Everyone is now familiar with LEDs. Modern technology is simply unthinkable without them. These are LED lights and lamps, indication of operating modes of various household appliances, backlighting of the screens of computer monitors, televisions and many other things that you can’t immediately remember. All of the listed devices contain visible light-emitting diodes of various colors: red, green, blue (RGB), yellow, white. Modern technologies make it possible to obtain almost any color.
In addition to visible LEDs, there are infrared and ultraviolet LEDs. The main area of application of such LEDs is automation and control devices. Enough to remember. If the first remote control models were used exclusively to control televisions, now they are used to control wall heaters, air conditioners, fans and even kitchen appliances, such as multicookers and bread makers.
So what is an LED?
In fact, it is not much different from the usual one - the same p-n junction, and the same basic property of one-way conductivity. As we studied the pn junction, it turned out that in addition to one-way conductivity, this very junction has several additional properties. During the evolution of semiconductor technology, these properties were studied, developed and improved.
The Soviet radiophysicist (1903 - 1942) made a great contribution to the development of semiconductors. In 1919, he entered the famous and still known Nizhny Novgorod Radio Laboratory, and from 1929 he worked at the Leningrad Institute of Physics and Technology. One of the scientist’s areas of activity was the study of the weak, barely noticeable glow of semiconductor crystals. It is on this effect that all modern LEDs work.
This faint glow occurs when current is passed through the pn junction in the forward direction. But now this phenomenon has been studied and improved so much that the brightness of some LEDs is such that you can simply go blind.
The color range of LEDs is very wide, almost all the colors of the rainbow. But the color is not obtained by changing the color of the LED housing. This is achieved by adding dopant impurities to the pn junction. For example, the introduction of a small amount of phosphorus or aluminum produces colors of red and yellow hues, while gallium and indium emit light from green to blue. The LED housing can be transparent or matte; if the housing is colored, then it is simply a light filter that matches the color of the p-n junction.
Another way to obtain the desired color is to introduce a phosphor. A phosphor is a substance that produces visible light when exposed to other radiation, even infrared. A classic example of this is fluorescent lamps. In the case of LEDs, white color is obtained by adding a phosphor to a blue crystal.
To increase the emission intensity, almost all LEDs have a focusing lens. Often the end of a transparent body, which has a spherical shape, is used as a lens. In infrared LEDs, sometimes the lens appears opaque, smoky gray in color. Although recently infrared LEDs have been produced simply in a transparent case, these are the ones used in various remote control systems.
Bi-color LEDs
Also known to almost everyone. For example, a charger for a mobile phone: while charging is in progress, the indicator lights up red, and when charging is complete, it lights up green. This indication is possible thanks to the existence of two-color LEDs, which can be of different types. The first type is three-terminal LEDs. One package contains two LEDs, for example, green and red, as shown in Figure 1.
Figure 1. Bi-color LED connection diagram
The figure shows a fragment of a circuit with a two-color LED. In this case, a three-terminal LED with a common cathode is shown (sometimes with a common anode) and its connection to. In this case, you can turn on either one or the other LED, or both at once. For example, it will be red or green, and if two LEDs are turned on at once, it will turn out yellow. If you use PWM modulation to adjust the brightness of each LED, you can get several intermediate shades.
In this circuit, you should pay attention to the fact that limiting resistors are included separately for each LED, although it would seem that you can get by with just one by including it in the common output. But with this switching on, the brightness of the LEDs will change when one or two LEDs are turned on.
What voltage is needed for an LED? This question can be heard quite often, asked by those who are not familiar with the specifics of LED operation or simply by people who are very far from electricity. In this case, it is necessary to explain that the LED is a device controlled by current, not voltage. You can turn on the LED at least at 220V, but the current through it should not exceed the maximum permissible. This is achieved by connecting a ballast resistor in series with the LED.
But still, remembering the voltage, it should be noted that it also plays a big role, because LEDs have a high forward voltage. If for a conventional silicon diode this voltage is about 0.6...0.7V, then for an LED this threshold starts from two volts and above. Therefore, the LED cannot be lit with a voltage of 1.5V.
But with this connection, meaning 220V, we should not forget that the reverse voltage of the LED is quite small, no more than a few tens of volts. Therefore, special measures are taken to protect the LED from high reverse voltage. The easiest way is to counter-connect a protective diode in parallel, which may also not be particularly high-voltage, for example KD521. Under the influence of alternating voltage, the diodes open alternately, thereby protecting each other from high reverse voltage. The circuit diagram for connecting the protective diode is shown in Figure 2.
Figure 2. Connection diagram parallel to the LED protective diode
Two-color LEDs are also available in a package with two terminals. In this case, the color of the glow changes when the direction of the current changes. A classic example is the indication of the direction of rotation of a DC motor. It should not be forgotten that a limiting resistor must be connected in series with the LED.
Recently, a limiting resistor is simply built into the LED, and then, for example, on the price tags in the store they simply write that this LED is rated at 12V. The flashing LEDs are also marked by voltage: 3V, 6V, 12V. There is a microcontroller inside these LEDs (you can even see it through the transparent case), so any attempts to change the blinking frequency do not produce results. With this marking, you can turn on the LED directly to the power supply at the specified voltage.
Developments of Japanese radio amateurs
It turns out that amateur radio is practiced not only in the countries of the former USSR, but also in such an “electronic country” as Japan. Of course, even an ordinary Japanese radio amateur is unable to create very complex devices, but individual circuit solutions deserve attention. You never know in what scheme these solutions might be useful.
Here is an overview of relatively simple devices that use LEDs. In most cases, control is carried out from microcontrollers, and there is no escape from this. Even for a simple circuit, it is easier to write a short program and solder the controller in a DIP-8 package than to solder several microcircuits, capacitors and transistors. Another attractive thing about this is that some microcontrollers can operate without any attached parts at all.
Bi-color LED control circuit
An interesting scheme for controlling a powerful two-color LED is offered by Japanese radio amateurs. More precisely, it uses two powerful LEDs with a current of up to 1A. But, we must assume that there are also powerful two-color LEDs. The diagram is shown in Figure 3.
Figure 3. Control circuit for a powerful two-color LED
The TA7291P chip is designed to control low-power DC motors. It provides several modes, namely: forward rotation, reverse rotation, stop and braking. The output stage of the microcircuit is assembled using a bridge circuit, which allows you to perform all of the above operations. But it was worth applying some imagination and, lo and behold, the microcircuit has a new profession.
The logic of the microcircuit is quite simple. As can be seen in Figure 3, the microcircuit has 2 inputs (IN1, IN2) and two outputs (OUT1, OUT2), to which two powerful LEDs are connected. When the logical levels at inputs 1 and 2 are the same (00 or 11 makes no difference), then the output potentials are equal, both LEDs are off.
At different logical levels at the inputs, the microcircuit operates as follows. If one of the inputs, for example, IN1, has a low logical level, then the output OUT1 is connected to the common wire. The cathode of LED HL2 is also connected to the common wire through resistor R2. The voltage at the OUT2 output (if there is a logical one at the IN2 input) in this case depends on the voltage at the V_ref input, which allows you to adjust the brightness of the HL2 LED.
