Tutorial #Electrónica Básica. Cap 09. Reguladores de Tension


Hello friends, how are you? After this holiday period I return with this chapter nine of the tutorial And I’m going to dedicate it to the voltage regulators Here you have a voltage regulator This is for 12 volts, the truth, all of the 78xx series have the same shape, and the way to know what voltage is each is through the code they have printed Later we will see how to interpret that code And the operation of this voltage regulator is very simple I already said that this is 12 volts, and the only condition … …Is that we must put at its input a voltage of at least 14 volts, ie about two volts above its nominal value (12 volts) Let’s see these two power supplies, they are classic type. We want to get 12 volts DC from 220V AC What secondary voltage will we choose in this transformer? A 12-volt one to “match” the output? NO… When we rectify an alternating current we know that it rises in a square root of two factor (1.41), practically 50% more. Thus… These 12 volts are going to rise to 18 Better put a 9 volt secondary and will rise to about 12-13 volts, more in line with what we are looking for We measure with the tester here and we see that, indeed, there is that voltage, but here there is a trap The tester hardly consumes current when it is measuring on the voltage scale And that’s why the voltage appears in the tester. But if we put a charge to make it work, we will see that the voltage of 12 volts drops, and will drop more as the current increases That is, this power supply is NOT stabilized Many circuits demand to be fed by stable voltage, regardless of whether the consumption fluctuates And that is done by adding more components in this area of the source Components such as zener diodes, resistors, transistors … that made the power supply more complex, but in return the goal was achieved: stabilize the voltage Today (well, for a long time) it is not necessary to be an engineer. We will simply connect the source output, from the capacitor … …to this component, which will stabilize the voltage As you can see, these 12 volts are from the almost 18 volts of the secondary. It is necessary to supply the stabilizer at least 2 volts above the nominal voltage, which in this case is 12 volts And so, the voltage will remain stable, regardless of consumption And regardless of the source’s own oscillations. This, of course, is up to a limit If the output consumes so much current that a short-circuit condition is created, the protection mechanism of the regulator will come into operation And the voltage will drop to practically zero, which is normal. These voltage regulators we just saw are the 78xx and 79xx family 78xx and 79xx are non-adjustable regulators, they offer a fixed output voltage. And there are two types We have the 78xx family and the 79xx family, the 78xx are for positive voltages, and the 79xx for negative voltages For example, in this power supply we are acting on the positive line, therefore, we must put a 78xx In contrast, in this power supply is acting on the negative line, and does not serve a 78xx, you have to put a 79xx In this symmetrical power supply we have a positive, a negative, and a zero or center line Well, we put a 78xx on the positive, and a 79xx on the negative Here we have a drawing of how these components are, with the arrangement of their terminals Be careful with the detail that 78xx and 79xx do not have the same order of terminals In the case of the 78xx we have: input, mass (common) and output. In the case of the 79xx is: mass (common), input and output. And this is the symbol that is usually used in the schemes to represent these regulators And for how many voltage values are these regulators made? For many values. Here is the list: From 5 volts to 24, and separated by very little, so we can find a value that will serve us. I have found that some manufacturers make the regulators for 3.3 and 27 volts, but I do not find them in any store, so the values in this list are the usual ones It is very important the detail that I said before For each regulator, make sure that the voltage at the input is at least two volts above the rated voltage For example, in this case we have 12 volts at the output, we are using a 7812. In this input we should have a minimum of 14 volts for the regulator to work correctly If the regulator were a 7805 (5 volts at its output), we would need at least 7 volts here There is a limitation on the input voltage. It can not be greater than 35 volts, except for the 24 volt regulator, which admits a maximum of 40 volts at the input. Regarding the performances, we have already talked about the voltages, it is now time to talk about the current The best way to know a component data is to consult the datasheet I have put in my blog a couple of links to those spec sheets, one for the 78xx and one for the 79xx In advance, I tell you that there are at least three options If the code is 78xx, ensure at least one ampere and a half of maximum current If the code has an “S” inserted, it will be two amps And if the code has a “T” inserted, we go to 3 amps I recommend giving these regulators a heatsink to prevent them from being broken by excess heat We have to put thermal paste and with screw and nut we fix the heatsink to the regulator Let’s try these regulators. In this protoboard there is a 7805 and I will feed it with a variable voltage that I will bring from a power supply I will apply from 0 to 20 volts This tester is connected to the input, so it will measure from 0 to 20 volts And this tester is measuring the output, so when we reach 5 volts should not increase the voltage, even if the input voltage increases up to 20 volts or more I turn on the power supply and I will go up the voltage … Correct: Almost 20 volts at the input, but a very constant voltage of 5 volts at the output. And so far what is basic as regards fixed voltage regulators. There are also variable or adjustable regulators We just saw the 78xx and the 79xx To obtain different voltages we had to change the component In the case of variable regulators there is no change of component. A potentiometer is coupled and voltages can be obtained in a wide range, from almost zero volts to tens of volts …which make them quite more versatile One of the best known is the LM317 Let’s see how to take advantage of this regulator In this regulator there is also a version to work with positive voltages (LM317) as shown in this diagram And there is another version, with another name (LM337), to work with negative tensions. The rest of the circuit is the same in both The most common is the 317 for positive voltages This potentiometer applied to pin # 1 of the LM317 allows us to obtain a voltage between 1.2 and 37 volts The maximum current depends on the version of the regulator (there are several) but it is normal to be 1.5 amps Here you have an LM317 regulator, with the same encapsulation as the 78xx and 79xx The LM317 voltage regulator can be present in other packages such as tiny SMD and larger TO-3 The arrangement of the terminals changes depending on whether it is 317 (positive) or 337 (negative) The same thing happened with the 78xx and 79xx, terminals 2 and 3 are interchanged (here is the