
- Excellent protection of the integral stabilizer from possible overheating.
Power dissipation and input voltage of the LM317 stabilizer
The voltage at the input of the stabilizer should not exceed 40 volts, and there is also one more condition - the minimum input voltage must exceed the desired output voltage by 2 volts.
The LM317 chip in the TO-220 package is able to operate stably at a maximum load current of up to 1.5 amperes. If you do not use a high-quality heat sink, then this value will be lower. The power released by the microcircuit during its operation can be approximately determined by multiplying the current at the output and the difference between the input and output potential.
The maximum allowable power dissipation without a heat sink is approximately 1.5 W at an ambient temperature of 30 degrees Celsius or less. With good heat dissipation from the LM317 case (no more than 60 gr.), the power dissipation can be 20 watts.
When placing a chip on a heatsink, it is necessary to isolate the chip body from the heatsink, for example, with a mica gasket. Also, for efficient heat dissipation, it is desirable to use heat-conducting paste.
Selection of resistance for the stabilizer LM317
For accurate operation of the microcircuit, the total value of the 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 is allowed (8 ... 10 mA).
The value of the variable resistance R2 is directly related to the output voltage range. Usually, its resistance should be approximately 10 ... 15% of the total resistance of the remaining resistors (R1 and R2), or you can choose its resistance experimentally.
The location of the resistors on the board can be arbitrary, but it is desirable for better stability to place them 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 chip to voltage changes, but also reduces the effect of electrical interference when the stabilizer board is placed near places with powerful electromagnetic radiation.
Recently, interest in current stabilizer circuits has grown significantly. And first of all, this is due to the leading positions of artificial lighting sources based on LEDs, for which a stable current supply is a vital point. The simplest, cheapest, but at the same time powerful and reliable current stabilizer can be built on the basis of one of the integrated circuits (IM): lm317, lm338 or lm350.
Datasheet for lm317, lm350, lm338
Before proceeding directly to the circuits, consider the features and technical characteristics of the above linear integrated stabilizers (LIS).
All three IMs have a similar architecture and are designed to build on their basis not complex current or voltage stabilizer circuits, including those used with LEDs. The differences between the microcircuits lie in the technical parameters, which are presented in the comparative table below.
| LM317 | LM350 | LM338 | |
|---|---|---|---|
| Adjustable output voltage range | 1.2…37V | 1.2…33V | 1.2…33V |
| Maximum current load | 1.5A | 3A | 5A |
| Maximum allowable input voltage | 40V | 35V | 35V |
| Indicator of possible stabilization error | ~0,1% | ~0,1% | ~0,1% |
| Maximum power dissipation* | 15-20W | 20-50W | 25-50W |
| Operating temperature range | 0° - 125°С | 0° - 125°С | 0° - 125°С |
| Datasheet | LM317.pdf | LM350.pdf | LM338.pdf |
* - depends on the IM manufacturer.
All three microcircuits have built-in protection against overheating, overload and possible short circuit.
Integrated stabilizers (ICs) are produced in a monolithic package of several options, the most common being the TO-220. The microcircuit has three outputs:
- ADJUST. Output for setting (adjusting) the output voltage. In the current stabilization mode, it is connected to the positive of the output contact.
- OUTPUT. Output with low internal resistance to form the output voltage.
- INPUT. Output for supply voltage.
Schemes and calculations
ICs are most widely used in LED power supplies. Consider the simplest current stabilizer (driver) circuit, consisting of only two components: a microcircuit and a resistor. 
The voltage of the power source is applied to the input of the IM, the control contact is connected to the output through a resistor (R), and the output contact of the microcircuit is connected to the anode of the LED.
If we consider the most popular IM, Lm317t, then the resistance of the resistor is calculated by the formula: R \u003d 1.25 / I 0 (1), where I 0 is the output current of the stabilizer, the value of which is regulated by the passport data on the LM317 and should be in the range of 0.01 -1.5 A. It follows that the resistance of the resistor can be in the range of 0.8-120 ohms. The power dissipated in the resistor is calculated by the formula: P R \u003d I 0 2 ×R (2). The inclusion and calculations of IM lm350, lm338 are completely similar.
The calculated data obtained for the resistor are rounded up, according to the nominal range.
Fixed resistors are manufactured with a small variation in resistance value, so it is not always possible to obtain the desired output current value. For this purpose, an additional tuning resistor of the appropriate power is installed in the circuit. 
This slightly increases the price of the regulator assembly, but ensures that the necessary current is received to power the LED. When the output current stabilizes more than 20% of the maximum value, a lot of heat is generated on the microcircuit, so it must be equipped with a radiator.