In this case, the voltage V_ref is obtained from PWM pulses from the microcontroller using the integrating chain R1C1, which regulates the brightness of the LED connected to the output. The microcontroller also controls the inputs IN1 and IN2, which allows you to obtain a wide variety of shades of light and LED control algorithms. The resistance of resistor R2 is calculated based on the maximum permissible current of the LEDs. How to do this will be described below.
Figure 4 shows the internal structure of the TA7291P chip and its block diagram. The diagram is taken directly from the datasheet, so it shows an electric motor as a load.
Figure 4.
Using the block diagram, it is easy to trace the current paths through the load and methods of controlling the output transistors. The transistors are switched on in pairs, diagonally: (upper left + lower right) or (upper right + lower left), which allows you to change the direction and speed of the engine. In our case, light one of the LEDs and control its brightness.
The lower transistors are controlled by signals IN1, IN2 and are simply designed to turn the bridge diagonals on and off. The upper transistors are controlled by the Vref signal, they regulate the output current. The control circuit, shown simply as a square, also contains circuit protection against short circuits and other unforeseen circumstances.
Ohm's law, as always, will help in these calculations. Let the initial data for the calculation be the following: supply voltage (U) 12V, current through the LED (I_HL) 10mA, the LED is connected to a voltage source without any transistors or microcircuits as a power-on indicator. The voltage drop across the LED (U_HL) is 2V.
Then it is quite obvious that the limiting resistor will receive voltage (U-U_HL), - two volts were “eaten” by the LED itself. Then the resistance of the limiting resistor will be
R_o = (U-U_HL) / I_HL = (12 - 2) / 0.010 = 1000(Ω) or 1KOhm.
Don't forget about the SI system: voltage in volts, current in amperes, result in Ohms. If the LED is turned on by a transistor, then in the first bracket the voltage of the collector-emitter section of the open transistor should be subtracted from the supply voltage. But, as a rule, no one ever does this; accuracy up to hundredths of a percent is not needed here, and it will not work due to the scattering of the parameters of the parts. All calculations in electronic circuits give approximate results, the rest has to be achieved by debugging and tuning.
Tri-color LEDs
In addition to two-color ones, recently they have become widespread. Their main purpose is decorative lighting on stages, at parties, at New Year's celebrations or at discos. Such LEDs have a body with four terminals, one of which is a common anode or cathode, depending on the specific model.
But one or two LEDs, even three-color ones, are of little use, so you have to combine them into garlands, and to control the garlands use all kinds of control devices, which are most often called controllers.
Assembling garlands of individual LEDs is boring and uninteresting. Therefore, in recent years, the industry has begun to produce strips based on three-color (RGB) LEDs. If single-color tapes are produced at a voltage of 12V, then the operating voltage of three-color tapes is often 24V.
LED strips are marked by voltage because they already contain limiting resistors, so they can be connected directly to a voltage source. Sources for are sold in the same place as the tapes.
Special controllers are used to control three-color LEDs and strips to create various lighting effects. With their help, it is possible to simply switch LEDs, adjust brightness, create various dynamic effects, as well as draw patterns and even paintings. The creation of such controllers attracts many radio amateurs, naturally those who know how to write programs for microcontrollers.
Using a three-color LED, you can get almost any color, because the color on a TV screen is also obtained by mixing only three colors. Here it is appropriate to recall another development of Japanese radio amateurs. Its circuit diagram is shown in Figure 5.
Figure 5. Three-color LED connection diagram
A powerful 1W three-color LED contains three emitters. With the resistor values indicated in the diagram, the glow color is white. By selecting resistor values, a slight change in shade is possible: from cold white to warm white. In the author's design, the lamp is designed to illuminate the interior of a car. Surely they (the Japanese) should be sad! In order not to worry about maintaining polarity, a diode bridge is provided at the input of the device. The device is mounted on a breadboard and is shown in Figure 6.
Figure 6. Development board
The next development of Japanese radio amateurs is also of an automotive nature. This device for illuminating the license plate, of course, with white LEDs is shown in Figure 7.
Figure 7. Diagram of a device for illuminating the license plate on white LEDs
The design uses 6 powerful, ultra-bright LEDs with a maximum current of 35mA and a luminous flux of 4lm. To increase the reliability of the LEDs, the current through them is limited to 27 mA using a voltage stabilizer chip connected as a current stabilizer circuit.
LEDs EL1...EL3, resistor R1, together with microcircuit DA1 form a current stabilizer. A stable current through resistor R1 maintains a voltage drop across it of 1.25V. The second group of LEDs is connected to the stabilizer through exactly the same resistor R2, so the current through the group of LEDs EL4...EL6 will also be stabilized at the same level.
Figure 8 shows a converter circuit for powering a white LED from one galvanic cell with a voltage of 1.5V, which is clearly not enough to light the LED. The converter circuit is very simple and is controlled by a microcontroller. In essence, the microcontroller is a pulse frequency of about 40KHz. To increase the load capacity, the microcontroller pins are connected in pairs in parallel.
Figure 8.
The scheme works as follows. When pins PB1, PB2 are low, outputs PB0, PB4 are high. At this time, capacitors C1, C2 are charged to approximately 1.4V through diodes VD1, VD2. When the state of the controller outputs changes to the opposite, the sum of the voltages of two charged capacitors plus the battery voltage will be applied to the LED. Thus, almost 4.5V will be applied to the LED in the forward direction, which is quite enough to light the LED.
Such a converter can be assembled without a microcontroller, simply on a logic chip. Such a diagram is shown in Figure 9.
Figure 9.
A square wave generator is assembled on element DD1.1, the frequency of which is determined by the ratings R1, C1. It is at this frequency that the LED will flash.
When the output of element DD1.1 is high, the output of DD1.2 is naturally high. At this time, capacitor C2 is charged through diode VD1 from the power source. The charging path is as follows: plus the power supply - DD1.1 - C2 - VD1 - DD1.2 - minus the power supply. At this time, only battery voltage is applied to the white LED, which is not enough to light the LED.
When the level at the output of element DD1.1 becomes low, a high level appears at the output of DD1.2, which leads to the blocking of diode VD1. Therefore, the voltage on capacitor C2 is summed with the battery voltage and this sum is applied to resistor R1 and LED HL1. This amount of voltage is quite enough to turn on the HL1 LED. Then the cycle repeats.
How to test an LED
If the LED is new, then everything is simple: the terminal that is slightly longer is the positive one or the anode. It is this that must be connected to the positive of the power source, naturally not forgetting about the limiting resistor. But in some cases, for example, the LED was soldered from an old board and its leads are the same length, a continuity test is required.
Multimeters behave somewhat incomprehensibly in such a situation. For example, a DT838 multimeter in semiconductor testing mode can simply light up the LED being tested slightly, but the indicator shows a break.
Therefore, in some cases, it is better to check LEDs by connecting them through a limiting resistor to a power source, as shown in Figure 10. The resistor value is 200...500 Ohm.
Figure 10. LED test circuit
Figure 11. Sequence of LEDs
Calculating the resistance of the limiting resistor is easy. To do this, you need to add up the forward voltage on all LEDs, subtract it from the power source voltage, and divide the resulting remainder by the given current.
