In previous articles, various issues of connecting LEDs have been described. But you can’t write everything in one article, so you have to continue this topic. Here we will talk about various ways turning on the LEDs.

As stated in the referenced articles, i.e. the current through it must be limited by a resistor. How to calculate this resistor has already been told, we will not repeat here, but we will give the formula, just in case, again.

Picture 1.

Here Upit. - supply voltage, Upad. - voltage drop across the LED, R - resistance of the limiting resistor, I - current through the LED.

However, despite all the theory, the Chinese industry produces all kinds of souvenirs, key rings, lighters, in which the LED is turned on without a limiting resistor: just two or three disk batteries and one LED. In this case, the current is limited by the internal resistance of the battery, which is simply not enough power to burn the LED.

But here, in addition to burnout, there is another unpleasant property - the degradation of LEDs, which is most characteristic of white and blue LEDs: after a while, the brightness of the glow becomes quite insignificant, although the current through the LED flows quite sufficient, at the nominal level.

It cannot be said that it does not shine at all, the glow is barely noticeable, but this is no longer a flashlight. If at rated current degradation occurs no earlier than after a year of continuous glow, then at an overestimated current, this phenomenon can be expected in half an hour. Such an inclusion of the LED should be called bad.

Such a scheme can only be explained by the desire to save on one resistor, solder, and labor costs, which, with a mass production scale, is apparently justified. In addition, a lighter or a keychain is a disposable, cheap thing: the gas ran out or the battery ran out - the souvenir was simply thrown away.

Figure 2. The scheme is bad, but it is used quite often.

Very interesting things happen (of course, by accident) if, according to this scheme, the LED is connected to a power supply with an output voltage of 12V and a current of at least 3A: a dazzling flash occurs, a rather loud bang, smoke is heard, and a suffocating smell remains. So I remember this parable: “Is it possible to look at the Sun through a telescope? Yes, but only twice. Once with the left eye, once with the right. By the way, connecting an LED without a limiting resistor is the most common mistake for beginners, and I would like to warn about it.

To correct this situation, to extend the life of the LED, the circuit should be slightly changed.

Figure 3. Good scheme, correct.

It is this scheme that should be considered good or correct. To check whether the value of the resistor R1 is correctly indicated, you can use the formula shown in Figure 1. We will assume that the voltage drop across the LED is 2V, current 20mA, supply voltage 3V due to the use of two AA batteries.

In general, there is no need to strive to limit the current at the level of the maximum allowable 20mA, you can power the LED with a lower current, well, at least 15 ... 18 milliamps. In this case, there will be a very slight decrease in brightness, which the human eye, due to the characteristics of the device, will not notice at all, but the service life of the LED will increase significantly.

Another example of poor switching on of LEDs can be found in various flashlights, which are already more powerful than key fobs and lighters. In this case, a certain number of LEDs, sometimes quite large, are simply connected in parallel, and also without a limiting resistor, which again acts as the internal resistance of the battery. Such flashlights quite often get into repairs precisely because of the burnout of the LEDs.

Figure 4. A very bad switching circuit.

It would seem that the circuit shown in Figure 5 can correct the situation. Just one resistor, and things seemed to be on the mend.

Figure 5. This is already a little better.

But such an inclusion will not help much. The fact is that in nature it is simply not possible to find two identical semiconductor devices. That is why, for example, transistors of the same type have a different gain, even if they are from the same production batch. Thyristors and triacs are also different. Some open easily, while others are so hard that they have to be abandoned. The same can be said about LEDs - it is simply impossible to find two absolutely identical, especially three or a whole bunch.

Note on the topic. In the DataSheet for the SMD-5050 LED assembly (three independent LEDs in one housing), the inclusion shown in Figure 5 is not recommended. Like, due to the spread of the parameters of individual LEDs, a difference in their glow can be noticeable. And it would seem, in one case!

Of course, LEDs do not have any gain, but there is such an important parameter as forward voltage drop. And even if the LEDs are taken from the same technological batch, from the same package, then there simply will not be two identical ones in it. Therefore, the current for all LEDs will be different. The LED that has the most current, and sooner or later exceeds the rated current, will burn out before anyone else.

