Resistance Help

I need to know howto get a 4.5volt 30mA LED to hook up to a 12volt power source. I want to be able to use the formula, i looked at another post and it was dead wrong. It said i should use a 330 ohm resistor but that got me 9.6 some volts. Thanks~Hadji

i put the led without the proper resistor and it is working fine, does that mean leds dont need resistors, or is it a matter of hours before these leds burn out or something?~Hadji

it will burn out  give it time it will burn out too much voltage

You can get 12 volt led's though.Sam

a 250 ohm resistor shouold give you what you want..you are only droping the voltage by 7.5 volts your led has an internal resistance of 150 ohms so when i divide the 12 volts by the .030 ma current draw we get a 450 ohm resistance now we subtract that from the working resistance which is 150 ohms and we get 300 ohms that is your resistor size to give you 4.5 volts.

just corecting my post i had corected above now THE RESISTOR SIZE IS 300 OHMS NOT 250 MY MISTAKE.YOU NEED A 300 OHM RESISTOR TO GIVE 4.5 VOLTS. AND HE WAS NOT OFF BY MUCH THAT WILL GIVE YOU WHAT YOU NEED A 330 OHM IS CLOSE.

what i mean is a led has an internal resistance of 150 ohms using math we can figure this out. everything thing electronic has resistance it has too. the reason this will work is the led has 150 ohms of resistance so we can subtract that and get our answer.

to calculate resistor value for an LED in a DC circuit, you use the LED's forward current (I) rating (mA), and the desired voltage (E) (2.1V, 5V, 9V whatever) and the resistor value (R) is your unknown.then just use Ohm's Law-GlassWolf

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