R = (U - (U_HL_1 + U_HL_2 + U_HL_3)) / I
Let's assume that the power supply voltage is 12V and the voltage drop across the LEDs is 2V, 2.5V and 1.8V. Even if the LEDs are taken from the same box, there can still be such a scatter!
According to the conditions of the problem, the current is set to 20 mA. All that remains is to substitute all the values into the formula and learn the answer.
R = (12- (2 + 2.5 + 1.8)) / 0.02 = 285Ω
Figure 12. Parallel connection of LEDs
On the left fragment, all three LEDs are connected through one current-limiting resistor. But why is this scheme crossed out, what are its shortcomings?
This is where the variation in LED parameters comes into play. The greatest current will flow through the LED that has a smaller voltage drop, that is, a smaller internal resistance. Therefore, with this switching on, it will not be possible to achieve a uniform glow of the LEDs. Therefore, the correct circuit should be considered the circuit shown in Figure 12 on the right.
Due to low energy consumption, theoretical durability and lower prices, incandescent and energy-saving lamps are rapidly replacing them. But, despite the declared service life of up to 25 years, they often burn out without even serving the warranty period.
Unlike incandescent lamps, 90% of burnt-out LED lamps can be successfully repaired with your own hands, even without special training. The examples presented will help you repair failed LED lamps.
Before you start repairing an LED lamp, you need to understand its structure. Regardless of the appearance and type of LEDs used, all LED lamps, including filament bulbs, are designed the same. If you remove the walls of the lamp body, you can see the driver inside, which is a printed circuit board with radio elements installed on it.
Any LED lamp is designed and works as follows. The supply voltage from the contacts of the electric cartridge is supplied to the terminals of the base. Two wires are soldered to it, through which voltage is supplied to the driver input. From the driver, the DC supply voltage is supplied to the board on which the LEDs are soldered.
The driver is an electronic unit - a current generator that converts the supply voltage into the current required to light the LEDs.
Sometimes, to diffuse light or protect against human contact with unprotected conductors of a board with LEDs, it is covered with diffusing protective glass.
About filament lamps
In appearance, a filament lamp is similar to an incandescent lamp. The design of filament lamps differs from LED lamps in that they do not use a board with LEDs as light emitters, but a sealed glass flask filled with gas, in which one or more filament rods are placed. The driver is located in the base.
The filament rod is a glass or sapphire tube with a diameter of about 2 mm and a length of about 30 mm, on which 28 miniature LEDs coated in series with a phosphor are attached and connected. One filament consumes about 1 W of power. My operating experience shows that filament lamps are much more reliable than those made on the basis of SMD LEDs. I believe that over time they will replace all other artificial light sources.
Examples of LED lamp repairs
Attention, the electrical circuits of the LED lamp drivers are galvanically connected to the phase of the electrical network and therefore extreme care should be taken. Touching an unprotected part of a person’s body to exposed parts of a circuit connected to an electrical network can cause serious damage to health, including cardiac arrest.
LED lamp repair
ASD LED-A60, 11 W on SM2082 chip
Currently, powerful LED light bulbs have appeared, the drivers of which are assembled on SM2082 type chips. One of them worked for less than a year and ended up being repaired. The light went out randomly and came on again. When you tapped it, it responded with light or extinguishing. It became obvious that the problem was poor contact.
To get to the electronic part of the lamp, you need to use a knife to pick up the diffuser glass at the point of contact with the body. Sometimes it is difficult to separate the glass, since when it is seated, silicone is applied to the fixing ring.
After removing the light-scattering glass, access to the LEDs and the SM2082 current generator microcircuit became available. In this lamp, one part of the driver was mounted on an aluminum LED printed circuit board, and the second on a separate one.
An external inspection did not reveal any defective soldering or broken tracks. I had to remove the board with LEDs. To do this, the silicone was first cut off and the board was pryed off by the edge with a screwdriver blade.
To get to the driver located in the lamp body, I had to unsolder it by heating two contacts with a soldering iron at the same time and moving it to the right.
On one side of the driver circuit board, only an electrolytic capacitor with a capacity of 6.8 μF for a voltage of 400 V was installed.
On the reverse side of the driver board, a diode bridge and two series-connected resistors with a nominal value of 510 kOhm were installed.
In order to figure out which of the boards the contact was missing, we had to connect them, observing the polarity, using two wires. After tapping the boards with the handle of a screwdriver, it became obvious that the fault lies in the board with the capacitor or in the contacts of the wires coming from the base of the LED lamp.
Since the soldering did not raise any suspicions, I first checked the reliability of the contact in the central terminal of the base. It can be easily removed if you pry it over the edge with a knife blade. But the contact was reliable. Just in case, I tinned the wire with solder.
It is difficult to remove the screw part of the base, so I decided to use a soldering iron to solder the soldering wires coming from the base. When I touched one of the soldering joints, the wire became exposed. A “cold” solder was detected. Since there was no way to get to the wire to strip it, I had to lubricate it with FIM active flux and then solder it again.
After assembly, the LED lamp consistently emitted light, despite hitting it with the handle of a screwdriver. Checking the light flux for pulsations showed that they are significant with a frequency of 100 Hz. Such an LED lamp can only be installed in luminaires for general lighting.
Driver circuit diagram
LED lamp ASD LED-A60 on SM2082 chip
The electrical circuit of the ASD LED-A60 lamp, thanks to the use of a specialized SM2082 microcircuit in the driver to stabilize the current, turned out to be quite simple.
The driver circuit works as follows. The AC supply voltage is supplied through fuse F to the rectifier diode bridge assembled on the MB6S microassembly. Electrolytic capacitor C1 smoothes out ripples, and R1 serves to discharge it when the power is turned off.
From the positive terminal of the capacitor, the supply voltage is supplied directly to the LEDs connected in series. From the output of the last LED, the voltage is supplied to the input (pin 1) of the SM2082 microcircuit, the current in the microcircuit is stabilized and then from its output (pin 2) goes to the negative terminal of capacitor C1.
Resistor R2 sets the amount of current flowing through the HL LEDs. The amount of current is inversely proportional to its rating. If the value of the resistor is decreased, the current will increase; if the value is increased, the current will decrease. The SM2082 microcircuit allows you to adjust the current value with a resistor from 5 to 60 mA.
LED lamp repair
ASD LED-A60, 11 W, 220 V, E27
The repair included another ASD LED-A60 LED lamp, similar in appearance and with the same technical characteristics as the one repaired above.
When turned on, the lamp came on for a moment and then did not shine. This behavior of LED lamps is usually associated with a driver failure. So I immediately started disassembling the lamp.
The light-scattering glass was removed with great difficulty, since along the entire line of contact with the body it was, despite the presence of a retainer, generously lubricated with silicone. To separate the glass, I had to look for a pliable place along the entire line of contact with the body using a knife, but still there was a crack in the body.
To gain access to the lamp driver, the next step was to remove the LED printed circuit board, which was pressed along the contour into the aluminum insert. Despite the fact that the board was aluminum and could be removed without fear of cracks, all attempts were unsuccessful. The board held tight.
It was also not possible to remove the board together with the aluminum insert, since it fit tightly to the case and was seated with the outer surface on silicone.