In connection with this unfortunate event, all possible current will go through the two surviving LEDs, naturally exceeding the nominal one. After all, the resistor was calculated “for three”, for three LEDs. Over current will also cause increased heating of the LED crystals, and the one that turns out to be “weaker” also burns out. The last LED also has no choice but to follow the example of his comrades. Such is the chain reaction.

In this case, the word "burn" means simply breaking the circuit. But it may happen that one of the LEDs will get an elementary short circuit, shunting the other two LEDs. Naturally, they will definitely go out, although they will remain alive. The resistor with such a malfunction will heat up intensely and in the end, perhaps, will burn out.

To prevent this from happening, the circuit needs to be slightly changed: for each LED, install its own resistor, which is shown in Figure 6.

Figure 6. And this is how LEDs will last a very long time.

Here everything is as required, everything is according to the rules of circuitry: the current of each LED will be limited by its resistor. In such a circuit, the currents through the LEDs are independent of each other.

But even this inclusion does not cause much enthusiasm, since the number of resistors is equal to the number of LEDs. I wish there were more LEDs and fewer resistors. How to be?

The way out of this situation is quite simple. Each LED should be replaced with a string of LEDs connected in series, as shown in Figure 7.

Figure 7 Parallel connection garlands.

The price for such an improvement will be an increase in the supply voltage. If only three volts are enough for one LED, then even two LEDs connected in series cannot be lit from such a voltage. So what voltage is needed to turn on a string of LEDs? Or in other words, how many LEDs can be connected to a power supply with a voltage of, for example, 12V?

Comment. The name "garland" hereinafter should be understood not only as a Christmas tree decoration, but also as any LED lighting device in which the LEDs are connected in series or in parallel. The main thing is that the LED is not alone. A garland, it is also a garland in Africa!

To get the answer to this question, it is enough to simply divide the supply voltage by the voltage drop across the LED. In most cases, this voltage is assumed to be 2V in calculations. Then it turns out 12/2=6. But we must not forget that some part of the voltage must remain for the quenching resistor, at least 2 volts.

It turns out that only 10V remains for the LEDs, and the number of LEDs will become 10/2=5. In this state of affairs, in order to obtain a current of 20mA, the limiting resistor must have a rating of 2V / 20mA \u003d 100Ω. The power of the resistor in this case will be P=U*I=2V*20mA=40mW.

Such a calculation is quite fair if the direct voltage of the LEDs in the garland, as indicated, is 2V. It is this value that is often taken in calculations as some average. But in fact, this voltage depends on the type of LEDs, on the color of the glow. Therefore, when calculating garlands, one should focus on the type of LEDs. Voltage drops for LEDs different types are given in the table shown in Figure 8.

Figure 8. Voltage drop across LEDs of different colors.

Thus, with a power supply voltage of 12V, minus the voltage drop across the current-limiting resistor, a total of 10 / 3.7 = 2.7027 white LEDs can be connected. But you can’t cut a piece from an LED, so you can only connect two LEDs. This result is obtained if we take the maximum value of the voltage drop from the table.

If we substitute 3V into the calculation, then it is quite obvious that it is possible to connect three LEDs. In this case, each time you have to painstakingly recalculate the resistance of the limiting resistor. If real LEDs turn out to have a voltage drop of 3.7V, or maybe higher, three LEDs may not light up. So it's better to stop at two.

It doesn’t matter in principle what color the LEDs will be, it’s just that when calculating, you will have to take into account different voltage drops depending on the color of the LED glow. The main thing is that they are designed for one current. It is impossible to assemble a serial garland of LEDs, some of which are with a current of 20mA, and the other part of 10 milliamps.

It is clear that at a current of 20mA, LEDs with a rated current of 10mA will simply burn out. If, however, the current is limited to 10mA, then 20 milliamps will not light up brightly enough, just like in a switch with an LED: you can see it at night, but not during the day.

To make life easier for themselves, radio amateurs develop various calculator programs that facilitate all kinds of routine calculations. For example, programs for calculating inductances, filters of various types, current stabilizers. There is such a program for calculating LED garlands. A screenshot of such a program is shown in Figure 9.

