Calculation of damping resistance. Calculation and selection of resistance for an LED
An LED is a device that emits light when current passes through it.
Depending on the type of material used to make the device, LEDs can emit light of different colors. These miniature, reliable, economical devices are used in technology, lighting and advertising purposes.
The LED has the same current-voltage characteristic as a conventional semiconductor diode. Moreover, as the forward voltage across the LED increases, the current passing through it increases sharply.
For example, for a green LED type WP710A10LGD from Kingbright, when the applied forward voltage changes from 1.9 V to 2 V, the current changes by a factor of 5 and reaches 10 mA. Therefore, when an LED is directly connected to a voltage source, with a small change in voltage, the LED current can increase to a very large value, which will lead to the burning of the p-n junction and the LED.
carried out using letters and numbers, with the help of which you can determine the quality characteristics of devices.
Therefore, when connecting LEDs in parallel, each device usually has its own limiting resistor connected in series. Calculating the resistance and power of such a resistor is no different from the previously considered case.
When switching LEDs on in series, it is necessary to switch on devices of the same type.
In addition, it must be taken into account that the source voltage must be no less than the total operating voltage of the entire group of LEDs.
The calculation of the current-limiting resistor for LEDs in series connection is considered the same as before. The exception is that when calculating, instead of the value Usv, the value Usv*N is used. In this case, N is the number of devices turned on.
Conclusions:
- LEDs are widely used devices used in technology, lighting and advertising.
- To avoid failure of LEDs due to their sensitivity to voltage changes, limiting resistors are often used for them.
- The calculation of the resistance value of the limiting resistor is done based on Ohm's law.
Calculation of a resistor for connecting LEDs on video
The LED has very little internal resistance; if it is connected directly to the power supply, the current will be high enough to burn it out. The copper or gold threads that connect the crystal to the external pins can withstand small surges, but if they exceed too much, they burn out and power stops flowing to the crystal. Online LED resistor calculation is based on its rated operating current.
- 1. Online calculator
- 2. Basic parameters
- 3. Features of cheap LEDs
Online calculator
Draw up a connection diagram in advance to avoid errors in calculations. The online calculator will show you the exact resistance in Ohms. As a rule, it will turn out that resistors with this value are not produced, and you will be shown the nearest standard value. If you cannot make an accurate selection of resistance, then use a larger value. A suitable value can be made by connecting the resistance in parallel or in series. You don't have to calculate the resistance for an LED if you use a powerful variable or trimming resistor. The most common type is 3296 at 0.5W. When using a 12V power supply, up to 3 LEDs can be connected in series.
Resistors come in different accuracy classes, 10%, 5%, 1%. That is, their resistance can vary within these limits in a positive or negative direction.
Do not forget to take into account the power of the current-limiting resistor, this is its ability to dissipate a certain amount of heat. If it is small, it will overheat and fail, thereby breaking the electrical circuit.
To determine the polarity, you can apply a small voltage or use the diode test function on a multimeter. Different from resistance measurement mode, usually supplied from 2V to 3V.
Main settings
Also, when calculating LEDs, you should take into account the spread of parameters; for cheap ones they will be maximum, for expensive ones they will be more the same. To check this parameter, you need to enable them under equal conditions, that is, sequentially. By reducing the current or voltage, reduce the brightness to slightly glowing points. Visually, you will be able to estimate that some will glow brighter, others dimly. The more evenly they burn, the less spread. The LED resistor calculator assumes that the characteristics of the LED chips are ideal, that is, the difference is zero.
The drop voltage for common low-power models up to 10W can be from 2V to 12V. As power increases, the number of crystals in a COB diode increases; each has a drop. The crystals are connected in chains in series, then they are combined into parallel circuits. At powers from 10W to 100W the reduction increases from 12V to 36V.
This parameter must be indicated in the technical characteristics of the LED chip and depends on the purpose:
- colors blue, red, green, yellow;
- three-color RGB;
- four-color RGBW;
- two-tone, warm and cool white.
