How to choose the resistance for the LED. Resistor Resistance Calculation for LEDs: Online Calculator. Calculation of the resistor for serial connection of LEDs
LED is a semiconductor element which is used for lighting. It is used in lanterns, lamps, lamps and other lighting fixtures. The principle of its operation is that when current flows through a light emitting diode, photons are released from the surface of the semiconductor material, and the diode begins to glow.Reliable operation of the LED depends on the current flowing through it. At underestimated values, it simply will not shine, and if the current value is exceeded, the characteristics of the element will deteriorate, up to its destruction. At the same time, they say that the LED burned out. In order to exclude the possibility of failure of this semiconductor, it is necessary to select a resistor in a circuit with a resistor included in it. It will limit the current in the circuit at optimal values.
LED resistance is non-linear. This means that when the voltage applied to this element changes, the current flowing through it will change non-linearly. You can verify this by finding the volt-ampere characteristic of any diode, including a light-emitting one. When power is applied below the opening voltage of the p-n junction, the current through the LED is low and the element does not work. As soon as this threshold is exceeded, the current through the element rapidly increases, and it begins to glow.
If a source of power connected directly to the LED, the diode will fail, as it is not designed for such a load. To prevent this from happening, you need to limit the current flowing through the LED with ballast resistance, or lower the voltage on the semiconductor that is important to us.
Consider the simplest connection diagram (Figure 1). The DC power supply is connected in series through a resistor to the desired LED, the characteristics of which must be known. This can be done on the Internet by downloading the description (information sheet) for a specific model, or by finding the desired model in reference books. If it is not possible to find a description, you can roughly determine the voltage drop across the LED by its color:
- Infrared - up to 1.9 V.
- Red - from 1.6 to 2.03 V.
- Orange - from 2.03 to 2.1 V.
- Yellow - from 2.1 to 2.2 V.
- Green - from 2.2 to 3.5 V.
- Blue - 2.5 to 3.7 V.
- Purple - 2.8 to 4 V.
- Ultraviolet - from 3.1 to 4.4 V.
- White - from 3 to 3.7 V.
Figure 1 - LED connection diagram
The current in the circuit can be compared to the movement of fluid through a pipe. If there is only one flow path, then the current strength (flow rate) in the entire circuit will be the same. This is exactly what happens in the circuit in Figure 1. According to Kirchhoff's law, the sum of the voltage drops on all elements included in the circuit for the flow of one current is equal to the EMF of this circuit (in Figure 1 it is indicated by the letter E). From this we can conclude that the voltage falling across the current-limiting resistor should be equal to the difference between the supply voltage and its drop across the LED.
Since the current in the circuit must be the same, the current through the resistor and through the LED is the same. For the stable operation of a semiconductor element, increasing its reliability and durability, the current through it must be of certain values \u200b\u200bspecified in its description. If a description cannot be found, an approximate current in the circuit of 10 milliamps can be assumed. After determining these data, it is already possible to calculate the resistance value of the resistor for the LED. It is determined by Ohm's law. The resistance of a resistor is equal to the ratio of the voltage drop across it to the current in the circuit. Or in symbolic form:
R \u003d U (R) / I,
where, U(R) is the voltage drop across the resistor
I - current in the circuit
Calculation of U (R) on a resistor:
U(R) = E - U(Led)
where, U (Led) is the voltage drop across the LED element.
Using these formulas, you will get the exact value of the resistance of the resistor. However, the industry produces only standard resistance values, the so-called series of ratings. Therefore, after the calculation, you will have to make a selection of the existing resistance value. You need to choose a slightly larger resistor than it turned out in the calculation, so you get protection against accidental overvoltage in the network. If it is difficult to select an element close in value, you can try connecting two resistors in series or in parallel.
If you choose a resistance of less power than you need in the circuit, it will simply fail. The calculation of the power of a resistor is quite simple, you need to multiply the voltage drop across it by the current flowing in this circuit. Then you need to choose a resistance with a power not less than the calculated one.
Calculation example
We have a supply voltage of 12V, a green LED. It is necessary to calculate the resistance and power of the current-limiting resistor. The voltage drop on the green LED we need is 2.4 V, the rated current is 20 mA. From here we calculate the voltage drop across the ballast resistor.
U (R) \u003d E - U (Led) \u003d 12V - 2.4V \u003d 9.6V.
Resistance value:
R \u003d U (R) / I \u003d 9.6V / 0.02A \u003d 480 Ohms.
Power value:
P \u003d U (R) ⋅ I \u003d 9.6V ⋅ 0.02A \u003d 0.192 W
From a number of standard resistances, we select 487 ohms (E96 series), and the power can be selected as 0.25 watts. Such a resistor must be ordered.
In the event that you need to connect several LEDs in series, you can also connect them to a power source using only one resistor, which will dampen the excess voltage. Its calculation is made according to the above formulas, however, instead of one forward voltage U (Led), you need to take the sum of the forward voltages of the desired LEDs.
If you want to connect several light-emitting elements in parallel, then for each of them you need to calculate your own resistor, since each of the semiconductors can have its own forward voltage. The calculations for each circuit in this case are similar to the calculation of one resistor, since they are all connected in parallel to the same power source, and its value for calculating each circuit is the same.
Calculation steps
To make correct calculations, you need to do the following:
- Finding out the forward voltage and current of the LED.
- Calculate the voltage drop across the desired resistor.
- Resistor resistance calculation.
- Selection of resistance from the standard range.
- Calculation and selection of power.
You can do this simple calculation yourself, but it's easier and more time-efficient to use a calculator to calculate the resistor for the LED. If you enter such a query into a search engine, there are many sites offering automated counting. All the necessary formulas are already built into this tool and work instantly. Some services also immediately offer a selection of elements. You will only need to choose the most suitable calculator for calculating LEDs, and thus save your time.
The online LED calculator is not the only way to save time in calculations. The calculation of transistors, capacitors and other elements for various circuits has long been automated on the Internet. It remains only to correctly use the search engine to solve these problems.
LEDs are the optimal solution for many lighting tasks at home, office and production. Pay attention to Ledz lamps. This is the best ratio of price and quality of lighting products, using them, you do not have to make calculations and assemble lighting equipment yourself.
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Powering LEDs is not as simple as it might seem. They are extremely sensitive to the mode in which they work and do not tolerate overloads. The most important thing to remember is that semiconductor emitting diodes are powered by a stable current, not voltage. Even a perfectly stabilized voltage will not provide support for a given mode, this is a consequence of the internal structure and principle of operation of semiconductors. Nevertheless, with the right approach, LEDs can be connected to power through a current-limiting or additional resistor. Its calculation is reduced to an elementary selection of such resistance, on which extra Volts will fall at a given current value. Let's look at how to calculate its face value manually or use an online calculator.
Although the main parameter for powering an LED is current, there is also one such as voltage drop. This is the amount necessary for it to ignite. Based on it, the limiting resistor is calculated.
Typical voltages of LEDs of different types:
| Color | Voltage, V |
| White | 2.8-3.2 for low power, 3.0 and higher for high power (more than 0.5W) |
| Red | 1.6-2.0 |
| Green | 1.9-4.0 |
| Blue | 2.8-3.2 |
| Yellow orange | 2.0-2.2 |
| IR | Up to 1.9 |
| UV | 3.1-4.4 |
Attention! If you cannot find documentation for an existing item, use the online calculator to take the data from this table.
To shorten the theory, let's immediately calculate in practice the resistance for connecting a white LED to a 12V car on-board circuit. Its actual value with the engine running reaches 14.2 V, and sometimes even higher, which means we take it for calculations.
Then the calculation of the resistance for the LED is performed according to:
An average of 3 volts should fall on the LED, which means you need to compensate:
Ures=14.2-3=11.2 V
For a conventional 5 mm LED, the rated current is 20 mA or 0.02 A. We calculate the resistance of the resistor, on which 11.2 V should fall at a given current:
R=11.2/0.02=560 ohm or closest up
In order to achieve stable power and brightness, an L7805 or L7812 stabilizer is additionally installed in the power circuit and a calculation is made relative to the supply 5 or 12 Volts, respectively.
Ures=220-3=217 V
R=217/0.02=10850 Ohm
Since any diode passes current in one direction, reverse voltage will cause it to fail. This means that another similar or shunting conventional low-power rectifier diode, for example, 1n4007, is installed in parallel with the LED.
Using our online calculator, you can calculate the resistance for one or more LEDs connected in series or in a chain in parallel:
If there are several LEDs, then:
- For a series connection, the resistor is calculated taking into account the sum of the drops on each element.