I decided to try removing the driver board from the base side. To do this, first, a knife was pryed out of the base and the central contact was removed. To remove the threaded part of the base, it was necessary to slightly bend its upper flange so that the core points would disengage from the base.
The driver became accessible and was freely extended to a certain position, but it was not possible to remove it completely, although the conductors from the LED board were sealed off.
The LED board had a hole in the center. I decided to try to remove the driver board by hitting its end through a metal rod threaded through this hole. The board moved a few centimeters and hit something. After further blows, the lamp body cracked along the ring and the board with the base of the base separated.
As it turned out, the board had an extension whose shoulders rested against the lamp body. It looks like the board was shaped this way to limit movement, although it would have been enough to fix it with a drop of silicone. Then the driver would be removed from either side of the lamp.
The 220 V voltage from the lamp base is supplied through a resistor - fuse FU to the MB6F rectifier bridge and is then smoothed out by an electrolytic capacitor. Next, the voltage is supplied to the SIC9553 chip, which stabilizes the current. Parallel connected resistors R20 and R80 between pins 1 and 8 MS set the amount of LED supply current.
The photo shows a typical electrical circuit diagram provided by the manufacturer of the SIC9553 chip in the Chinese datasheet.
This photo shows the appearance of the LED lamp driver from the installation side of the output elements. Since space allowed, to reduce the pulsation coefficient of the light flux, the capacitor at the driver output was soldered to 6.8 μF instead of 4.7 μF.
If you have to remove the drivers from the body of this lamp model and cannot remove the LED board, you can use a jigsaw to cut the lamp body around the circumference just above the screw part of the base.
In the end, all my efforts to remove the driver turned out to be useful only for understanding the LED lamp structure. The driver turned out to be OK.
The flash of the LEDs at the moment of switching on was caused by a breakdown in the crystal of one of them as a result of a voltage surge when the driver was started, which misled me. It was necessary to ring the LEDs first.
An attempt to test the LEDs with a multimeter was unsuccessful. The LEDs did not light up. It turned out that two light-emitting crystals connected in series are installed in one case, and in order for the LED to start flowing current, it is necessary to apply a voltage of 8 V to it.
A multimeter or tester turned on in resistance measurement mode produces a voltage within 3-4 V. I had to check the LEDs using a power supply, supplying 12 V to each LED through a 1 kOhm current-limiting resistor.
There was no replacement LED available, so the pads were shorted with a drop of solder instead. This is safe for driver operation, and the power of the LED lamp will decrease by only 0.7 W, which is almost imperceptible.
After repairing the electrical part of the LED lamp, the cracked body was glued with quick-drying Moment super glue, the seams were smoothed by melting the plastic with a soldering iron and leveled with sandpaper.
Just for fun, I did some measurements and calculations. The current flowing through the LEDs was 58 mA, the voltage was 8 V. Therefore, the power supplied to one LED was 0.46 W. With 16 LEDs, the result is 7.36 W, instead of the declared 11 W. Perhaps the manufacturer has indicated the total power consumption of the lamp, taking into account losses in the driver.
The service life of the ASD LED-A60, 11 W, 220 V, E27 LED lamp declared by the manufacturer raises serious doubts in my mind. In the small volume of the plastic lamp body, with low thermal conductivity, significant power is released - 11 W. As a result, the LEDs and driver operate at the maximum permissible temperature, which leads to accelerated degradation of their crystals and, as a consequence, to a sharp reduction in their time between failures.
LED lamp repair
LED smd B35 827 ERA, 7 W on BP2831A chip
An acquaintance shared with me that he bought five light bulbs like in the photo below, and after a month they all stopped working. He managed to throw away three of them, and, at my request, brought two for repairs.
The light bulb worked, but instead of bright light it emitted a flickering weak light with a frequency of several times per second. I immediately assumed that the electrolytic capacitor had swollen; usually, if it fails, the lamp begins to emit light like a strobe.
The light-scattering glass came off easily, it was not glued. It was fixed by a slot on its rim and a protrusion in the lamp body.
The driver was secured using two solders to a printed circuit board with LEDs, as in one of the lamps described above.
A typical driver circuit on the BP2831A chip taken from the datasheet is shown in the photograph. The driver board was removed and all simple radio elements were checked; they all turned out to be in good order. I had to start checking the LEDs.
The LEDs in the lamp were installed of an unknown type with two crystals in the housing and inspection did not reveal any defects. By connecting the leads of each LED in series, I quickly identified the faulty one and replaced it with a drop of solder, as in the photo.
The light bulb worked for a week and was repaired again. Shorted the next LED. A week later I had to short-circuit another LED, and after the fourth I threw out the light bulb because I was tired of repairing it.
The reason for the failure of light bulbs of this design is obvious. LEDs overheat due to insufficient heat sink surface, and their service life is reduced to hundreds of hours.
Why is it permissible to short-circuit the terminals of burnt-out LEDs in LED lamps?
The LED lamp driver, unlike a constant voltage power supply, produces a stabilized current value at the output, not a voltage. Therefore, regardless of the load resistance within the specified limits, the current will always be constant and, therefore, the voltage drop across each of the LEDs will remain the same.
Therefore, as the number of series-connected LEDs in the circuit decreases, the voltage at the driver output will also decrease proportionally.
For example, if 50 LEDs are connected in series to the driver, and each of them drops a voltage of 3 V, then the voltage at the driver output is 150 V, and if you short-circuit 5 of them, the voltage will drop to 135 V, and the current will not change.
But the efficiency of the driver assembled according to this scheme will be low and the power loss will be more than 50%. For example, for an LED light bulb MR-16-2835-F27 you will need a 6.1 kOhm resistor with a power of 4 watts. It turns out that the resistor driver will consume power that exceeds the power consumption of LEDs and placing it in a small LED lamp housing will be unacceptable due to the release of more heat.
But if there is no other way to repair an LED lamp and it is very necessary, then the resistor driver can be placed in a separate housing; anyway, the power consumption of such an LED lamp will be four times less than incandescent lamps. It should be noted that the more LEDs connected in series in a light bulb, the higher the efficiency will be. With 80 series-connected SMD3528 LEDs, you will need an 800 Ohm resistor with a power of only 0.5 W. The capacitance of capacitor C1 will need to be increased to 4.7 µF.
Finding faulty LEDs
After removing the protective glass, it becomes possible to check the LEDs without peeling off the printed circuit board. First of all, a careful inspection of each LED is carried out. If even the smallest black dot is detected, not to mention blackening of the entire surface of the LED, then it is definitely faulty.
When inspecting the appearance of the LEDs, you need to carefully examine the quality of the soldering of their terminals. One of the light bulbs being repaired turned out to have four LEDs that were poorly soldered.
The photo shows a light bulb that had very small black dots on its four LEDs. I immediately marked the faulty LEDs with crosses so that they were clearly visible.
Faulty LEDs may not have any changes in appearance. Therefore, it is necessary to check each LED with a multimeter or pointer tester turned on in resistance measurement mode.
There are LED lamps in which standard LEDs are installed in appearance, in the housing of which two crystals connected in series are mounted at once. For example, lamps of the ASD LED-A60 series. To test such LEDs, it is necessary to apply a voltage of more than 6 V to its terminals, and any multimeter produces no more than 4 V. Therefore, checking such LEDs can only be done by applying a voltage of more than 6 (recommended 9-12) V to them from the power source through a 1 kOhm resistor .
