Which Resistor to Use with this LED?
This is a very simple question, so simple that we usually don’t think too much about it and use whatever we know is working. The voltage might be 5V, 9V, 12V or any other. We know we have to insert a resistor in series with the LED to limit the current. What should be the value of this resistor? If you browse the Internet to see what other people are doing you will find a wide range of opinions:
For 5 V, people use values ranging between 150 Ohms and 2.2 K. Many use 330 Ω!
For 9 V, people use values ranging between 330 Ω and 1.5 K. Many use 470 Ω!
For 12V, people use values ranging between 470 Ohms and 3.9 K. Many use 1K!
And so on… And the most interesting is that all those values seem to work… somehow. As long as the LED is ON and bright, people seem to be happy!
But is it optimal? Which one is the best choice? If it works, does it matter? To answer those questions we need to do some testing. The calculation indeed is very simple, it is only another application of Ohm’s Law:
To calculate R, you just need to know the values of VF and IF which depend on the type of LED you selected and the brightness you want.
You will find a large number of “LED Resistor Calculators” on the Internet. They will ask you to enter VF, IF and VCC and will apply those value to the above formula. Some will suggest values if you don’t know them, unfortunately many will suggest 20 mA for IF!
Finding the correct values takes a little bit of investigation and this is what we are going to do in this article.
Forward Current (IF) vs. Forward Voltage (VF)
A LED characteristic looks like the characteristic of any other diode. If you apply a small voltage across the LED, starting with zero and increasing slowly, there will be no current (or very little) until the voltage reaches a certain value (around 2 V for most LEDs), then the current increases sharply as the voltage increases (make sure you have a resistor in series to limit the current). Most diode manufacturers specify VF as the voltage across the Diode when the Forward Current (IF) is around 20 or 25 mA.
We are used with silicon diodes where VF is around 0.6 to 0.7 Volts. However, for LEDs VF is higher, around 2V for most LED’s and up to 3 to 3.2 V for some types of LEDs. There is not much you can do regarding VF as it depends on the type of LED you are using. All what you need is to know it or, if you don’t know it, to measure it!
How to select IF?
The value of IF depends on the luminous intensity you want. The relation between the Forward Current and the Luminous Intensity is another LED’s characteristic as shown in this picture. The Luminous Intensity is usually given as a relative value compared to what it would be when IF = 20mA. On this graph, for example, we see that when IF = 10mA the Luminous Intensity is half of what it is when IF = 20 mA. Indeed for this particular LED the characteristic is almost linear and you will find out that it is the same for most LEDs.
If this is the relative value, what about the “real” value of the Luminous Intensity? It is usually expressed in mini candela (mcd) and also expressed in function of the viewing angle and wavelength. This data is presented on different graphs and tables in the LED Datasheet. It is beyond the scope of this article but could be an interesting topic if you want to use the LEDs for other applications than just indicate the presence of a voltage… It is also useful to compare different types of LEDs.
So, how to select the right value of the Forward Current? For our applications we want to use as less current as possible for a reasonable brightness. I tend to be a bit greedy with the current allowed for the ON/OFF LEDs in my projects and would never go up to 20 mA! For a long time, 10 mA maximum was my standard but now, with modern LEDs 5 mA maximum became my new standard. If the circuit is low power and needs to work on battery I would go for less current. Some LEDs still provide sufficient brightness under 1 mA or less.
To illustrate the above I put a few LED’s from my junk drawers under test, and checked how they behave, starting with 5 mA and then looking at what happens when we change the current. All it takes is a variable power supply, two multi meters and a resistor. Of course also the LED to be tested:
You may wonder why, for most of those LEDs, there is no visible change in brightness between 5 mA and 10 mA as the Luminous Intensity is supposed to double based on the graph we have just seen? The reason is because the brightness perception of the human eye vs. the luminance is not linear. The human eye is more sensitive at low luminance than at high luminance. We can see this by the fact that the LED’s brightness visibly increases when we increase the current from zero until a point after which it doesn’t seem to increase any more, even if we double the current. This is represented by the graph below.
For example, this blue LED is already very bright at 5 mA, then we do not see much difference if we increase the current up to 10 mA or more.
This red LED already provides a reasonable brightness with only 1 mA!
And this is the one I will select for my project! I will settle for 2 mA. What will be the value of the resistor for 12 V? If you found 5K you are correct. I will select a resistor of 4.7 K to be a little bit on the brighter side.
Here it is with 12 V and 4.7 K resistor:
Can we use LEDs for AC voltages?
This is a question many people would ask. Diodes usually can stand a certain amount of reverse voltage without breaking down. This maximum reverse voltage, for example, is 75V for the all too famous 1N4148 diode and much higher for rectifying diodes, which can stand up to several hundred Volts. The maximum reverse voltage of LEDs, however, is very low, usually around 5 V! If you connect a LED, with its serial resistor, directly to an AC voltage, for example 12 VAC, your LED will be damaged during the negative half period of the AC voltage. During this half period the voltage will be reversed. The current in the resistor will be very low as the LED doesn’t conduct, so the voltage dropped across the resistor will be low as well. The reverse voltage across the LED will be nearly equal to the peak AC voltage, permanently damaging the LED. But no panic, there are a few solutions. One of the solutions consists of connecting a diode (reverse polarity) in parallel with the LED as shown in this picture. The diode will clamp the reverse voltage to 0.7 V, protecting your LED.
What about the resistor’s power rating?
This is a point that is often overlooked, because the power dissipated in the resistor is usually very low and we tend to think that any size of resistor will do. Not always though; it is important to consider the resistor’s power rating if you use very small resistors, such as 1/8W or SMDs. Let’s take an example of a LED under 12V for which we selected a Forward Current of 5 mA:
A 1/8 W (125 mW) resistor cannot be used and even a ¼ W resistor (250 mW) would be a bit too close for comfort!
LEDs are not all the same. We often buy them in bulk without knowing their exact specifications and we tend to use more current than necessary. For low power battery operated circuits you want to make sure that the LED used to indicate that the circuit is ON doesn’t use more power than the circuit itself! For other applications, where it doesn’t seem to matter much, it is good engineering practice to ensure that you don’t use more current than needed. In any case it is useful to know the basic characteristics of your LEDs, and it is so simple to measure!
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