LM317T: power supply circuit for high-power, adjustable

On the microassembly LM317T , the power supply unit (PSU) scheme is simplified many times. First, there is the opportunity to make adjustments. Secondly, the stabilization of food is produced. And according to many radio amateurs, this microassembly at times exceeds domestic counterparts. In particular, its resource is very large, it can not be compared to any other element.

Power supply base - transformer

It is necessary to use a step-down transformer as a voltage converter. It can be taken from almost any home appliances - tape recorders, televisions, etc. You can also use transformers of the TVK-110 brand, which were installed in the block of personnel scanning of black and white TVs. True, they have an output voltage of only 9 V, and the current is rather small. And if you need to feed a powerful consumer, it clearly is not enough.

But if you want to make a powerful PSU, then it's more reasonable to use power transformers. Their power should be at least 40 watts. In order to make a power supply for the DAC on the microassembly of the LM317T, you will need an output voltage of 3.5-5 V. It is this value that must be maintained in the microcontroller power circuit . It is possible that the secondary winding will need to be slightly changed. Primary thus is not rewound, only its isolation (on necessity) is carried out.

Rectifier cascade

The rectifier block is an assembly of semiconductor diodes. There is nothing complicated in it, only it is necessary to decide what type of straightening should be used. The rectifier circuit can be:

• Half-wave;
• Full-wave;
• Roadway;
• With doubling, tripling, tension.

The latter is reasonable to use if, for example, you have 24 V at the transformer output, and you need to get 48 or 72. At the same time, the output current inevitably decreases, this should be taken into account. For a simple power supply, the rectifier bridge circuit is the most suitable. The microassembly LM317T used for power supply is not powerful. The reason for this - the power of the chip itself is only 2 watts. The bridge circuit also allows you to get rid of pulsations, and its efficiency is an order of magnitude higher (if compared with a half-wave circuit). It is allowed to use both diode assemblies and separate elements in the rectifier cascade.

Power Supply Enclosure

As a material for the case, it is more reasonable to use plastic. It is easy to handle, deformable when heated. In other words, it is easy to give any shape to the workpieces. And it does not take a lot of time to drill holes. But you can work a little and make a beautiful, reliable body of sheet aluminum. Of course, there will be more trouble with it, but the appearance will be terrific. After manufacturing the case from sheet aluminum, it can be carefully cleaned, primed and applied to several layers of paint and varnish.

In addition, you will immediately kill two birds with one stone - get a beautiful body and provide additional cooling to the microassembly. On LM317T power supply is built on the principle that stabilization is carried out with the allocation of a large amount of heat. For example, you have at the output of the rectifier 12 volts, and stabilization should give out 5 V. This difference, 7 volts, goes to the heating of the microassembly body. Consequently, it needs qualitative cooling. And the aluminum case will contribute to this. However, you can do more advanced - mount a thermal switch on the radiator, which will control the cooler.

Voltage stabilization circuit

So, you have a microassembly LM317T, the circuit of the power unit on it before your eyes, now you need to determine the purpose of its outputs. They have only three - the input (2), the output (3) and the mass (1). Turn the case face-to-face, numbering from left to right. That's all, now it remains to stabilize the voltage. And it is not difficult to do this if the rectifier unit and the transformer are already ready. As you know, the minus from the rectifier is fed to the first output of the assembly. With the plus of the rectifier, voltage is applied to the second terminal. From the third, the stabilized voltage is removed. And the input and output must be installed electrolytic capacitors with a capacity of 100 uF and 1000 uF, respectively. That's all, only at the output it is desirable to put a constant resistance (of the order of 2 kOhm), which will allow electrolytes to be discharged faster after shutdown.

The scheme of the power supply unit with the possibility of voltage regulation

Make an adjustable power supply on the LM317T is easy, it does not require special knowledge and skills. So, you already have a power supply with a stabilizer. Now you can slightly upgrade it to change the output voltage, depending on what you need. To do this, it is enough to disable the first output of the microassembly from the minus supply. On the output, consistently connect two resistors - a constant (nominal 240 Ohm) and an alternating (5 kΩ). At the point of their connection, the first microassembly pin is connected. Such simple manipulations allow making an adjustable power supply. And the maximum voltage applied to the input LM317T, can be 25 volts.

Additional Features

With the use of the microassembly LM317T, the power supply circuit becomes more functional. Of course, during the operation of the power supply, you will need to monitor the main parameters. For example, the current consumed or the output voltage (especially this is relevant for the circuit with the adjustment). Therefore, on the front panel you need to mount indicators. In addition, you need to know whether the power supply is connected to the network. The duty to notify you about inclusion in the grid is better placed on the LED. This design is quite reliable, only the power for it should be taken from the output of the rectifier, and not microassemblies.

To monitor current and voltage, you can use the dial indicators with a graduated scale. But in case you want to make a power supply that will not yield to laboratory power, you can also use LCD displays. True, to measure the current and voltage on the LM317T the power supply circuit is complicated, since it is necessary to use a microcontroller and a special driver - a buffer element. It allows you to connect the LCD display to the I / O ports of the controller.