LED's How to



LED circuit symbol

LED in real - to the left is the L-shaped cathode, which is always connetected to the ground side. On this side the housing is flat. This is how the LED looks in a circuit-diagram -again the cathode (and with this ground) is to the left side.


LED's last forever - as long as you don't connect them directly to your powersupply. Normal LED's go with 2,2V - the higher brightness types want something more. If you give them to much, there lifespan will be shorten rapidly! 

Most likely they drive with a current of 20mA = 0,02A. With this low consumption you can hang fifty and more of them on your PC-powersupply and it won't hurt anything. Fifty LED's consume as much power as two harddiskdrives, so there is no need to use additional powersupplies.


LED connection-circuit To achieve the right voltage, you need to put a resistor (R in the circuit diagram) before the LED. To calculate the resistors value, you use the formula:


R = Resistor in Ohm     U=Voltage that is to much    I=Current that is needed by the LED

Let's have an example:

You got a blue LED that wants 3,5V and 20mA. So here we go:

1) As the circuit works with 5V, there is 1,5V that is to much.
2) The LED draws 20mA, but we need to calculate in Ampere, so this is 20mA / 1000 = 0,02A.
3) So we calculate: R=U/I comes to R=1,5V / 0,02A which results in 75 Ohm. If a resistor with this value is not available, just take the next nearest one you can get.


Some typical LED-values and resistors needed for 5V powersupply

Below is a table with some common values -don't take them for granted, there are many types out there so I can only list some examples of LED's I've got. Best is to check with your electronic-catalogue/dealer if you've got the same values.

LED 5mm superbright

Voltage (U)

Current (I)

Resistor (R)

Blue 3,5 V 20 mA 75 Ohm
White 3,5 V 20 mA 75 Ohm
Green 3,5 V 20 mA 75 Ohm
Yellow 2,0 V 20 mA 150 Ohm
Red 2,2 V 20 mA 150 Ohm


LED 5mm, normal brightness, also RGB-LED

Voltage (U)

Current (I)

Resistor (R)

Blue 3,0 V 20 mA 100 Ohm
Green 2,2 V 20 mA 150 Ohm
Red 1,7 V 20 mA 160 Ohm


Why resistors? I heard of putting them simply serial/parallel?

Well, under some circumstances this works.So let's have a look:

LED's serial Two LED's serial connected. So what's the trick? When you put two devices in serial, they divide the voltage between them. E.g. when you put two green LED's in serial, they both would get 2,5V. It's a bit more then the 2,2V they are build for, but not so much that it'd kill them.

Heck, why should I use resistors anyway? Naturally, the values of LED are likely to differ, even if they are from the same production. There is a good chance that one LED will look brighter then the other. Mixing up different LED's isn't any good, e.g. when you mix up a red LED and a green, the red LED would be much more brighter and the green barely lit!

Unless you use the same LED-types and colors and want to lit something where the LED's are not directly seen, I recommend not to go this route. Also, if you want to have e.g. two blue LED's, you are lost anyway, 'cause you need 3V + 3V = 6V instead of only 5V.


Parallel? Yes, but only this way! All are connected to the same ground, but everyone gets his own resistor! By putting them parallel without a resistor on your supply, you would surely kill them no matter if you take two or one hundred of them!