- For a parallel connection, the resistance is calculated taking into account the sum of the currents of each light emitting diode.
Also, one should not forget about the power of the resistor, for example, in the second example with connecting the circuit to a 220V network, a power equal to:
P=217*0.02=4.34W
In this case, it will be a rather large resistor. To reduce this power, you can limit the current even more, for example, to 0.01A, which will reduce this power by half. In any case, the rated power of the resistance must be greater than that which will be released during its operation.
For long and stable operation of the emitter, when connected to the network, use a voltage slightly higher than the nominal voltage, that is, 230-240 V, in the calculations.
If it is difficult for you to calculate or you are not sure about something, then our online calculator for calculating the resistor for the LED will quickly tell you which resistor from the standard size range is needed, as well as its minimum power.
The operation of the LED is based on the emission of light quanta that occur when current flows through it. Depending on this, the brightness of the glow of the element also changes. With a small current, it shines dimly, and with a large current, it flares up and burns out. To limit the current flowing through it, the easiest way is to use resistance. It is not difficult to perform the correct calculation of the resistor, but it should be remembered that it only limits, but does not stabilize the current.
Principle of operation and properties
LED is a device having the ability to emit light. On printed circuit boards and diagrams, it is denoted by the Latin letters LED (Light Emitting Diode), which means “light emitting diode” in translation. Physically, it is a crystal placed in a case. Classically, it is considered to be a cylinder, one side of which has a convex rounded shape, which is a hemisphere lens, and the other is a flat base, and conclusions are located on it.
With the development of solid state technology and the reduction of process technology, the industry began to produce SMD diodes designed for surface mounting. Despite this, the physical principle of the LED has not changed and is the same for any type and color of the device.
The manufacturing process of the radiation device can be described as follows. At the first stage, a crystal is grown. This happens by placing an artificially made sapphire in a chamber filled with a gaseous mixture. The composition of this gas includes dopants and a semiconductor. When the chamber is heated, the resulting substance is deposited on the plate, while the thickness of such a layer does not exceed several microns. After the end of the deposition process by sputtering, contact pads are formed and this entire structure is placed in the housing.
Due to the peculiarities of production, LEDs do not have the same parameters and characteristics. Therefore, although manufacturers are trying to sort devices that are close in value, often in the same batch there are products that differ in color temperature and operating current.
Radio element device
A light-emitting diode or LED-diode is a semiconductor radio element, the operation of which is based on the properties of an electron-hole transition. When a current passes through it in the forward direction, recombination processes occur at the interface between two materials, accompanied by radiation in the visible spectrum.
For a very long time, the industry could not produce a blue LED, which is why it was impossible to obtain a white glow emitter. Only in 1990, researchers at the Japanese corporation Nichia Chemical Industries invented a technology for obtaining a crystal that emits light in the blue spectrum. This automatically made it possible to obtain white by mixing green, red and blue.
The process of radiation is based on the release of energy during the recombination of charges in the zone of the electron-hole transition. It is formed by contact of two semiconductor materials with different conductivity. As a result of injection, the transition of minor charge carriers, a barrier layer is formed.
On the side of the material with n-conductivity, a barrier of holes appears, and on the side with p-conductivity, of electrons. There is a balance. When voltage is applied in forward bias, there is a massive movement of charges into the band gap on both sides. As a result, they collide and energy is released in the form of light emission.
This light may or may not be visible to the human eye. It depends on the composition of the semiconductor, the amount of impurities, and the band gap. Therefore, the visible spectrum is achieved by fabricating multilayer semiconductor structures.
Characteristics of LEDs
The color of the glow depends on the type of semiconductor and the degree of its doping, which determines the width of the band gap of the p-n junction. The service life of LEDs primarily depends on the temperature conditions of its operation. The higher the heating of the device, the faster its aging occurs. And the temperature, in turn, is related to the current passing through the LED. The lower the power of the light source, the longer its service life. Aging is expressed as a decrease in the brightness of a light fixture. Therefore, it is so important to choose the right resistance for the LED.
