LM317: all about the adjustable linear voltage regulator

LM317

Un voltage regulator or voltage regulator it is a small electronic device to be able to make the voltage constant in a circuit. It is frequently seen in components such as power supplies and power adapters. In this case, the LM317 is a small adjustable linear voltage regulator encapsulated in a shield similar to what we saw in the case of transistors.

Muchos electronics or makers often use the LM317 frequently for some projects where you need to work with a stable voltage or where it transforms from one type of voltage to another, etc. In these cases, the destabilized voltage signal or the effects on the signal when switching from alternating current to direct current is not suitable for supplying direct current circuits if it is not previously treated with this type of device.

LM317

LM317 internal circuit diagram

El LM317 It is very popular with the adjustable linear voltage regulators out there. One of the best known manufacturers of this electronic device is TI (Texas Instruments). It is a fairly simple device, but very practical for circuits because it is capable of obtaining a non-regular voltage at its input and supplying a voltage in much more regular conditions at its output.

It is not the first of the adjustable sliders in history, in fact, it is one of the latest in a series of improvements to a series of sliders. It all started with the LM117, the first of all. Then would come the LM337 that I talk about in the last paragraph of this section and then the LM317 would follow, which has become the most popular of them all.

You can usually manage stresses of 1,2 to 37 volts, with currents of 1.5 A. All this in a very small size and with only three pins or pins. One of them is the input marked with the letters IN, another the output or OUT and finally the setting or ADJ. If we take the LM317 head-on, the center pin is the output. The sides will be ADJ (left) and IN (right).

If you are looking for the LM317 complement, that is, a voltage regulator device but for negative voltages, since the LM317 only works with positive ones, then you can opt for the LM337. That would be the right solution if you want to regulate negative voltages.

Related article:
2N2222 transistor: everything you need to know

Technical details and datasheet

LM317 Datasheet (Capture)

The LM317 has a series of outstanding technical characteristics on the table:

  • Voltage regulator type: adjustable
  • Voltages: from 1.25 to 37v
  • Output current: 1.5 A
  • Overheat protection
  • Package: It has different types of packaging, such as SOT-223, TO-220, and TO-263.
  • Voltage tolerance output 1%
  • La current limitation does not depend on temperature
  • Noise protection input (RR = 80dB)
  • Can work at high temperatures, up to 125ºC

You already know that all the complete technical details you can get in the datasheets provided by manufacturers. You may download the PDF for the LM317 from the official TI website from this link.

Example of use

power supply (circuit)

There multitude of practical circuits using an LM317, but perhaps one of the most striking when you study electronics is when they teach you how a standard power supply works, since all the operation is very good in a very practical and intuitive way.

I would like you to pay attention to the image in this section, it is about a basic circuit of a power supply. In it you will see that there are a series of stages that I am now going to detail, and in each one a small inserted graph that shows how the voltage signal passes through that part of the circuit:

  1. Transformer: at the beginning we have a transformer with two spirals marked as N1 and N2. What the transformer achieves is to convert an input voltage, for example the 220v alternating current that we have in a plug to which we connect the power supply. And that high AC voltage transforms it into a somewhat lower voltage, depending on the application. For example, you can transform those 220v to 12v to power an electronic device. You can check that the input Ve is an alternating high voltage signal and at the output of the transitor you also have an alternating current but with a lower voltage (V1).
  2. Diode bridge: then we see four diodes connected in a particular way. It is known as a diode bridge and that 12v alternating voltage will enter through the bridge to be rectified. If we look at the graph, we have gone from a sinusoidal AC signal to curves only of positive voltage, eliminating the negative part.
  3. Condenser: The capacitor smooths the output of the bridge signal, that is, those little jumps represented in the graph will be absorbed by the capacitor's capacity and then the voltage will gradually drop. The result is a line with some curves, but much smoother. It is becoming more like a completely straight line, that is, a direct current.
  4. Stabilizer: it is the last stage, and although it is called that, it is a voltage regulator like the LM317. Obtaining a completely corrected signal upon departure. That is to say, those small voltage jumps that the previous capacitor or stage gave, have now been completely smoothed out and it is a totally straight line. That is, we have a constant voltage of 12v in our case. Therefore, now we can say that we have a direct current.

This is how a power supply gets go from AC to DC, such as the one that a PC may have inside or such as mobile phone chargers, etc. I think it was the most graphic example to learn about what exactly the voltage regulator does, instead of explaining it in a theoretical way that is perhaps something more abstract and complicated to understand.

Therefore, in all those circuits in which a voltage needs to be stabilized and correct small signal flaws, you can always use a voltage regulator like the LM317. If you have an oscilloscope or software simulator at home, you could test the same circuit in the image and do tests at different points in the circuit to see how the signal passes from one state to another.

I hope this post will be of great help to you ... and the LM317 has no secrets for you now.


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  1.   Luis said

    Very well explained everything. Thank you very much, I think I will mount it soon. a cordial greeting