Features of cheap LEDs
Before choosing a resistor for an LED using an online calculator, you should make sure of the parameters of the diodes. The Chinese sell a lot of LEDs on Aliexpress, passing them off as branded ones. The most popular models are SMD3014, SMD 3528, SMD2835, SMD 5050, SMD5630, SMD5730. All the bad stuff is usually made under the Epistar brand.
For example, most often the Chinese cheat on SMD5630 and SMD5730. The numbers in the markings only indicate the case size of 5.6mm by 3.0mm. In branded ones, such a large case is used to install powerful 0.5W crystals, so buyers of SMD5630 diodes directly associate it with 0.5W power. The cunning Chinese takes advantage of this and installs a cheap and weak crystal in the 5630 case with an average power of 0.1W, while indicating the energy consumption of 0.5W.
Chinese LED corn lamps
A good example would be car lamps and LED corn lamps, which contain a large number of weak and low-quality LED chips. The average buyer believes that the more LEDs, the better the light and the higher the power.
Car lamps on the weakest ice 0.1W
To save money, my LED colleagues are looking for decent LEDs on Aliexpress. They look for a good seller who promises certain parameters, order, and wait a month for delivery. After tests, it turns out that the Chinese seller cheated and sold junk. You'll be lucky if the seventh time you get decent diodes and not junk. Usually they make 5 orders, and without achieving results, they go to place an order in a domestic store that can make an exchange.
LED is a semiconductor element, which is used for lighting. Used in lanterns, lamps, lamps and other lighting devices. 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 LED operation depends on current flowing through it. If the values are too low, it simply will not shine, and if the current value is exceeded, the characteristics of the element will deteriorate, even to the point of destruction. At the same time they say that the LED has burned out. In order to eliminate the possibility of failure of this semiconductor, it is necessary to select it in a circuit with a resistor included in it. It will limit the current in the circuit to optimal values.
LED resistance is nonlinear. This means that when the voltage applied to this element changes, the current flowing through it will change nonlinearly. You can verify this if you find the volt-ampere characteristic of any diode, including light-emitting diodes. 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. Once this threshold is exceeded, the current through the element rapidly increases and it begins to glow.
If power supply connect directly to the LED, the diode will fail, since it is not designed for such a load. To prevent this from happening, you need to limit the current flowing through the LED with a ballast resistance, or lower the voltage on a semiconductor that is important to us.
Let's consider the simplest connection diagram (Figure 1). The DC power source 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 a 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 approximately 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 – from 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 liquid 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 across all elements included in a circuit flowing one current is equal to the EMF of this circuit (in Figure 1 indicated by the letter E). From this we can conclude that the voltage dropping 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 obtained through both the resistor and the LED is the same. For stable operation of a semiconductor element, increasing its reliability and durability, the current through it must be of certain values indicated in its description. If a description cannot be found, you can take the approximate value of the current in the circuit as 10 milliamps. After determining this data, you can already calculate the resistor value 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 = 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 resistor resistance. However, the industry produces only standard resistance values, the so-called rating series. Therefore, after the calculation, you will have to select the existing resistance value. You need to select a slightly larger resistor than what was calculated, this way you will have protection against accidental excess voltage in the network. If it is difficult to select an element that is close in value, you can try connecting two resistors in series or in parallel.
If you choose a resistance with less power than needed in the circuit, it will simply fail. Calculating 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 select 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 across the green LED we need is 2.4 V, the rated current is 20 mA. From here we calculate the voltage dropping across the ballast resistor.
U (R) \u003d E - U (Led) \u003d 12V - 2.4V \u003d 9.6V.
Resistance value:
R = U (R)/ I = 9.6V/0.02A = 480 Ohm.
Power value:
P = U (R) ⋅ I = 9.6V ⋅ 0.02A = 0.192 W
From a number of standard resistances we select 487 Ohms (E96 series), and the power can be selected at 0.25 W. This resistor must be ordered.
If you need to connect several LEDs in series, you can also connect them to the power source using only one resistor, which will suppress excess voltage. Its calculation is carried out using the above formulas, however, instead of one forward voltage U (Led), you need to take the sum of the forward voltages of the required LEDs.
