LED forward voltage

[Berserk87]

ive seen/am seeing a lot of guides to random electronics projects that shows leds in line with a 5V source, then a resistor.

which usually ends up being 5V @ ~20mA.

but looking at led spec sheets, the max forward voltage (for ultra brights) is 3.8V.

am i missing something?

do you count the leds ~1.5V voltage drop when your looking at it. so it ends up being 3.5V forward if your connecting it to 5V?

 

[Diver300]

The normal thing to do is to take the typical forward voltage drop and subtract that from the supply voltage.

Single red LEDs are never more than about 2 V forward voltage drop. What colour are the ultrabrights that you are quoting?

If you have a red LED with a drop of 2 V and you want to light if at 20 mA from a 5 V supply, there is (5 - 2) = 3 V across the resistor, so you want 150 Ω.

On a 12 V supply, it is usual to put a few LEDs in series. So to light four of the same LEDs from 12 V the total drop is 8 V, so there is 4 V left on the resistor, which should be 200 Ω.

You could go to 5 LEDs but not on a car battery because the voltage regulation is poor, so the brightness would vary too much.

 

[Berserk87]

ok, i was just double checking.

ive been up for a long time, and obvious things are going over my head...

edit:

o ya, im using ultra bright blue/green/white.

 

[audioguru]

If your LEDs have a forward voltage of 3.8V (seems like a max rating, not a typical rating) then with a 5V supply the resistor will have 5V - 3.8V= 1.2V across it.
Ohm's Law is used to calculate the value of the current-limiting resistor. For 20mA, R= 1.2V/20mA= 60 ohms. Use 62 ohms as the nearest standard value.