The LED is checked like a regular diode; in one direction the resistance should be equal to tens of megaohms, and if you swap the probes (this changes the polarity of the voltage supply to the LED), then it should be small, and the LED may glow dimly.
When checking and replacing LEDs, the lamp must be fixed. To do this, you can use a suitable sized round jar.
You can check the serviceability of the LED without an additional DC source. But this verification method is possible if the light bulb driver is working properly. To do this, it is necessary to apply supply voltage to the base of the LED light bulb and short-circuit the terminals of each LED in series with each other using a wire jumper or, for example, the jaws of metal tweezers.
If suddenly all the LEDs light up, it means that the shorted one is definitely faulty. This method is suitable if only one LED in the circuit is faulty. With this method of checking, it is necessary to take into account that if the driver does not provide galvanic isolation from the electrical network, as for example in the diagrams above, then touching the LED solders with your hand is unsafe.
If one or even several LEDs turn out to be faulty and there is nothing to replace them with, then you can simply short-circuit the contact pads to which the LEDs were soldered. The light bulb will work with the same success, only the luminous flux will decrease slightly.
Other LED lamp faults
If checking the LEDs showed their serviceability, then the reason for the light bulb’s inoperability lies in the driver or in the soldering areas of the current-carrying conductors.
For example, in this light bulb a cold solder connection was found on the conductor supplying power to the printed circuit board. The soot released due to poor soldering even settled on the conductive paths of the printed circuit board. The soot was easily removed by wiping with a rag soaked in alcohol. The wire was soldered, stripped, tinned and re-soldered into the board. I was lucky with the repair of this light bulb.
Of the ten failed bulbs, only one had a faulty driver and a broken diode bridge. The driver repair consisted of replacing the diode bridge with four IN4007 diodes, designed for a reverse voltage of 1000 V and a current of 1 A.
Soldering SMD LEDs
To replace a faulty LED, it must be desoldered without damaging the printed conductors. The LED from the donor board also needs to be desoldered for replacement without damage.
It is almost impossible to desolder SMD LEDs with a simple soldering iron without damaging their housing. But if you use a special tip for a soldering iron or put an attachment made of copper wire on a standard tip, then the problem can be easily solved.
LEDs have polarity and when replacing, you need to install it correctly on the printed circuit board. Typically, printed conductors follow the shape of the leads on the LED. Therefore, a mistake can only be made if you are inattentive. To seal an LED, it is enough to install it on a printed circuit board and heat its ends with the contact pads with a 10-15 W soldering iron.
If the LED burns out like carbon, and the printed circuit board underneath is charred, then before installing a new LED, you must clean this area of the printed circuit board from burning, since it is a current conductor. When cleaning, you may find that the LED solder pads are burnt or peeled off.
In this case, the LED can be installed by soldering it to adjacent LEDs if the printed traces lead to them. To do this, you can take a piece of thin wire, bend it in half or three times, depending on the distance between the LEDs, tin it and solder it to them.
Repair of LED lamp series "LL-CORN" (corn lamp)
E27 4.6W 36x5050SMD
The design of the lamp, which is popularly called a corn lamp, shown in the photo below is different from the lamp described above, therefore the repair technology is different.
The design of LED SMD lamps of this type is very convenient for repair, since there is access to test the LEDs and replace them without disassembling the lamp body. True, I still disassembled the light bulb for fun in order to study its structure.
Checking the LEDs of an LED corn lamp is no different from the technology described above, but we must take into account that the SMD5050 LED housing contains three LEDs at once, usually connected in parallel (three dark points of the crystals are visible on the yellow circle), and during testing all three should glow.
A faulty LED can be replaced with a new one or short-circuited with a jumper. This will not affect the reliability of the lamp, only the luminous flux will decrease slightly, unnoticeably to the eye.
The driver of this lamp is assembled according to the simplest circuit, without an isolating transformer, so touching the LED terminals when the lamp is on is unacceptable. Lamps of this design must not be installed in lamps that can be reached by children.
If all the LEDs are working, it means the driver is faulty, and the lamp will have to be disassembled to get to it.
To do this, you need to remove the rim from the side opposite the base. Using a small screwdriver or a knife blade, try in a circle to find the weak spot where the rim is glued the worst. If the rim gives way, then using the tool as a lever, the rim will easily come off around the entire perimeter.
The driver was assembled according to the electrical circuit, like the MR-16 lamp, only C1 had a capacity of 1 µF, and C2 - 4.7 µF. Due to the fact that the wires going from the driver to the lamp base were long, the driver was easily removed from the lamp body. After studying its circuit diagram, the driver was inserted back into the housing, and the bezel was glued into place with transparent Moment glue. The failed LED was replaced with a working one.
Repair of LED lamp "LL-CORN" (corn lamp)
E27 12W 80x5050SMD
When repairing a more powerful lamp, 12 W, there were no failed LEDs of the same design and in order to get to the drivers, we had to open the lamp using the technology described above.
This lamp gave me a surprise. The wires leading from the driver to the socket were short, and it was impossible to remove the driver from the lamp body for repair. I had to remove the base.
The lamp base was made of aluminum, cored around the circumference and held tightly. I had to drill out the mounting points with a 1.5 mm drill. After this, the base, pryed off with a knife, was easily removed.
But you can do without drilling the base if you use the edge of a knife to pry it around the circumference and slightly bend its upper edge. You should first put a mark on the base and body so that the base can be conveniently installed in place. To securely fasten the base after repairing the lamp, it will be enough to put it on the lamp body in such a way that the punched points on the base fall into the old places. Next, press these points with a sharp object.
Two wires were connected to the thread with a clamp, and the other two were pressed into the central contact of the base. I had to cut these wires.
As expected, there were two identical drivers, feeding 43 diodes each. They were covered with heat shrink tubing and taped together. In order for the driver to be placed back into the tube, I usually carefully cut it along the printed circuit board from the side where the parts are installed.
After repair, the driver is wrapped in a tube, which is fixed with a plastic tie or wrapped with several turns of thread.
In the electrical circuit of the driver of this lamp, protection elements are already installed, C1 for protection against pulse surges and R2, R3 for protection against current surges. When checking the elements, resistors R2 were immediately found to be open on both drivers. It appears that the LED lamp was supplied with a voltage that exceeded the permissible voltage. After replacing the resistors, I didn’t have a 10 ohm one at hand, so I set it to 5.1 ohms, and the lamp started working.
Repair of LED lamp series "LLB" LR-EW5N-5
The appearance of this type of light bulb inspires confidence. Aluminum body, high quality workmanship, beautiful design.
The design of the light bulb is such that disassembling it without the use of significant physical effort is impossible. Since the repair of any LED lamp begins with checking the serviceability of the LEDs, the first thing we had to do was remove the plastic protective glass.
The glass was fixed without glue on a groove made in the radiator with a collar inside it. To remove the glass, you need to use the end of a screwdriver, which will go between the fins of the radiator, to lean on the end of the radiator and, like a lever, lift the glass up.