If you need to connect several light-emitting elements in parallel, then for each of them you need to calculate its own resistor, since each of the semiconductors may 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 the correct calculations, you must do the following:
- Finding out the forward voltage and current of the LED.
- Calculation of the voltage drop across the desired resistor.
- Calculation of resistor resistance.
- 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 a resistor for an LED. If you enter such a query into a search engine, you will find many sites offering automated calculations. 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. All that remains is to competently use the search engine to solve these problems.
LEDs are the optimal solution for many lighting problems 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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This is what the LED looks like in real life:
And this is how it is indicated in the diagram: 
What is the LED used for?
LEDs emit light when electrical current passes through them.
They were invented in the 70s of the last century to change light bulbs, which often burned out and consumed a lot of energy.
Connection and soldering
The LEDs must be connected in the correct way, taking into account their polarity + for the anode and k for the cathode. The cathode has a short lead, a shorter leg. If you see the inside of the LED, the cathode has a larger electrode (but this is not an official method).
LEDs can be damaged by the heat of soldering, but the risk is small if you solder quickly. No special precautions need to be taken when soldering most LEDs, but it can be useful to grab the LED leg with tweezers for heat dissipation.
Checking LEDs
Never connect LEDs directly to a battery or power source!
The LED will burn out almost instantly because too much current will burn it out. LEDs must have a limiting resistor. For quick testing, a 1k ohm resistor is suitable for most LEDs if the voltage is 12V or less. Don't forget to connect the LEDs correctly, observing the polarity! 
LED colors
LEDs come in almost every color: red, orange, amber, amber, green, blue and white. Blue and white LED are a little more expensive than other colors.
The color of LEDs is determined by the type of semiconductor material from which it is made, and not by the color of the plastic of its housing. LEDs of any color come in a colorless case, in which case the color can only be found out by turning it on...
Multicolor LEDs
A multicolor LED is designed simply; as a rule, it is red and green combined into one housing with three legs. By changing the brightness or the number of pulses on each crystal, you can achieve different glow colors.
LED resistor calculation
An LED must have a resistor connected in series in its circuit to limit the current passing through the LED, otherwise it will burn out almost instantly...
Resistor R is determined by the formula:
R = (V S - V L ) / I
VS = supply voltage
V L = forward voltage calculated for each type of diode (usually 2 to 4 volts)
I = LED current (for example 20mA), this should be less than the maximum allowed for your diode
If the resistance size cannot be selected accurately, then take a resistor of a larger value. In fact, you will hardly notice the difference... the brightness of the glow will decrease quite slightly.
For example: If the supply voltage is VS = 9V, and there is a red LED (V = 2V) requiring I = 20mA = 0.020A,
R = (- 9 V) / 0.02A = 350 Ohm. In this case, you can select 390 Ohms (the closest standard value, which is larger).
Calculating an LED Resistor Using Ohm's Law
Ohm's law states that the resistance of a resistor is R = V / I, where:
V = voltage across resistor (V = S - V L in this case)
I = current through resistor
So R = (V S - V L ) / I
Serial connection of LEDs.
If you want to connect several LEDs at once, this can be done in series. This reduces energy consumption and allows you to connect a large number of diodes at the same time, for example, as some kind of garland.
All LEDs that are connected in series must be of the same type. The power supply must have sufficient power and provide the appropriate voltage.
Calculation example:
Red, yellow and green diodes - when connected in series, a supply voltage of at least 8V is required, so a 9-volt battery will be an almost ideal source.
V L = 2V + 2V + 2V = 6V (three diodes, their voltages are summed).
If supply voltage VS is 9V and diode current = 0.015A,
Resistor R = (V S - V L ) / I= (9 - 6) /0.015 = 200 Ohm
We take a 220 Ohm resistor (the nearest standard value, which is larger).
Avoid connecting LEDs in parallel!
Connecting several LEDs in parallel using one resistor is not a good idea...
As a rule, LEDs have a range of parameters, each requiring slightly different voltages... which makes such a connection practically unworkable. One of the diodes will glow brighter and take on more current until it fails. This connection greatly accelerates the natural degradation of the LED crystal. If LEDs are connected in parallel, each LED must have its own limiting resistor.