The main characteristics of LED diodes include:
Connection methods
For the trouble-free operation of the LED, the value of the operating current is very important. Incorrect connection of radiation sources or a significant spread in their parameters during joint operation will lead to an excess of the current flowing through them and further burnout of the devices. This is due to an increase in temperature, due to which the LED crystal is simply deformed, and the p-n junction breaks through. Therefore, it is so important to limit the amount of current supplied to the light source, that is, to limit the supply voltage.
The easiest way to do this is to use a resistance connected in series with the emitter circuit. In this capacity, an ordinary resistor is used, but it must have a certain value. Its large value will not be able to provide the necessary potential difference for the occurrence of the recombination process, and a smaller value will burn it. In this case, you need not only to know how to calculate the resistance for the LED, but also to understand how to put it correctly, especially if the circuit is saturated with radio elements.
In an electrical circuit, both one LED and several can be used. In this case, there are three schemes for their inclusion:
- single;
- consistent;
- parallel.
single element
When only one LED is used in an electrical circuit, then one resistor is placed in series with it. As a result of such a connection, the total voltage applied to this circuit will be equal to the sum of the drops in the potential difference across each element of the circuit. If we designate these losses on the resistor as Ur, and on the LED as Us, then the total voltage of the EMF source will be equal to: Uo = Ur + Us.
Paraphrasing Ohm's law for the network section I \u003d U / R, the formula is obtained: U \u003d I * R. Substituting the resulting expression into the formula for finding the total voltage, we get:
Uo = IrRr + IsRs, where
- Ir is the current flowing through the resistor, A.
- Rr - calculated resistance of the resistor, Ohm.
- Is is the current passing through the LED, A.
- Rs is the internal impedance of the LED, Ohm.
The value of Rs varies depending on the operating conditions of the radiation source and its value depends on the current strength and potential difference. This dependence can be seen by studying the current-voltage characteristic of the diode. At the initial stage, there is a smooth increase in current, and Rs has a high value. After that, the impedance drops sharply and the current rapidly increases even with a slight increase in voltage.
If you combine the formulas, you get the following expression:
Rr = (Uo - Us) / Io, Ohm
This takes into account that the strength of the current flowing in the series circuit of the circuit section is the same at any point, that is, Io = Ir = Is. This expression is also suitable for connecting LEDs in series, because it also uses only one resistor for the entire circuit.
Thus, to find the desired resistance, it remains to find out the value of Us. The value of the voltage drop across the LED is a reference value and it has its own for each radio element. To obtain data, you will need to use the datasheet on the device. Datasheet is a set of information sheets that contain comprehensive information about the parameters, operating modes, as well as radio element switching schemes. Produced by the manufacturer of the product.
parallel circuit
With a parallel connection, the radio elements contact each other at two points - nodes. For this type of circuit, two rules are true: the strength of the current entering the node is equal to the sum of the strengths of the currents emanating from the node, and the potential difference at all points of the nodes is the same. Based on these definitions, we can conclude that in the case of a parallel connection of LEDs, the required resistor, located at the beginning of the node, is found by the formula: Rr = Uo / Is1 + In, Ohm, where:
- Uo is the potential difference applied to the nodes.
- Is1 is the current flowing through the first LED.
- In is the current passing through the nth LED.
But such a circuit with a common resistance located in front of the parallel connection of the LEDs is not used. This is due to the fact that in the event of a burnout of one emitter, according to the law, the current entering the node will remain unchanged. And this means that it will be distributed between the remaining working elements and at the same time more current will flow through them. As a result, a chain reaction will occur and all semiconductor emitters will eventually burn out.
Therefore, it will be correct to use your own resistor for each parallel branch with its own LED and calculate the resistor for the LED separately for each arm. This approach is also advantageous in that radioelements with different characteristics can be used in the circuit.
The calculation of the resistance of each arm is similar to a single inclusion: Rn = (Uo - Us) / In, Ohm, where:
- Rn is the desired resistance of the n-th branch.
- Uo - Us - voltage drop difference.
- In - current through the n-th LED.
Calculation example
Let the electrical circuit be powered by a constant voltage source equal to 32 volts. In this circuit, there are two brand LEDs connected in parallel to each other: Cree C503B-RAS and Cree XM-L T6. To calculate the required impedance, you will need to find out from the datasheet the typical voltage drop across these LEDs. So, for the first one it is 2.1 V at a current of 0.2, and for the second it is 2.9 V at the same current strength.