Checking the LEDs with a tester showed they are working properly, therefore, the driver is faulty and we need to get to it. The aluminum board was secured with four screws, which I unscrewed.
But contrary to expectations, behind the board there was a radiator plane, lubricated with heat-conducting paste. The board had to be returned to its place and the lamp continued to be disassembled from the base side.
Due to the fact that the plastic part to which the radiator was attached was held very tightly, I decided to go the proven route, remove the base and remove the driver through the opened hole for repair. I drilled out the core points, but the base was not removed. It turned out that it was still attached to the plastic due to the threaded connection.
I had to separate the plastic adapter from the radiator. It held up just like the protective glass. To do this, a cut was made with a hacksaw for metal at the junction of the plastic with the radiator and by turning a screwdriver with a wide blade, the parts were separated from each other.
After unsoldering the leads from the LED printed circuit board, the driver became available for repair. The driver circuit turned out to be more complex than previous light bulbs, with an isolation transformer and a microcircuit. One of the 400 V 4.7 µF electrolytic capacitors was swollen. I had to replace it.
A check of all semiconductor elements revealed a faulty Schottky diode D4 (pictured below left). There was an SS110 Schottky diode on the board, which was replaced with an existing analog 10 BQ100 (100 V, 1 A). The forward resistance of Schottky diodes is two times less than that of ordinary diodes. The LED light came on. The second light bulb had the same problem.
Repair of LED lamp series "LLB" LR-EW5N-3
This LED lamp is very similar in appearance to the "LLB" LR-EW5N-5, but its design is slightly different.
If you look closely, you can see that at the junction between the aluminum radiator and the spherical glass, unlike the LR-EW5N-5, there is a ring in which the glass is secured. To remove the protective glass, use a small screwdriver to pry it at the junction with the ring.
Three nine super-bright crystal LEDs are installed on an aluminum printed circuit board. The board is screwed to the heatsink with three screws. Checking the LEDs showed their serviceability. Therefore, the driver needs to be repaired. Having experience in repairing a similar LED lamp "LLB" LR-EW5N-5, I did not unscrew the screws, but unsoldered the current-carrying wires coming from the driver and continued disassembling the lamp from the base side.
The plastic connecting ring between the base and the radiator was removed with great difficulty. At the same time, part of it broke off. As it turned out, it was screwed to the radiator with three self-tapping screws. The driver was easily removed from the lamp body.
The screws that fasten the plastic ring of the base are covered by the driver, and it is difficult to see them, but they are on the same axis with the thread to which the transition part of the radiator is screwed. Therefore, you can reach them with a thin Phillips screwdriver.
The driver turned out to be assembled according to a transformer circuit. Checking all elements except the microcircuit did not reveal any failures. Consequently, the microcircuit is faulty; I couldn’t even find a mention of its type on the Internet. The LED light bulb could not be repaired; it will be useful for spare parts. But I studied its structure.
Repair of LED lamp series "LL" GU10-3W
At first glance, it turned out to be impossible to disassemble a burnt-out GU10-3W LED light bulb with protective glass. An attempt to remove the glass resulted in its chipping. When great force was applied, the glass cracked.
By the way, in the lamp marking, the letter G means that the lamp has a pin base, the letter U means that the lamp belongs to the class of energy-saving light bulbs, and the number 10 means the distance between the pins in millimeters.
LED light bulbs with a GU10 base have special pins and are installed in a socket with a rotation. Thanks to the expanding pins, the LED lamp is pinched in the socket and held securely even when shaking.
In order to disassemble this LED light bulb, I had to drill a hole with a diameter of 2.5 mm in its aluminum case at the level of the surface of the printed circuit board. The drilling location must be chosen in such a way that the drill does not damage the LED when exiting. If you don’t have a drill at hand, you can make a hole with a thick awl.
Next, a small screwdriver is inserted into the hole and, acting like a lever, the glass is lifted. I removed the glass from two light bulbs without any problems. If checking the LEDs with a tester shows their serviceability, then the printed circuit board is removed.
After separating the board from the lamp body, it immediately became obvious that the current-limiting resistors had burned out in both one and the other lamp. The calculator determined their nominal value from the stripes, 160 Ohms. Since the resistors burned out in LED bulbs of different batches, it is obvious that their power, judging by the size of 0.25 W, does not correspond to the power released when the driver operates at the maximum ambient temperature.
The driver circuit board was well filled with silicone, and I did not disconnect it from the board with the LEDs. I cut off the leads of the burnt resistors at the base and soldered them to more powerful resistors that were on hand. In one lamp I soldered a 150 Ohm resistor with a power of 1 W, in the second two in parallel with 320 Ohms with a power of 0.5 W.
In order to prevent accidental contact of the resistor terminal, to which the mains voltage is connected, with the metal body of the lamp, it was insulated with a drop of hot-melt adhesive. It is waterproof and an excellent insulator. I often use it to seal, insulate and secure electrical wires and other parts.
Hot-melt adhesive is available in sticks with diameters of 7, 12, 15 and 24 mm in different colors, from transparent to black. It melts, depending on the brand, at a temperature of 80-150°, which allows it to be melted using an electric soldering iron. It is enough to cut a piece of the rod, place it in the right place and heat it. Hot-melt glue will acquire the consistency of May honey. After cooling it becomes hard again. When reheated it becomes liquid again.
After replacing the resistors, the functionality of both bulbs was restored. All that remains is to secure the printed circuit board and protective glass in the lamp body.
When repairing LED lamps, I used “Installation” moment liquid nails to secure printed circuit boards and plastic parts. The glue is odorless, adheres well to the surfaces of any materials, remains plastic after drying, and has sufficient heat resistance.
It is enough to take a small amount of glue on the end of a screwdriver and apply it to the places where the parts come into contact. After 15 minutes the glue will already hold.
When gluing the printed circuit board, in order not to wait, holding the board in place, since the wires were pushing it out, I additionally fixed the board at several points using hot glue.
The LED lamp began to flash like a strobe light
I had to repair a couple of LED lamps with drivers assembled on a microcircuit, the malfunction of which was the light blinking at a frequency of about one hertz, like in a strobe light.
One instance of the LED lamp began to blink immediately after being turned on for the first few seconds and then the lamp began to shine normally. Over time, the duration of the lamp's blinking after switching on began to increase, and the lamp began to blink continuously. The second instance of the LED lamp suddenly began blinking continuously.
After disassembling the lamps, it turned out that the electrolytic capacitors installed immediately after the rectifier bridges in the drivers had failed. It was easy to determine the malfunction, since the capacitor housings were swollen. But even if the capacitor looks free of external defects in appearance, then the repair of an LED light bulb with a stroboscopic effect still needs to begin with its replacement.
After replacing the electrolytic capacitors with working ones, the stroboscopic effect disappeared and the lamps began to shine normally.
Online calculators for determining resistor values
by color marking
When repairing LED lamps, it becomes necessary to determine the resistor value. According to the standard, modern resistors are marked by applying colored rings to their bodies. 4 colored rings are applied to simple resistors, and 5 to high-precision resistors.
Despite the high cost, the energy consumption of semiconductor lamps (LED) is much less than that of incandescent lamps, and their service life is 5 times longer. The LED lamp circuit operates with a supply of 220 volts, when the input signal causing the glow is converted to an operating value using a driver.