function of those terminals). Beware of this detail Let’s try the LM317 on the protoboard And this schematic is of the datasheet, the manufacturer recommends to put a capacitor of 100 nf to the input … Also recommends an electrolytic capacitor between 1 and 10 μF at the output. Of course, respecting the polarity of the capacitor And these two resistors (this is a potentiometer) serve to regulate (at terminal #1) the output voltage. Let’s try it on protoboard Here we have the circuit, the voltage regulator LM317 The adjustable resistor, the two capacitors, the resistor that is connected to terminal 2 and 1 and some jumper The wires for the input current are these two, negative and positive, connected to a power supply This power supply is now supplying about 12 volts Set the potentiometer to minimum Let’s see what voltage reading we get at the output. This wire is the negative of the output, common with the input. And the positive output, terminal 2 of the LM317, is this wire. We connected it to the tester We actually get the minimum voltage value that the manufacturer tells us on the datasheet: 1.2 volts, despite having in the input… 12 volts If we now act on the potentiometer, the voltage will rise 2, .. 3 volts … and so up to a maximum … … and we see that we lose a volt. It is normal. We have 11 volts at the output while in the input there are 12 Now let’s increase the input voltage from 12 volts to 20 volts. It’s the maximum I get from the power supply It happens the same as before, at the output we have a volt less than the input. We have 18.55 volts at the output We have in the input 19.90 volts, yes … And … with this input voltage of 20 volts can we reach the minimum of 1.2 volts as before? Yes, and accurately: 1.24 volts … If we want to obtain, for example, 5 volts to charge a mobile phone, adjust the potentiometer until obtaining 5 volts, or a few tenths of volt more You have to adjust very slowly … 5.09 volts, this would be a correct voltage This voltage regulator can deliver up to 1.5 amps, there are more (and for less) current. The usual are 1.5 amps If we intend to consume enough current, it is advisable to put the LM317 a heatsink to avoid failures due to excess heat With this we arrive at the end of the subject ” adjustable voltage regulators” We have focused attention on the LM317 regulator and its negative counterpart LM337 There are many more types of adjustable regulators. Sometimes a regulator with other characteristics is required either in voltage or current. How do we ask for such a regulator? What code does the regulator have? It’s the same with me. We can not know by heart all the components A trick is to use a search engine on the web, I give you an example that happened to me a week ago I needed an NPN transistor that would support at least 25 amps, and I had no idea what transistor code to use to order it in the store Well, we enter the search engine, we write: Transistor, NPN 25 A … and you get a listing And I chose one of them, type TIP35C I went to the datasheet to consult that transistor and, indeed, it was what I was looking for. I go to the store, I ask for the transistor, and they serve it to me without problems This is a good trick to find components if you do not know your code but you know the parameters that must have (tensions, currents …) Well, let us now turn to the corner of theory with an interesting phenomenon: The Doppler effect What you have just seen, or rather “listening”, is a clear manifestation of the Doppler effect As the car approached the one who was recording the video, the sound of the cláxon increased its frequency And when the car reaches and exceeds that observer, the sound has an increasingly lower tone, the frequency decreases However, the occupants of that car hear the sound of the horn with the same tone at all times And if the car is stationary, even if it is far away, the sound remains constant So it is clear that the speed of the car is related to this phenomenon. The question is: The speed of the car and the speed of sound … Do they add up? The answer is no When the car approaches, the speed of sound remains the same, so it happens that there will be more waves per unit time, which means a higher frequency. When the car goes away, the opposite happens It is not subtracted to the speed of sound the speed of the car There will simply be fewer waves per unit time, which is equivalent to a lower frequency This also happens with electromagnetic waves, such as light and radio waves If an object approaches towards us at a very high speed, the light emitted by that object turns towards the blue, that is to say, a greater frequency … and when that object reaches us and begins to move away, its light moves towards the red, that is, at a lower frequency The Doppler effect is used in daily life for example in traffic radars, which are used to detect excess speed The radar emits a microwave beam and directs it to the vehicle whose speed is to be measured And the vehicle reflects radar waves with the “printed” Doppler effect on these waves The radar has only to measure the frequency difference between the emitted wave and the reflected wave by the vehicle, and with mathematical calculations the speed of the vehicle is deduced Of course, these radars must be very well calibrated and preserved, because otherwise, the readings are real nonsense, since here we are working with very small magnitudes Regarding the Doppler effect and the light, it is not necessary that an object moves at speeds close to that of the light so that it takes place to the red shift if it moves away, or to the blue if it approaches For some time there has been a means to detect this phenomenon of red or blue displacement for relatively slow objects, for example, the displacement of galaxies This phenomenon of red or blue displacement can be measured and this led to an astounding discovery a little less than a century ago And is that all galaxies are moving away from us There are billions of galaxies, and practically none comes close to us This discovery at first caused bewilderment, but it soon became apparent that the universe was expanding, as if it were the result of an explosion Therefore, if we “rewind” backwards in time, something that expands, it turns out that it is approaching This can be simulated with the simile of the globe, this would be the universe … I have drawn with a marker some points, would be the galaxies Now, as time goes on, the balloon swells, and we will see how these points move away from each other. Each point can “think” that it is special, but not: Simply the balloon expands Now the points are more separated from each other than before. If I go back in time, all points will approach each other And this is what led to the big bang or theory of the big explosion All matter was concentrated in one point, a point that mathematicians call “singularity” That point exploded and gave rise to the universe we know today 15,000,000,000 years after that explosion (the estimated age of the universe), and still continues to expand It is fascinating that a physical effect as simple as the Doppler effect can serve humanity to clarify the origin of the universe

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