LED Circuit Question

[jaftwo]

Trying to get back into electronics and was looking at some basic LED circuits at this site: http://www.circuitdiagram.org/simple-basic-led-circuit.html

The first figure with that 10 ohm resistor just doesn't seem right -- is it?

 

[Audioguru]

The website you found says no details. The LED has a maximum allowed current and has a range of forward voltage that are shown on its datasheet.

If the forward voltage of the blue LED is 3.3V then the current in the LED and in the 10 ohms resistor is (3V - 3.3V)/10 ohms= ZERO and the LED will not light or it will be very dim.
If the forward voltage of a red LED is 1.8V then the current is (3V - 1.8V)/10 ohms= 120mA and the LED will blow up.
Most 5mm diameter LEDs are spec'd to use a current of 20mA. Then a 1.8V red LED with a 3V battery will use a resistor that is (3V - 1.8V)/20mA= 60 ohms.

 

[bertus]

Hello,

The led resistor can be calculated:
Rled = ( Vsupply - Vfled ) / Ifled
The Vfled will be different for each color.
The attached PDF will show you rough figures for the colors.

Bertus