Figure 9. Screenshot of the program "Calculation_of_resistance_of_resistor__Ledz_".

The program works without installation in the system, you just need to download and use it. Everything is so simple and clear that no explanation for the screenshot is required at all. Naturally, all LEDs must be of the same color and with the same current.

Limiting resistors are, of course, good. But only when it is known that this garland will be powered by a constant voltage of 12V, and the current through the LEDs will not exceed the calculated value. But what if there is simply no source with a voltage of 12V?

Such a situation may arise, for example, in a truck with an on-board network voltage of 24V. A current stabilizer will help to get out of such a crisis situation, for example, “SSC0018 - Adjustable stabilizer current 20..600mA". Its appearance is shown in Figure 10. Such a device can be bought in online stores. The issue price is 140 ... 300 rubles: it all depends on the imagination and impudence of the seller.

Figure 10. Adjustable Current Stabilizer SSC0018

Specifications of the stabilizer are shown in Figure 11.

Figure 11. SSC0018 Current Stabilizer Specifications

The current stabilizer SSC0018 was originally designed for use in LED lamps, but can also be used to charge small batteries. Using the SSC0018 is quite simple.

The load resistance at the output of the current stabilizer can be zero, you can simply short-circuit the output terminals. After all, stabilizers and current sources are not afraid of short circuits. In this case, the output current will be nominal. If you set 20mA, then that's how much it will be.

From the foregoing, we can conclude that a milliammeter can be "directly" connected to the output of the current stabilizer direct current. Such a connection should be started from the largest measurement limit, because no one knows what current is adjusted there. Then, by simply rotating the tuning resistor, set the required current. In this case, of course, do not forget to connect the current stabilizer SSC0018 to the power supply. Figure 12 shows the wiring diagram of the SSC0018 for powering LEDs connected in parallel.

Figure 12. Wiring to power LEDs connected in parallel

Here everything is clear from the diagram. For four LEDs with a current consumption of 20mA for each, a current of 80mA must be set at the output of the stabilizer. At the same time, at the input of the SSC0018 stabilizer, a voltage slightly greater than the voltage drop across one LED, as mentioned above, will be required. Of course, a higher voltage is also suitable, but this will only lead to additional heating of the stabilizer microcircuit.

Comment. If, in order to limit the current with a resistor, the voltage of the power source must exceed the total voltage on the LEDs slightly, only two volts, then for the normal operation of the SSC0018 current regulator, this excess should be slightly higher. No less than 3 ... 4V, otherwise the regulating element of the stabilizer simply will not open.

Figure 13 shows the connection of the SSC0018 stabilizer when using a garland of several series-connected LEDs.

Figure 13. Powering a serial string through the SSC0018 stabilizer

The figure is taken from the technical documentation, so let's try to calculate the number of LEDs in the garland and constant pressure required from the power supply.

The current indicated in the diagram, 350mA, allows us to conclude that the garland is assembled from powerful white LEDs, because, as mentioned a little higher, the main purpose of the SSC0018 stabilizer is lighting sources. The voltage drop on the white LED is in the range of 3 ... 3.7V. For calculation, you should take the maximum value of 3.7V.

The maximum input voltage of the SSC0018 is 50V. We subtract from this value 5V, necessary for the operation of the stabilizer itself, 45V remains. This voltage can "light up" 45/3.7=12.1621621... LEDs. Obviously, this should be rounded up to 12.

The number of LEDs may be less. Then the input voltage will have to be reduced (while the output current will not change, it will remain 350mA as it was adjusted), why should 50V be applied to 3 LEDs, even powerful ones? Such a mockery can end badly, because powerful LEDs are by no means cheap. What voltage is required to connect three powerful LEDs those who wish, and they will always be found, can count themselves.

Adjustable current stabilizer SSC0018 device is quite good. But the question is, is it always necessary? And the price of the device is somewhat embarrassing. What could be the way out of this situation? Everything is very simple. An excellent current stabilizer is obtained from integrated voltage regulators such as the 78XX or LM317 series.

