Choosing a resistor for a diode

Publish date 2021-08-12
Often , the LED diode was associated with control lamps in RTV equipment, household appliances, in industrial controllers or in cars, but it is thanks to its small dimensions, durability and very low power consumption compared to the corresponding light bulbs that it began to take over the market. What is an LED? LED Light Emitting Diode means a light emitting diode, also known as a light emitting diode, it is a semiconductor optoelectronic device that converts electricity into electromagnetic radiation energy, i.e. light. The simplest The LED is a semiconductor pn junction which, when biased in the direction of conduction with a sufficiently high voltage, emits electromagnetic radiation in the range from infrared (IR) through the spectrum of visible light to ultraviolet (UV) radiation. Construction and principles of operation of the LED diode The main element of an LED is a semiconductor (crystal) that conducts electricity in only one direction. There are two electrodes under the light source housing. After connecting the positive pole of the power supply to the anode and the negative pole to the cathode, the current flows. LED operation it is based on the recombination of charge carriers, consisting in the transition of electrons from a higher energy level to a lower one, while maintaining momentum by them. The electron energy is converted into quanta of electromagnetic radiation. It should be added that LEDs also use the phenomenon of luminescence, in other words, the emission of light waves under the influence of non-thermal factors. It is formed in the semiconductor pn junction In the "n" layer, electrons are accumulated, and in the "p" layer, the so-called holes. When a current is applied, the electrons jump to the holes, giving off energy in the form of light.

Calculation of the resistor for the LED

At the beginning, it should be remembered that when connecting the LED to the system, it is necessary to use a resistor , because without it, the diode will be completely damaged as a result of drawing too much current. The forward current is the parameter that determines the maximum current that can flow through the diode without the risk of damaging it. It depends on the material of a given diode (for a rectifying diode it is silicon, and for LEDs, the semiconductor material depends on the color of a given diode). Forward voltage is the minimum voltage at which the selected LED will glow due to current flow. Information on the maximum current conduction characterizing a given diode can be found in catalog information, but it can be difficult, especially when we do not know the origin of our diode. Typically LEDs have a maximum forward current of 20-30mA, but most traditional LEDs shine well at less than 10mA, most typically 2-3mA. The voltage on the diode depends on the color of the light, and therefore also on the material from which it is made. So how to choose a resistor for a diode. It is not difficult if we apply the basic laws of electrical circuits. Second law of Kirchhoff says that the sum of the voltage drops on individual elements of the system is equal to the supply voltage of this system. Using this law, the sum of the voltage drops across the diode and resistor must be equal to the supply voltage.

U a = U diode + U R

After transforming the formula, the voltage across the resistor is equal to the difference between the supply voltage and the voltage across the diode

U R = U su - U diode

Once we know the voltage that is deposited on the resistor, we are able to determine which resistor to choose so that the diode is not damaged. According to the formula, the resistance is the quotient of the voltage on the resistor and the current flowing in the system. After substituting voltage and current in the formula, the resistance we need can be calculated from the formula. R = U / I


We have a yellow LED diode which has no markings on it. We read the forward voltage from the table, it will be reasonable to 2.1 V. We set the current flowing in the system to 8 mA. The supply voltage is 12V According to the formula, the resistor should be: R = (12-2.1) / 0.008 = 1237.5 Ohm We will not find a resistor with this resistance in stores, so we have to use a different resistor with a resistance close to the calculated one. Here, a 1.2 kOhm resistor was used. This will not have too much effect on the current flowing in the system, as it will change by such a small amount that you should not worry about it. You can see what the difference will be by converting the formula to resistance. So: I = U / R, hence I = (12-2.1) / 1237.5 = 8mA I = (12-2.1) / 1200 = 8.25mA (Fig No.1) The forward voltage of the diode is the average value of the read voltages from the table. We can check how the current flowing in the system will change if we substitute the extreme values from the table. We assume, of course, that our power source is ideally 12V and our resistor is ideally 1.2kOhm. Maximum current = (12-2) / 1200 = 8.33mA Minimum current = (12-2.3) / 1200 = 8.083mA As you can see, the maximum difference between the current values is about 0.25mA, so only approximately 3%. Not only is the current flowing in the system important, it is also worth paying attention to the supply voltage, because when it is too low, voltage fluctuations will drastically change the current flowing. We must remember that the highest possible voltage should be deposited on the resistor, which will result in better stabilization of the diode's forward current. Here, however, the golden mean may be the best solution, because the higher the voltage is deposited on the resistor, the greater the power drawn from the power source.

LEDs in InterElcom

An example of a diode with a wide range of applications is LED 3mm diffusion green. This diode is one of the the most popular LEDs. It emits radiation in the range of visible light. Perfect for creating a variety of projects. We are an authorized distributor of electronic components, including LEDs. We sell wholesale, ensuring the highest quality. We place particular emphasis on ensuring that the products we offer are available immediately and at a competitive price. We follow the needs of our clients to constantly supplement our offer with new products. Producers of electronic components and devices cooperating with us are leading brands from all over the world. As an independent distributor, InterElcom also has the largest optoelectronics warehouse in Poland. This makes InterElcom products one of the best available on the market. Many years of experience, as well as the continuous development of InterElcom employees, ensure professional service and comprehensive assistance in the selection of electronic components. We want the customer to be aware of the choice of a dedicated solution when deciding on the products from our offer.   Sources: