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The first thing I would like to tell you is that there is already a lot of information about what resistance to putting on an LED. This post is not designed to calculate the value of resistance without further, but to understand why this value and how to adapt it to your circuit.
If you are starting with this of electronics, surely you have already asked yourself the question of What resistance does an LED carry? It is one of the typical problems that come to everyone who starts with this DIY electronics so calm.
What Resistance Does an LED Carry?
To answer this question, you must first know why an LED needs a resistance and, consequently, what effect the LED is causing on your circuit (apart from glowing lol).
What Effect Does (At the Electric Level) Add an LED to a Circuit?
Surely you already know that electronic circuits are governed by a law known as Ohm’s law. This law establishes that the voltage drop in an element of the circuit is equal to the current (current) that flows through it multiplied by the resistance that that component exerts to the passage of the current.
In your circuit, the voltage with which you are going to work is the one that gives you the source to which you have it connected (either a battery, battery …) and the resistance are made up of all the elements that hinder the passage of the current (including your LED).
In the same way that your components present resistance to the passage of current, they also cause a voltage drop.
Note: Not all components act according to the above. There are, for example, elements that increase the voltage and current, such as a transistor.
An LED is a component in which the voltage that falls depends on the intensity of the current flowing through it, as long as this current circulates in the correct direction (if it circulates in the opposite direction, the LED melts).
However, unless you make the current flowing through your LED vary a lot, you can consider that the voltage drop is constant and, depending on the type of LED you have (color and size), its value will be between 1, 8V and 3.6V. In addition to the voltage drop that generates, your LED also has an internal resistance, only very small.
Concluding: Assembling an LED in a project causes a constant voltage drop without adding scarce resistance to the circuit.
What happens if you put the LED without resistance?
Suppose you have a red 1.8V LED and a 9V battery. Let’s see what happens if you connect them without the resistance:
As you can see, if you do not connect the resistance you are making 3.6 Amps circulate through your LED. This is a real madness. We are talking about really dangerous current values.
A standard LED supports an intensity of up to 20mA, that is, the maximum current that your LED can support is running 180 times. Even in the hypothetical case that you had an LED that could withstand that current (they do not exist) A 9V battery like the one you can buy in a supermarket does not reach 1A of charge, that is, the battery would be used immediately.
The solution (as you imagine) is to add resistance in series, that is, force the current to pass through the LED and the resistance, limiting, reducing, the current flowing through your LED.
LED resistance value
At this point of the post, it is quite likely that you have already realized how to calculate the value of the resistance. Simply apply Ohm’s law considering that the maximum intensity that can circulate through the LED is 20mA.
If you want your LED to shine brightly but without running the risk of it melting, a good intensity value would be 17mA. This is because the components are not perfect and their nominal values (those that appear in the specification sheets) reflect the average value of that component (they are indicative). In the case of resistance, depending on the material from which the value is made, it will oscillate between 5% and 10%. To all this, we must add (among other things) that when heating the components a (cause of the current that circulates through them), their resistivity also varies.
Another important fact that you should know is that there are no commercial resistances of all the values, so you will have to carry out your project with the one that most closely approximates the ideal value.
Definitely. From paper to reality there is a slight difference. This is something that will happen in all the projects that you do, so it is advisable that you always get used to leaving a safety margin. In this case, 17mA is a fair value.
If we go back to the example of the 9V battery and the 1.8V red LED, the resistance value you need is R = 423.53Ω.
As you can imagine, they do not sell resistances of that value. However, the 470Ω resistors are easy to find. Also, as we choose a resistance value a little higher than the ideal, the intensity will be a little lower (just take a look at Ohm’s law), so you make sure that the intensity is always below 20mA.
Although you think you already know what resistance to putting an LED, you still have another concept: the power dissipated.
Power Dissipated in Resistance
You already know how to calculate the Ohms you need so that your LED circulates a reasonable current. However, you still have to find the value of dissipated power that must be able to withstand your resistance.
The calculation of the power is really simple; it suffices to apply the formula of the power P = I x V.
You have to take into account that, if the value of the resistance that you are going to put is not the one that you calculated (because that commercial value did not exist), the current will not be the same and you will have to calculate it again.