Substituting these values into the formula for a series circuit, you get the following result:
- R1 \u003d (U0-Us1) / I \u003d (32-2.1) / 0.2 \u003d 21.5 ohms.
- R2 \u003d (U0-Us2) / I \u003d (32-2.9) / 0.2 \u003d 17.5 ohms.
The nearest values are selected from the standard series. They will be: R1 = 22 ohms and R2 = 18 ohms. If desired, you can also calculate the power dissipated by the resistors using the formula: P \u003d I * I * U. For the found resistors, it will be P \u003d 0.001 W.
Browser based online calculators
With a large number of LEDs in the circuit, calculating the resistance for each is a rather tedious process, especially since a mistake can be made in this case. Therefore, it is easiest to use online calculators for calculations.
They are a program written to run in a browser. On the Internet you can find many such calculators for LEDs. but the principle of operation is the same. You will need to enter reference data in the proposed forms, select a connection scheme and click the "Result" or "Calculation" button. After that, it remains only to wait for a response.
By manually recalculating it, you can check it, but there will not be much point in this, since the programs use similar formulas when calculating.
LEDs are modern, economical, reliable radio elements used for light indication. We think everyone and everything knows about it! It is on the basis of this experience that the desire to use LEDs is so high for designing a wide variety of electrical circuits, both in consumer electronics and for cars. But here certain difficulties arise. After all, the most common LEDs have a supply voltage of 3 ... 3.3 volts, and the on-board voltage of the car is 12 volts, while sometimes it rises to 14 volts. Of course, a logical assumption pops up here that in order to connect the LEDs to the 12 volt network of the machine, it will be necessary to lower the voltage. It is this topic, connecting the LED to the vehicle's on-board network and lowering the voltage, that the article will be devoted to.
Two basic principles on how you can connect an LED to 12 volts or lower the voltage on the load
Before moving on to specific circuits and their descriptions, I would like to talk about two fundamentally different, but possible options for connecting an LED to a 12-volt network.
The first, this is when the voltage drops due to the fact that additional consumer resistance is connected in series with the LED, which is a voltage stabilizer microcircuit. In this case, a certain part of the voltage is lost in the microcircuit, turning into heat. This means that the second, remaining, goes directly to our consumer - the LED. Because of this, it does not burn out, since not all of the total voltage passes through it, but only a part. The advantage of using a microcircuit is the fact that it is able to automatically maintain a given voltage. However, there are also disadvantages. You will not be able to reduce the voltage below the level for which it is designed. Second. Since the microcircuit has a certain efficiency, the drop relative to the input and output will differ by 1-1.5 volts down. Also, to use the chip, you will need to use a good dissipative heatsink installed on it. After all, in fact, the heat released from the microcircuit is the loss that we have not claimed. That is, what we cut off from a greater potential in order to get a smaller one.
Second a power supply option for the LED, when the voltage is limited by a resistor. This is akin to taking a large water pipe and narrowing it down. In this case, the flow (flow rate and pressure) would decrease significantly. In this case, only part of the voltage reaches the LED. This means that it can also work without the danger of being burned. The disadvantage of using a resistor is that it also has its own efficiency, that is, it also wastes unclaimed voltage into heat. In this case, it can be difficult to install a resistor on the heatsink. As a result, it is not always suitable for inclusion in the circuit. Also, the fact that the resistor does not support automatic holding of the voltage within the specified limit will also be a minus. When the voltage drops in the common circuit, it will apply an equally lower voltage to the LED. Accordingly, the reverse situation will occur with an increase in voltage in the common circuit.
Of course, both options are not ideal, because when working from portable energy sources, each of them will spend part of the useful energy on heat. And this is relevant! But what to do, such is the principle of their work. In this case, the power source will spend part of its energy not on useful action, but on heat. Here, the panacea is the use of pulse-width modulation, but this greatly complicates the circuit ... Therefore, we will still focus on the first two options, which we will consider in practice.
Connecting an LED through a resistance to 12 volts in a car (through a resistor)
Let's start, as in the paragraph above, with the option of connecting the LED to a voltage of 12 volts through a resistor. In order for you to better understand how the voltage drop occurs, we will give several options. When 3 LEDs are connected to 12 volts, 2 and 1.
Connecting 1 LED through a resistance to 12 volts in a car (through a resistor)
So we have an LED. Its supply voltage is 3.3 volts. That is, if we took a 3.3 volt power source and connected an LED to it, then everything would be great. But in our case, there is an increased voltage, which is not difficult to calculate by the formula. 14.5-3.3= 11.2 volts. That is, we need to initially reduce the voltage by 11.2 volts, and then only apply voltage to the LED. In order for us to calculate the resistance, we need to know how much current flows in the circuit, that is, the current consumed by the LED. On average, this is about 0.02 A. If you wish, you can see the rated current in the datasheet for the LED. As a result, according to Ohm's law it turns out. R=11.2/0.02=560 Ohm. The resistor value is calculated. Well, drawing a diagram is even easier.
The power of the resistor is calculated by the formula P=UI=11.2*0.02=0.224 W. We take the closest one according to the standard type order.
Connecting 2 LEDs through a resistor to 12 volts in a car (through a resistor)
By analogy with the previous example, everything is calculated in the same way, but with one condition. Since there are already two LEDs, the voltage drop across them will be 6.6 volts, and the remaining 14.5-6.6 \u003d 7.9 volts will remain with the resistor. Based on this, the scheme will be as follows.
Since the current in the circuit has not changed, the power of the resistor remains unchanged.
Connecting 3 LEDs through a resistor to 12 volts in a car (through a resistor)
And one more option, when almost all the voltage is extinguished by LEDs. So, the resistor at its face value will be even less. Only 240 ohms. A diagram for connecting 3 LEDs to the on-board network of the machine is attached.
Finally, it only remains for us to say that the voltage used in the calculations was not 12, but 14.5 volts. It is this increased voltage that usually occurs in the electrical network of the machine when it is started.
It is also not difficult to estimate that when connecting 4 LEDs, you will not need to use any kind of resistor at all, because each of the LEDs will have 3.6 volts, which is quite acceptable.
Connecting an LED through a voltage regulator to 12 volts in a car (via a microcircuit)
Now let's move on to a stabilized 12 volt LED power supply circuit. Here, as we have already said, there is a circuit that regulates its own internal resistance. Thus, the power supply of the LED will be stable, regardless of power surges on the on-board network. Unfortunately, the disadvantage of using a microcircuit is the fact that the minimum stabilized voltage that can be achieved will be 5 volts. It is with this voltage that you can find the most widely known microcircuits - stabilizers KR142 EN 5B or a foreign analogue L7805 or L7805CV. Here the difference is only in the manufacturer and the rated operating current from 1 to 1.5 A.
So, the remaining voltage from 5 to 3.3 volts will have to be extinguished according to the same example as in the previous cases, that is, by using a resistor. However, reducing the voltage with a resistor by 1.7 volts is no longer as critical as by 8-9 volts. Voltage stabilization in this case will still be observed! Here is the connection diagram of the stabilizer microcircuit.
As you can see, it is very simple. Everyone can implement it. No more difficult than soldering the same resistor. The only condition is the installation of a heatsink that will remove heat from the microcircuit. It is a must to install it. The diagram says that the microcircuit can power 10 chains with an LED, in fact, this parameter is underestimated. In fact, if about 0.02A passes through the LED, then it can power up to 50 LEDs. If you need to provide more power, then use a second such independent circuit. Using two chips connected in parallel is not correct. Since their characteristics are slightly different from each other, due to individual characteristics. As a result, one of the microcircuits will have a chance to burn out much faster, since its operating modes will be different - overestimated.
We have already talked about the use of similar microcircuits in the article "5 volt charger in the car". By the way, if you still decide to power the PWM LED, although it is hardly worth it, then this article will also reveal to you all the secrets of implementing such a project.
Summing up about connecting an LED to 12 volts in a car with your own hands
Summing up the connection of the LED to a 12 volt network, we can say about the simplicity of the circuit design. As with the case where a resistor is used, so with a microcircuit - a stabilizer. All this is easy and simple. At least, this is the simplest thing that you can meet in electronics. So everyone should master the connection of the LED to the on-board network of the car at 12 volts and for sure. If even this is not “too tough”, then more complex ones should not be taken at all.
Video on connecting the LED to the network in the car
And now, to make it easier for you to figure out what resistance value is needed and what power for your particular case, you can use the resistor selection calculator
This article will talk about current limiting resistor calculation for the LED.
