LED and Ohms law

I am doing the ohms law to light LEDs. I have misplaced the notes I had after a move. Can someone help?

I am going to be doing the following for 3 and 4 cells. Lighting up standard white and red LEDs. I can't seem to find my ohms law notes and I want to be able to configure the with calculation the value of the resistor I need to use to get the right current. The current is I think the 30-50ma range for standard stuff LEDs, but can someone point me to a primer again. I am probably just going to wire up some switches to run LEDs so I can make some simple night lights for my 2 year old and to put in some of his toys so the batteries last longer and give him something he can play again.

Check out jtice's LEDPro software. I believe it's in...

Never mind, here's the link.

I could use the software. But I really could use the primer. I have seen one somewhere and had it printed out. It is great and I learned a ton and it let me do a other calculations too. You know the saying, feed a man... I want to learn to fish myself if I can...

Ohm's Law:

E = I x R

Further info:

For a circuit with resistances in series, ADD the individual resistances to get the total circuit resistance.

R(total) = R(1) + R(2) + R(3) ...

In a series circuit, the resistances see the SAME current. So you take the circuit current and multiply that times the individual resistor and that gives you how much voltage is dropped by that component. Go around the circuit and add up all the voltage drops and you better get the supply voltage. /ubbthreads/images/graemlins/grin.gif

Remember that Ohm's Law applies, but that the voltage/current characteristics of LEDs are NOT linear. If you know two of E, I, or R for the LED, then you know it's characteristic at those conditions. But change any condition, including temperature or age of the LED, and the characteristics of the LED will be different even though Ohm's Law still applies at the new conditions.

Example (using easy numbers so I don't have to use a calculator): You measure 3 volts dropped by the LED and 500 mA current. The calculated effective resistance of the LED under those conditions is R=E/I=3/0.5=6 ohms. Boost the voltage to 4 volts and your LED might have 1 A going through it. The calculated resistance under the new conditions is R=E/I=4/1=4 ohms. The LED resistance is not constant for different conditions.