LED lamps 220 V
Whatever the supply voltage, a constant voltage of 1.8-4 V is supplied to one LED.
Types of LEDs
An LED is a semiconductor crystal made up of several layers that converts electricity into visible light. When its composition changes, radiation of a certain color is obtained. The LED is made on the basis of a chip - a crystal with a platform for connecting power conductors.
To reproduce white light, the “blue” chip is coated with a yellow phosphor. When the crystal emits radiation, the phosphor emits its own. Mixing yellow and blue light creates white.
Different chip assembly methods allow you to create 4 main types of LEDs:
- DIP - consists of a crystal with a lens located on top and two conductors attached. It is most common and is used for lighting, lighting decorations and displays.
- “Piranha” is a similar design, but with four terminals, which makes it more reliable for installation and improves heat dissipation. Mostly used in the automotive industry.
- SMD LED - placed on the surface, due to which it is possible to reduce dimensions, improve heat dissipation and provide many design options. Can be used in any light sources.
- COB technology, where the chip is soldered into the board. Due to this, the contact is better protected from oxidation and overheating, and the glow intensity is significantly increased. If an LED burns out, it must be completely replaced, since DIY repairs by replacing individual chips are not possible.
The disadvantage of the LED is its small size. To create a large, colorful light image, many sources are required, combined into groups. In addition, the crystal ages over time, and the brightness of the lamps gradually decreases. For high-quality models, the wear process is very slow.
LED lamp device
The lamp contains:
- frame;
- base;
- diffuser;
- radiator;
- LED block;
- transformerless driver.
220 volt LED lamp device
The figure shows a modern LED lamp using SOV technology. The LED is made as one unit, with many crystals. It does not require wiring of numerous contacts. It is enough to connect just one pair. When a lamp with a burnt-out LED is repaired, the entire lamp is replaced.
The shape of the lamps is round, cylindrical and others. Connection to the power supply is made through threaded or pin sockets.
For general lighting, luminaires with color temperatures of 2700K, 3500K and 5000K are selected. The spectrum gradations can be any. They are often used for advertising lighting and for decorative purposes.
The simplest driver circuit for powering a lamp from the mains is shown in the figure below. The number of parts here is minimal, due to the presence of one or two quenching resistors R1, R2 and the back-to-back connection of LEDs HL1, HL2. This way they protect each other from reverse voltage. In this case, the flickering frequency of the lamp increases to 100 Hz.
The simplest diagram for connecting an LED lamp to a 220 volt network
The supply voltage of 220 volts is supplied through the limiting capacitor C1 to the rectifier bridge, and then to the lamp. One of the LEDs can be replaced with a regular rectifier, but the flickering will change to 25 Hz, which will have a bad effect on vision.
The figure below shows a classic LED lamp power supply circuit. It is used in many models and can be removed for DIY repairs.
Classic scheme for connecting an LED lamp to a 220 V network
The electrolytic capacitor smooths out the rectified voltage, which eliminates flicker at a frequency of 100 Hz. Resistor R1 discharges the capacitor when the power is turned off.
DIY repair
A simple LED lamp with individual LEDs can be repaired by replacing faulty elements. It can be easily disassembled if you carefully separate the base from the glass body. There are LEDs inside. The MR 16 lamp has 27 of them. To access the printed circuit board on which they are located, you need to remove the protective glass by prying it off with a screwdriver. Sometimes this operation is quite difficult to do.
LED lamp 220 volts
Burnt-out LEDs are immediately replaced. The rest should be ringed with a tester or a voltage of 1.5 V should be applied to each. The serviceable ones should light up, and the rest must be replaced.
The manufacturer calculates the lamps so that the operating current of the LEDs is as high as possible. This significantly reduces their service life, but it is not profitable to sell “eternal” devices. Therefore, a limiting resistor can be connected in series to the LEDs.
If the lights blink, the cause may be a failure of capacitor C1. It should be replaced with another one with a rated voltage of 400 V.
Make it yourself
LED lamps are rarely made again. It is easier to make a lamp from a faulty one. In fact, it turns out that repairing and manufacturing a new product is one process. To do this, the LED lamp is disassembled and the burnt-out LEDs and driver radio components are restored. There are often original lamps on sale with non-standard lamps, which are difficult to find replacements in the future. A simple driver can be taken from a faulty lamp, and LEDs from an old flashlight.
The driver circuit is assembled according to the classic model discussed above. Only resistor R3 is added to it to discharge capacitor C2 when turned off and a pair of zener diodes VD2, VD3 to bypass it in case of an open circuit of the LEDs. You can get by with one zener diode if you choose the right stabilization voltage. If you select a capacitor for voltages greater than 220 V, you can do without additional parts. But in this case, its dimensions will increase and after the repair is done, the board with the parts may not fit into the base.
LED lamp driver
The driver circuit is shown for a lamp of 20 LEDs. If their number is different, it is necessary to select a capacitance value for capacitor C1 such that a current of 20 mA passes through them.
The power supply circuit for an LED lamp is most often transformerless, and care should be taken when installing it yourself on a metal lamp so that there is no phase or zero short circuit to the housing.
Capacitors are selected according to the table, depending on the number of LEDs. They can be mounted on an aluminum plate in the amount of 20-30 pieces. To do this, holes are drilled in it, and LEDs are installed on hot-melt adhesive. They are soldered sequentially. All parts can be placed on a printed circuit board made of fiberglass. They are located on the side where there are no printed tracks, with the exception of LEDs. The latter are attached by soldering the pins on the board. Their length is about 5 mm. The device is then assembled in the luminaire.
LED table lamp
220 V lamp. Video
You can learn about making a 220 V LED lamp with your own hands from this video.
A properly made homemade LED lamp circuit will allow you to operate it for many years. It may be possible to repair it. Power sources can be any: from a regular battery to a 220 volt network.
LEDs or LIGHT EMITTING DIODES (in the English version LED - Light Emitting Diode) are well known to every electronics engineer. These are semiconductor devices that convert electric current into light radiation. Their main advantages: high efficiency, close to monochrome radiation, miniature size, mechanical strength, high reliability, low heat generation, up to 10 years of operating time without turning off the power. Finally, LEDs are low-voltage devices, and therefore extremely electrically safe.
The first industrial samples of red LEDs appeared in 1962 (General Electric Corp.). In 1976, orange, green and yellow LEDs were developed, and in 1993 the first blue semiconductor emitters appeared (Nichia Corporation). In amateur designs, “red” and “green” LEDs are most often used, less often “blue” and “white”.
Typical efficiency values for standard LEDs range from 1 to 10%. For comparison, the efficiency of a steam engine is 5...7%. For powerful modern LEDs this figure reaches 12...35%.
In Table. Table 2.1 shows the parameters of low-power LEDs with a luminous intensity of no more than 1000 MKd. Their feature is a significant technological spread in the current-voltage characteristic (volt-ampere characteristic). As a consequence, for a particular LED, the forward current / PR and forward voltage V np are known only approximately. When calculating this, people usually turn a blind eye, since in most cases the LED is required to state the fact “on” or “off”.
