Resistor calculation for LED: resistance selection formulas, online calculator

The operation of an LED is based on the emission of light quanta that arise when a current flows through it. Depending on this, the brightness of the element's glow also changes. At a low current, it shines dimly, and at a large current, it flashes and burns out. The easiest way to limit the current flowing through it is to use a resistance. It is not difficult to correctly calculate the resistor, but it should be remembered that it only limits, but does not stabilize the current.

Working principle and properties

LED is a devicewith the ability to emit light. On printed circuit boards and circuits, it is denoted by the Latin letters LED (Light Emitting Diode), which means "light emitting diode". 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 hemispherical lens, and the other is a flat base, and the leads are located on it.

With the development of solid state technology and the reduction of the technological process, the industry began to produce SMD diodes intended for surface mount installation. Despite this, the physical principle of operation of the LED has not changed and is the same for any type and for the color of the device.

The manufacturing process of the radiation device can be described as follows. In the first stage, a crystal is grown. This happens by placing an artificially made sapphire in a chamber filled with a gaseous mixture. This gas contains 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, the contact pads are formed by the sputtering method and the whole structure is placed in the case.

Due to the peculiarities of production, there are no LEDs with the same parameters and characteristics. Therefore, although manufacturers try to sort devices that are close in value, often in the same batch there are products that differ in color temperature and operating current.

Radioelement device

A light-emitting diode or LED-diode is a semiconductor radioelement, the operation of which is based on the properties of an electron-hole junction. 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 emitter. Only in 1990, researchers at the Japanese corporation Nichia Chemical Industries invented a technology for producing a crystal that emits light in the blue spectrum. This automatically made it possible by mixing green, red and blue to get white.

The radiation process is based on the release of energy during the recombination of charges in the zone of the electron-hole transition. It is formed by the contact of two semiconductor materials with different conductivity. As a result of injection, transition of minority charge carriers, a blocking layer is formed.

On the side of the material with n-conductivity, a barrier of holes arises, and on the side with p-conductivity, of electrons. Balance comes. When voltage is applied in forward bias, there is a massive movement of charges into the forbidden zone on both sides. As a result, they collide and energy is released in the form of radiation of light.

This light may or may not be visible to the human eye. It depends on the composition of the semiconductor, the amount of impurities, the band gap. Therefore, the visible spectrum is achieved through the manufacture of multilayer semiconductor structures.

LED characteristics

The color of the glow depends on the type of semiconductor and the degree of its doping, which determines the width of the bandgap 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 it will age. 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 the device light. Therefore, it is so important to choose the right resistance for the LED.

The main characteristics of LED diodes include:

1. Current consumption. Single-chip LEDs consume a current of 0.02 A. At the same time, its value grows in direct proportion to the number of crystals. So, a diode with four crystals consumes a current of 0.08 A. It is because of this parameter of the diode that a limiting resistor is installed so that it does not burn out at a high current strength.
2. The magnitude of the voltage drop. This characteristic indicates how much energy is released on the LED, that is, by how many volts the voltage value will decrease when it is connected in parallel to the circuit. For example, if the drop is 3 volts and the input voltage is 9 volts, then when connected in parallel to the LED power supply, the output voltage will be 6 volts.
3. Light output. This characteristic shows the amount of light emitted by the device when the power consumption is equal to one watt.
4. Color temperature. It depends on the drive current, heat dissipation efficiency and ambient temperature. The intense flux of light associated with electrical power consumption also increases the temperature. At the same time, it should be noted that temperature drops significantly reduce the life of the LED.
5. Standard size. Its value depends on the size of the emitter. Accordingly, the larger the size of the LED, the greater its brightness and power.

Connection methods

For trouble-free operation of the LED, the operating current value is very important. Incorrect connection of radiation sources or a significant scatter of 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 will break 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 using a resistance connected in series to the emitter circuit. An ordinary resistor is used in this capacity, but it must have a certain value. Its large value will not be able to provide the required potential difference for the occurrence of the recombination process, and a smaller value will burn out. 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 radioelements.

One LED or several LEDs can be used in an electrical circuit. At the same time, 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 potential difference drops 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: Uo = Ur + Us.

Paraphrasing Ohm's law for the network section I = U / R, the formula is obtained: U = I * R. Substituting the resulting expression in the formula for finding the total voltage, we get:

Uo = IrRr + IsRs, where

• Ir is the current flowing through the resistor, A.
• Rr is the calculated resistance of the resistor, Ohm.
• Is is the current passing through the LED, A.
• Rs - LED internal impedance, Ohm.

The Rs value changes depending on the operating conditions of the radiation source and its value depends on the current strength and the potential difference. This dependence can be seen by studying the current-voltage characteristic of the diode. At the initial stage, the current increases smoothly, and Rs is high. After that, the impedance drops sharply and the current rises rapidly even with a slight increase in voltage.

If you combine the formulas, you get the following expression:

Rr = (Uo - Us) / Io, Ohm

In this case, it is taken into account that the strength of the current flowing in the serial circuit of the section of the circuit 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 for the entire circuit. resistor.

Thus, to find the required resistance, it remains to find out the value of Us. The voltage drop across the LED is a reference value and it has its own for each radio element. To get the data, you need to use the datasheet on the device. Datasheet is a set of information sheets that contain comprehensive information about the parameters, modes of operation, as well as the circuit for switching on the radio element. It is produced by the manufacturer of the product.

Parallel circuit

With a parallel connection, radioelements are in contact with each other at two points - nodes. For this type of circuit, two rules are valid: the current entering the node is equal to the sum 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 LEDs, the desired 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 unit will remain unchanged. This means that it will be distributed among the remaining working elements and more current will flow through them. The result will be a chain reaction and all semiconductor emitters will eventually burn out.

Therefore, it will be correct to use its own resistor for each parallel branch with its own LED and calculate the resistor for the LED separately for each leg. 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 connection: Rn = (Uo - Us) / In, Ohm, where:

• Rn is the required resistance of the n-th branch.
• Uo - Us - voltage drop difference.
• In is the current through the n-th LED.

Calculation example

Let the electrical circuit be powered from a 32 volt constant voltage source. In this circuit there are two LEDs of the brand connected in parallel to each other: Cree C503B-RAS and Cree XM-L T6. To calculate the required impedance, you need to find out from the datasheet the typical voltage drop across these LEDs. So, for the first, it is 2.1 V at a current of 0.2, and for the second - 2.9 V at the same current strength.

Substituting these values ​​into the daisy chain formula gives the following result:

• R1 = (U0-Us1) / I = (32-2.1) / 0.2 = 21.5 ohms.
• R2 = (U0-Us2) / I = (32-2.9) / 0.2 = 17.5 Ohm.

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 = I * I * U. For the resistors found, it will be P = 0.001 W.

Browser online calculators

With a large number of LEDs in the circuit, calculating the resistance for each is a rather tedious process, especially since you can make a mistake. Therefore, the easiest way to make calculations is to use online calculators.

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 their work 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, you just have to wait for an answer.

Having recalculated it manually, you can check it, but there will not be much sense in this, since when calculating the program, similar formulas are used.