Simple Leds normally require only a few volts to make them light up but what if you want to add one to a project that has 12 volts ? How do you work out what Resistor is needed to stop it burning out ?.
For that we need to use OHMs Law and once we understand OHMs law we can use different LEDs on different voltages without damaging the LEDS.
Two other important things to know is 1: What voltage does the LED need and 2: what Current is required. you can often find this information on the website or packaging that the led arrives in but for this tutorial I am going to make an example of a typical 5mm Red Led which requires 2.2volts at 25mA (mA is milliamps - 1000 milliamps is equal to 1 AMP) so 25mA is 0.025 AMPs.
So lets say we have a 12 volt battery and want to know what resistor is needed for the LED
The symbol used for resistance is Ω (omega symbol)
12volts -minus 2.2volts (for the led) leaves us 9.8volts
the LED has a current rating of 25mA It is important to remember that this is 0.025AMPS
so 9.8 divided by 0.025 = 392 and this number is the resistance in OHMs that would be ideal to use
However heres the catch, I have never ever seen a 392Ω resistor though I am sure they must exist but what you can do is use a slightly higher value resistor and it will still work. plus it's better to go slightly higher than lower as your LED will have a longer life. A 430Ω resistor will work just fine and they are easy to find though there is a 390Ω you could get away with that as it is very close.
so using the OHMs law we just did the equation V over I = R which is Volts divided by Current equals Resistance. Current in the Ohms Law Triangle is shown as the letter I because the letter C is reserved for Capacitance.
* always remember the Current is calculated in AMPS (which is 1000 milliamps).*
As long as you have 2 of the variables in the triangle you can always work out the 3rd.
V=IxR : Volts is Equal to the Current times Resistance
I=V/R : Current is Equal to the Voltage divided by the Resistance
R=V/I : Resistance Ω is Equal to Volts divided by Current
So lets try one more calculation:
A Blue LED on a 5volt power supply.
the Blue 5mm LED has a typical forward voltage of 3.4volts at 30mA
so 3.4 volts minus the 5volts power supply leaves us with 1.6volts across the resistor
1.6v divide by 0.03Amps = 53.3Ω Ohms
you can easily buy a 51Ω or 56Ω resistor so going higher would mean a 56Ω should work just fine.
Try it out yourself and see how easy it is once you understand the OHMs law Triangle.
You may never burn out another LED once you have mastered this.
Georg Ohm did his work on resistance in the years 1825 and 1826, and published his results in 1827. It is a simple yet valuable equation to know and understand when working with electricity and electronics.
There are many types of stick on LED ribbons that can be purchased that run on 12 volts and yet they can be 5 or 10 meters long plus you can cut them at the markers on the ribbon and it will still work.
Between the markers you will always find 3 LEDs and 1 Resistor. All that is happening there is the 3 LEDS have their Forward voltages added up and the resistor value is chosen in the same way as above. you just add up the forward voltages of the LEDs.
You do not have to add the mA of each LED together as that does not change wether its 1 or 3 LEDs used. The Current through those LEDs should remain at 20mA
I must admit I use to get confused about this when I was a Boy trying to learn electronics all those years ago ( to be honest LEDs were not easy to buy when I was a Boy as they had only been around for a couple of years so I was learning this calculation with filament bulbs and believe me I made lots of mistakes) "still do occasionally" :-)
1962- Nick Holonyak develops a red LED- the first on the visible spectrum. The First LED is created. 1964- IBM starts to implement LED's for the first time on a circuit board in an early computer.
In the image above there are 2 circuits but they are actually the same, It's just the values of components that are different. the circuit at the bottom of the picture is from an LED strip but notice that the circuit is just multiplied over and over for as long as the strip is going to be.
If I used 6 of the RED leds at 2volts each a resistor will not be required as that adds up to the 12 volts that is coming from the power supply but when you combine the voltages of all the LEDs you should try and keep that total value under 80% of what the power supply can give.
ie: 80% of 12 volts is 9.6 volts so try and keep the total value of the LEDs in the circuit above under 9.6 volts and let the resistor do its work. I realise this is not being adhered to in the LED Strip circuit above but you may find that manufacturers will actually change the resistor value to 120Ω so the LEDs run at 3.2volt each and 3 x 3.2volt makes 9.6volts and that will slightly under-run the LEDs and they will last much longer.
** The golden rule is try and keep the total of your string of LEDs below 80% of the power supply. **
So one last time for good luck: Add the voltages of all the LEDs together minus the power supply voltage then divide that number by the Current and your answer is the Resistance in OHMs Ω.
You may notice I have calculated the Resistive Power in the picture above and I will go into more detail on another page that will be dedicated to Formulas and calculations.