Resistor calculation for one LED
To power one LED, we need a power source, for example, two AA batteries of 1.5V each. Let's take a red LED, where the direct voltage drop at an operating current of 0.02 A (20mA) is -2 V. For ordinary LEDs, the maximum allowable current is 0.02 A. The LED connection diagram is shown in Fig. 1.
Why do I use the term "forward voltage drop", not the supply voltage. But the fact is that LEDs do not have a supply voltage parameter as such. Instead, the voltage drop characteristic of the LED is used, which means the amount of voltage at the output of the LED when the rated current passes through it. The voltage value indicated on the packaging reflects just the voltage drop. Knowing this value, you can determine the remaining voltage on the LED. It is this value that we need to use in the calculations.
The forward voltage drop for various LEDs depending on the wavelength is presented in Table 1.
Table 1 - Characteristics of LEDs
The exact value of the LED voltage drop can be found on the packaging for this LED or in the reference literature.
R \u003d (Un.p - Ud) / Id \u003d (3V-2V) / 0.02A \u003d 50 Ohm.
- Un.p – supply voltage, V;
- Ud - direct voltage drop across the LED, V;
Since there is no such resistance in the standard series, we select the nearest resistance from the nominal series E24 in the direction of increase - 51 Ohm.
In order to guarantee the long operation of the LED and eliminate the error in the calculations, I recommend using not the maximum allowable current - 20 mA, but a little less - 15 mA in the calculations.
This decrease in current will not affect the brightness of the LED for the human eye. In order for us to notice a change in the brightness of the LED, for example, by 2 times, we need to reduce the current by 5 times (according to the Weber-Fechner law).
As a result, we get the calculated resistance of the current-limiting resistor: R \u003d 50 Ohms and power dissipation P \u003d 0.02 W (20mW).
Calculation of the resistor for serial connection of LEDs
In the case of a series resistor calculation, all LEDs must be of the same type. The scheme for connecting LEDs in a serial connection is shown in Fig. 2.
For example, we want to connect to a 9 V power supply, three green LEDs, each 2.4 V, the operating current is 20 mA.
The resistance of the resistor is determined by the formula:
R \u003d (Un.p - Ud1 + Ud2 + Ud3) / Id \u003d (9V - 2.4V + 2.4V + 2.4V) / 0.02A \u003d 90 Ohm.
- Un.p – supply voltage, V;
- Ud1 ... Ud3 - direct voltage drop across the LEDs, V;
- Id is the operating current of the LED, A.
We select the nearest resistance from the nominal series E24 in the direction of increase - 91 Ohm.
Calculation of resistors for parallel-series connection of LEDs
Often in practice, we need to connect a large number of LEDs to the power supply, several dozen. If all the LEDs are connected in series through one resistor, then in this case the voltage at the power source will not be enough for us. The solution to this problem is the parallel-serial connection of LEDs, as shown in Fig. 3.
Based on the voltage of the power supply, the maximum number of LEDs that can be connected in series is determined.
Fig. 3 - LED connection diagram for parallel-serial connection
For example, we have a 12 V power supply, based on the voltage of the power supply, the maximum number of LEDs for one circuit will be: 10V / 2V = 5 pcs, given that the voltage drop on the LED (red) is 2 V.
Why we took 10 V, and not 12 V, is due to the fact that there will also be a voltage drop across the resistor and we must leave somewhere around 2 V.
The resistance of the resistor for one circuit, based on the operating current of the LEDs, is determined by the formula:
R \u003d (Un.p - Ud1 + Ud2 + Ud3 + Ud4 + Ud5) / Id \u003d (12V - 2V + 2V + 2V + 2V + 2V) / 0.02A \u003d 100 Ohm.
We select the nearest resistance from the nominal series E24 upwards - 110 ohms.
The number of such chains of five LEDs connected in parallel is practically unlimited!
Calculation of the resistor for parallel connection of LEDs
This connection is not desirable and I do not recommend using it in practice. This is due to the fact that each LED has a technological voltage drop, and even if all the LEDs are from the same package, this is not a guarantee that their voltage drop will be the same due to the production technology.
As a result, one LED will have more current than the others, and if it exceeds the maximum allowable current, it will fail. The next LED will burn out faster, as the remaining current will already pass through it, distributed among the other LEDs, and so on until all the LEDs fail.
This problem can be solved by connecting a resistor to each LED, as shown in Fig. 5.