To create such a current stabilizer based on a voltage stabilizer, you need only 2 parts. Actually, the stabilizer itself and one single resistor, the resistance and power of which will be calculated by the StabDesign program, a screenshot of which is shown in Figure 14.

Figure 14. Calculation of the current stabilizer using the program StabDesign.

The program does not require special explanations. In the Type drop-down menu, the type of stabilizer is selected, the required current is set in the In line and the Calculate button is pressed. The result is the resistance of the resistor R1 and its power. In the figure, the calculation was carried out for a current of 20mA. This is for the case when the LEDs are connected in series. For a parallel connection, the current is calculated in the same way as shown in Figure 12.

The LED garland is connected instead of the resistor Rn, which symbolizes the load of the current stabilizer. It is even possible to connect just one LED. In this case, the cathode is connected to a common wire, and the anode to the resistor R1.

The input voltage of the considered current stabilizer is in the range of 15 ... 39V, since the 7812 stabilizer with a stabilization voltage of 12V is used.

It would seem that this story about LEDs can be completed. But there are also LED strips, which will be discussed in the next article.

Boris Aladyshkin

P.S. If the article "Good and bad LED switching schemes" was useful for you, then click on the social media icon and share es link to article from with your friends!

Since the LED is a semiconductor device, the polarity must be observed when connected to the circuit. The LED has two outputs, one of which is the cathode ("minus"), and the other is the anode ("plus").

The LED will be on only when directly connected, as shown in the figure

When turned back on, the LED will not light up. Moreover, the failure of the LED is possible at low allowable values ​​of the reverse voltage.

The dependences of current on voltage for direct (blue curve) and reverse (red curve) inclusions are shown in the following figure. It is not difficult to determine that each voltage value corresponds to its own amount of current flowing through the diode. The higher the voltage, the higher the current value (and the higher the brightness). For each LED there are allowed values supply voltages Umax and Umaxarr (respectively for direct and reverse switching). When voltages above these values ​​are applied, an electrical breakdown occurs, as a result of which the LED fails. There is also a minimum value of the supply voltage Umin, at which the LED glows. The range of supply voltages between Umin and Umax is called the "working" zone, since this is where the operation of the LED is ensured.

1. There is one LED, how to connect it correctly in the simplest case?

In order to properly connect the LED in the simplest case, you need to connect it through a current-limiting resistor.

Example 1

There is an LED with an operating voltage of 3 volts and an operating current of 20 mA. It must be connected to a 5 volt source.

Calculate the resistance of the current limiting resistor

R = Uquenching / ILED
Uquenching = Upower - ULED
Usupply = 5 V
ULED = 3 V
ILED = 20mA = 0.02A
R \u003d (5-3) / 0.02 \u003d 100 Ohm \u003d 0.1 kOhm

That is, you need to take a resistor with a resistance of 100 ohms

P.S. You can use the on-line LED resistor calculator

2. How to connect multiple LEDs?

We connect several LEDs in series or in parallel, calculating the required resistance.

Example 1

There are LEDs with an operating voltage of 3 volts and an operating current of 20 mA. It is necessary to connect 3 LEDs to a source of 15 volts.

We make a calculation: 3 LEDs for 3 volts \u003d 9 volts, that is, a 15 volt source is enough to turn on the LEDs in series

The calculation is similar to the previous example.

R = Uquenching / ILED

Usupply = 15 V
ULED = 3 V
ILED = 20mA = 0.02A
R \u003d (15-3 * 3) / 0.02 \u003d 300 Ohm \u003d 0.3 kOhm

Example 2

Let there be LEDs with an operating voltage of 3 volts and an operating current of 20 mA. It is necessary to connect 4 LEDs to a source of 7 volts

We make a calculation: 4 LEDs for 3 volts \u003d 12 volts, which means we do not have enough voltage to connect the LEDs in series, so we will connect them in series-parallel. Let's divide them into two groups of 2 LEDs. Now we need to calculate the current-limiting resistors. Similarly to the previous paragraphs, we calculate the current-limiting resistors for each branch.