Flashing LEDs
Flashing LEDs look like regular LEDs, they can blink on their own because they contain a built-in integrated circuit. The LED flashes at low frequencies, usually 2-3 flashes per second. Such trinkets are made for car alarms, various indicators or children's toys.
Alphanumeric LED indicators
LED alphanumeric indicators are now used very rarely; they are more complex and more expensive than liquid crystal ones. Previously, this was practically the only and most advanced means of display; they were even installed on cell phones :)
(light-emitting diode) - emits light when electric current flows through it. The simplest circuit for powering LEDs consists of a power supply, an LED, and a resistor connected in series with it.
This is often called a ballast or current-limiting resistor. The question arises: “Why does an LED need a resistor?” A current limiting resistor is needed to limit the current flowing through the LED to protect it from burning out. If the power supply voltage is equal to the voltage drop across the LED, then such a resistor is not necessary.
Calculation of a resistor for an LED
The resistance of the ballast resistor is easy to calculate using Ohm's law and Kirchhoff's rules. To calculate the required resistor resistance, we need to subtract the rated voltage of the LED from the power supply voltage, and then divide this difference by the operating current of the LED:
- V - power supply voltage
- V LED - voltage drop across the LED
- I – LED operating current
Below is a table of the dependence of the operating voltage of the LED on its color:
Although this simple circuit is widely used in consumer electronics, it is not very efficient because excess power from the power supply is dissipated as heat in the ballast resistor. Therefore, more complex schemes () are often used, which are more efficient.
Let's use an example to calculate the resistor resistance for an LED.
We have:
- power supply: 12 volts
- LED voltage: 2 volts
- LED operating current: 30 mA
Let's calculate the current limiting resistor using the formula:
It turns out that our resistor should have a resistance of 333 Ohms. If it is not possible to select the exact value from, then it is necessary to take the nearest higher resistance. In our case it will be 360 Ohm (series E24).
Series connection of LEDs
Often several LEDs are connected in series to one voltage source. When identical LEDs are connected in series, their total current consumption is equal to the operating current of one LED, and the total voltage is equal to the sum of the drop voltages of all LEDs in the circuit.
Therefore, in this case, it is enough for us to use one resistor for the entire sequential chain of LEDs.
An example of calculating the resistance of a resistor for a series connection.
In this example, two LEDs are connected in series. One red LED with a voltage of 2V and one ultraviolet LED with a voltage of 4.5V. Let's say both have a current rating of 30 mA.
From Kirchhoff's rule it follows that the sum of the voltage drops in the entire circuit is equal to the voltage of the power source. Therefore, the voltage across the resistor must be equal to the voltage of the power source minus the sum of the voltage drop across the LEDs.
Using Ohm's law, we calculate the resistance value of the limiting resistor:
The resistor must have a value of at least 183.3 ohms.
Note that after subtracting the voltage drop we still have 5.5 volts left. This makes it possible to connect another LED (of course, having previously recalculated the resistor resistance)
Parallel connection of LEDs
You can also connect LEDs in parallel, but this creates more problems than with a serial connection.
Limiting the current of parallel-connected LEDs with one common resistor is not a good idea, since in this case all LEDs must have exactly the same operating voltage. If any LED has a lower voltage, then more current will flow through it, which in turn can damage it.
And even if all LEDs have the same specification, they may have different current-voltage characteristics due to differences in the manufacturing process. This will also result in a different current flowing through each LED. To minimize current differences, LEDs connected in parallel usually have a ballast resistor for each link.
Online calculator for calculating a resistor for an LED
This online calculator will help you find the required resistor value for an LED connected according to the following diagram:
note: the tenth separator is a period, not a comma
Formula for calculating resistor resistance online calculator
Resistor resistance= (U– U F)/ I F
- U- power supply;
- U F– forward voltage of the LED;
- I F– LED current (in milliamps).
Note: It is too difficult to find a resistor with the resistance that was obtained in the calculation. As a rule, resistors are produced in standard values (nominal range). If you cannot find the resistor you need, then select the nearest higher resistance value that you calculated.