Table 2.1. Parameters of low-power LEDs for general use
Conditional stresses 1.6; 1.7; 1.8; 3.5 V characterize the starting point of the rise in the current-voltage characteristic curve, respectively, for the “red”, “yellow”, “green” and “blue”/“white” indicators. It is these numbers that will be indicated in the future in electrical diagrams near the designation of LEDs. However, the actual operating voltage V pr is approximately 0.1…0.4 V greater than the initial one, which depends on the flowing current (Fig. 2.1).
Rice. 2.1. Typical current-voltage characteristics of low-power LEDs from Kingbright.
Important notes.
1. Do not set the constant forward current / PR through the LED close to the maximum limit specified in the datasheet. Typically this is 20 mA. Long-term operation with this current reduces long-term reliability. To obtain acceptable brightness, it is enough to set the current to 4...10 mA.
2. LEDs allow a pulsed operating mode, in which the forward current / PR can be increased 3...6 times to 60...120 mA while maintaining the average current for a period of no more than 20...25 mA. When making calculations, we must not forget that as the current increases, the voltage also increases. For example, for a “green” LED at a current of 15 mA, the voltage V PR = 2.1 V, and at a current of 75 mA V np = 2.7 V.
3. The red color of the indication does not guarantee that the LED belongs to the group with the conditional beginning of the I-V curve of 1.6 V (although in most cases this is exactly the case). A “red” LED may have a “green” I-V characteristic with a rise point of 1.8 V. It all depends on the chemical composition from which the emitter is made, and this parameter is a priori unknown when purchased on the radio market. The situation is similar with powerful “green” LEDs, which can have a “blue” I-V characteristic with a rise point of 3.5 V.
4. Some datasheets for LEDs indicate the maximum permissible reverse voltage U OBR = 2...5 V. But this is just a test voltage at which the reverse leakage current, equal to several tens of microamps, is checked at the manufacturer.
5. The LED fails not from high reverse voltage, but from exceeding the power dissipated on it. Studies have shown that green and red LEDs have a “zener diode” current-voltage characteristic with a fairly steep bend. At a reverse voltage of 12...35 V, a reversible breakdown of the n-p junction occurs. If the breakdown current does not exceed 2...4 mA, then the dissipation power remains within the limits regulated by the datasheet of 75...150 mW.
Practical conclusion - if the MK supply voltage is within 3..5 V, there is no fear of “confusing” the polarity when soldering the “red-orange-yellow-green” indicators. All of them are guaranteed to remain intact.
“Blue” and “white” LEDs are much more gentle in this regard. They are afraid of electrostatic potentials that can accumulate on clothing and on the human body. The reverse voltage for them should not exceed 5 V and they should be treated approximately the same way as field-effect transistors.
In Fig. 2.2, a...g shows diagrams for connecting single LEDs to one MK line. In Fig. 2.3, a...M shows diagrams for connecting single LEDs to several MK lines.
Rice. 2.2. Schemes for connecting single LEDs to one MK line (beginning):
a) standard current limiting circuit through the HL1 LED using resistor R1. For reference, the idealized MK has G 1H = 4.75 V at a load current of 5...10 mA and G 1H = 4.5 V at a load current of 20 mA;
b) similar to Fig. 2.2, a, but with inversion of the signal at the MK output For reference, the idealized MK has V OL = 0.15...0.3 V at load current
5.. . 10 mA and V OL = 0.4…0.5 V at a load current of 20 mA. If the MK outputs have a symmetrical load capacity, then between the circuits in Fig. 2.2, a and in Fig. 2.2, b no difference;
c) direct connection of the HL1 LED to the MK line is possible, but only with a low supply voltage. Operating point K PR = 2 V at / PR = 15 mA. However, in each specific case, you need to check the load capacity graphs of MK lines according to the datasheet;
d) connecting LED HL1 to an increased voltage source of +9 V through a quenching zener diode VD1. Check calculation - the sum of the supply voltage MK (5 V) and the stabilization voltage VD1 (5.6 V) must be greater than the difference between the increased voltage (9 V) and the voltage drop on the HL1 LED (1.7...1.9 V); ABOUT
About Fig. 2.2. Schemes for connecting single LEDs to one MK line (end):
e) LED HL1 has a built-in integral resistor that limits forward current. In the datasheet, instead of the resistor resistance, the permissible operating voltage of the LED is indicated at a current of no more than 20 mA. Classification number when ordering: 3; 5; 12 V;
e) it is assumed that the HL1 LED is located at a considerable distance from MK and is connected to contact pads XI, X? long wires. Resistors R1, R2 - current protection, in case of wire breakage and short circuit to the metal body of the device, which, as a rule, is connected to the GND circuit (“ground”);
g) the effect of smooth extinguishing of the HL1 LED. In the initial state, the MK output level is LOW, the LED is off. A HIGH level at the MK output quickly turns on the LED, and then smoothly decreases its brightness as capacitor C1 charges. Diode VD1 helps discharge capacitor C/ at a LOW level at the MK output.
Rice. 2.3. Schemes for connecting single LEDs to several MK lines (beginning):
a) LEDs HL1...HLn are switched on independently of each other at a HIGH level at the MK output. Resistors R1…Rn limit the currents through the LEDs and determine the brightness of their glow. The total current through the +5 V power pin at a HIGH level on all outputs should not exceed 100...300 mA (look in the datasheet for a specific MK);
b) similar to Fig. 2.3, a, but with an active LOW level and with a separate power supply for the LEDs. If the MK outputs have a symmetrical load capacity and the LED power supply is +5 V, then the circuits in Fig. 2.3, a and in Fig. 2.3, b are equivalent;
c) a typical technique for reducing the number of resistors. It is used if the simultaneous lighting of several indicators is not required, otherwise their brightness will decrease due to the increased voltage across the resistor R1\O
d) similar to Fig. 2.3, in, but with a “running zero” at the MK outputs;
e) the HL1 indicator lights up when the upper MK line is set to HIGH and the lower line is LOW, while other nodes can be connected to the MK outputs;
e) MK generates 8 gradations of brightness of the HL1 LED. Resistors R1…R3 determine the dynamic range and degree of linearity of the characteristic;
g) the ultra-bright HL1 LED requires increased current, which is achieved by paralleling the MK lines. At each of them, the levels must be set synchronously;
h) similar to Fig. 2.3, g, but with synchronous HIGH levels at the MK outputs;
i) LED HL1 indicates the presence of “running unit” pulses at three MK outputs; j) automatic continuation of a long cable. On MK lines, it is programmatically generated
“running unit” (on one line there is a HIGH level, on the others there is a LOW level). If any wire breaks, the LED in this circuit will be constantly extinguished; ABOUT
About Fig. 2.3. Schemes for connecting single LEDs to several MK lines (end):
l) in the initial state, all MK outputs have HIGH levels, the HL1, HL2, HL4 indicators are lit. In the event of an accident, the LOW level is set at one or more MK outputs, the corresponding indicator goes out, and HL3\m automatically starts to light) with a large number of LEDs, it makes sense to unload the MK power terminals by directing the incoming and outgoing currents to different circuits. In particular, LEDs HL1...HL8 reduce the load on the +5 V pin of MK, and LEDs HL9...HL16 reduce the load on the GND pin of MK.
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