R = Uquenching / ILED
Uquenching = Upower - N * ULED
Usupply = 7 V
ULED = 3 V
ILED = 20mA = 0.02A
R \u003d (7-2 * 3) / 0.02 \u003d 50 Ohm \u003d 0.05 kOhm

Since the LEDs in the branches have the same parameters, the resistances in the branches are the same.

Example 3

If there are LEDs of different brands, then we combine them in such a way that each branch has LEDs of only ONE type (or with the same operating current). In this case, it is not necessary to observe the same voltages, because we calculate our own resistance for each branch.

For example, there are 5 different LEDs:
1st red voltage 3 volts 20 mA
2nd green voltage 2.5 volts 20 mA
3rd blue voltage 3 volts 50 mA
4th white voltage 2.7 volts 50 mA
5th yellow voltage 3.5 volts 30 mA

Since we divide the LEDs into groups by current
1) 1st and 2nd
2) 3rd and 4th
3) 5th

we calculate resistors for each branch:
R = Uquenching / ILED
Uquenching = Upower - (ULEDY + ULEDX + ...)
Usupply = 7 V
ULED1 = 3 V
ULED2 = 2.5 V
ILED = 20mA = 0.02A
R1 = (7-(3+2.5))/0.02 = 75 Ohm = 0.075 kOhm

likewise
R2 = 26 Ohm
R3 = 117 Ohm

Similarly, you can arrange any number of LEDs

IMPORTANT NOTE!!!

When calculating the current-limiting resistance, numerical values ​​\u200b\u200bof which are not in the standard series of resistances are obtained, THEREFORE, we select a resistor with a resistance slightly larger than calculated.

3. What happens if there is a voltage source with a voltage of 3 volts (or less) and an LED with an operating voltage of 3 volts?

It is acceptable (BUT NOT DESIRABLE) to include an LED in a circuit without current-limiting resistance. The disadvantages are obvious - the brightness depends on the supply voltage. It is better to use dc-dc converters (voltage boost converters).

4. Is it possible to turn on several LEDs with the same operating voltage of 3 volts in parallel to each other to a source of 3 volts (or less)? In the "Chinese" lanterns, this is exactly what is done.

Again, this is acceptable in amateur radio practice. The disadvantages of such an inclusion: since the LEDs have a certain spread in parameters, the following picture will be observed, some will glow brighter, while others will be dimmer, which is not aesthetic, which is what we observe in the flashlights above. It is better to use dc-dc converters (voltage boost converters).

A Light Emitting Diode (LED) is a semiconductor diode capable of emitting light when a voltage is applied to it in the forward direction. In fact, it is a diode that converts electrical energy into light. Depending on the material from which the LED is made, it can emit light of different wavelengths (different colors) and have different electrical characteristics.

LEDs are used in many areas of our lives as a means of display. visual information. For example, in the form of single emitters or in the form of structures of several LEDs - seven-segment indicators, LED matrices, clusters, and so on. Also in recent years, LEDs have been actively occupying the segment of lighting devices. They are used in car headlights, lanterns, lamps and chandeliers.

Designation of the LED on the diagram

In electrical diagrams, an LED is indicated by the symbol of a diode with two arrows. The arrows point away from the diode, symbolizing light emission. Do not confuse with the photodiode, which has arrows pointing towards it.

On domestic schemes, the letter designation of a single LED is HL.

Conclusions and marking of the LED

A standard single-color LED has two terminals - an anode and a cathode. It is possible to determine which of the conclusions is the anode visually. For wire lead LEDs, the anode is usually longer than the cathode.

SMD LEDs have the same pins, but on the reverse side there is usually a marking in the form of a triangle or something like the letter T. The anode is the pin that faces one side of the triangle or the top of the letter T.

If it is not possible to determine visually where which conclusions are, you can ring the LED. To do this, you need a power source or adapter capable of delivering a voltage of about 5 volts. We connect any output of the LED to the minus of the source, and connect the second to the positive terminal of the source through a resistance of 200 - 300 Ohms. If the LED is connected correctly, it will light up. Otherwise, swap the conclusions in places and repeat the procedure.

You can do without a resistor if you do not connect the positive terminal of the power source, but quickly "strike" it on the output of the LED. But in general, it is impossible to apply a large voltage to the LED without limiting the current - it can fail!

LED voltage

An LED emits light when a voltage is applied to it in the forward direction: positive to the anode and negative to the cathode.

The minimum voltage at which the LED starts to glow depends on its material. The table below shows the voltage values ​​of the LEDs at a test current of 20 mA and the colors they emit. I took this data from the Vishay LED catalog, various datasheets and Wikipedia.

The highest voltage is required for blue and white LEDs, and the smallest for infrared and red.

The radiation of an infrared LED is not visible to the human eye, so these LEDs are not used as indicators. They are used in various sensors, backlights of video cameras. By the way, if you power the infrared LED and look at it through the camera of a mobile phone, then its glow will be clearly visible.

The table shown gives approximate LED voltage values. Usually this is enough to turn it on. The exact forward voltage of a particular LED can be found in its datasheet under Electrical Characteristics. It indicates the nominal value of the forward voltage at a given LED current. For example, let's look at the datasheet for the red SMD LED from Kingbright.

Volt-ampere characteristic of the LED

The current-voltage characteristic of an LED shows the relationship between applied voltage and LED current. The figure below shows a direct branch of the characteristic from the same datasheet.

If the LED is connected to a power source (to the anode +, to the cathode -) and gradually increase the voltage on it from zero, then the LED current will change according to this graph. It shows that after passing the "bend" point, the current through the LED will increase sharply with small changes in voltage. This is exactly the reason why the LED cannot be connected to any power source without a resistor, unlike an incandescent light bulb.

The higher the current, the brighter the LED glows. However, it is naturally impossible to increase the LED current to infinity. At high current The LED will overheat and burn out. By the way, if you immediately apply to the LED high voltage he can even slap like a weak firecracker!

Other characteristics of the LED

What other characteristics of the LED are of interest from the point of view of practical use?

Maximum power dissipation, maximum direct and pulse forward currents, and maximum reverse voltage. These characteristics show the limit values ​​​​of voltages and currents that should not be exceeded. They are described in the datasheet in the Absolute Maximum Ratings section.

If you apply voltage to the LED in the opposite direction, the LED will not light up, and in general it may fail. The fact is that with reverse voltage, a breakdown can occur, as a result of which reverse current LED will rise sharply. And if the power allocated to the LED (reverse current * to reverse voltage) exceeds the allowable one, it will burn out. In some datasheets, the reverse branch of the current-voltage characteristic is additionally given, from which it is clear at what voltage breakdown occurs.

Radiation intensity (light intensity)

Roughly speaking, this is a characteristic that determines the brightness of the LED glow at a given test current (usually 20 mA). It is designated - Iv, and is measured in microcandelas (mcd). The brighter the LED, the higher the Iv value. The scientific definition of light intensity is on Wikipedia.

Also of interest is the graph of the relative intensity of the LED radiation from the direct current. For some LEDs, for example, as the current increases, the radiation intensity grows less and less. The figure shows several examples.

Spectral characteristic

It determines in which wavelength range the LED emits, roughly speaking the color of the radiation. Usually, the peak value of the wavelength and a plot of the intensity of the LED emission from the wavelength are given. I rarely look at this data. I know, for example, that the LED is red and that's enough for me.

Climate characteristics

They determine the operating temperature range of the LED and the dependence of the LED parameters (forward current and radiation intensity) on temperature. If the LED is going to be used at high or low temperatures, you should pay attention to these characteristics.

How does an LED work?

The material of the article is designed for beginner electronics engineers, and therefore I deliberately do not touch on the physics of the LED. The realization that an LED emits photons as a result of the recombination of charge carriers in area p-n transition, does not carry any useful information for the practical use of LEDs. And not only for use, but also for understanding in principle.

However, if you want to delve into this topic, then I give you the direction where to dig - Pasynkov V.V., Chirkin L.K. "Semiconductor Devices" or Zi.S "Physics of Semiconductor Devices". These are VUZ'ovskie textbooks - everything is grown-up there.

About connecting LEDs in the following material ...

Shared an article - got an LED beam of goodness!