Cuitcuts and Electronics

[AND_YOU_ARE]

Hear is my problem:

I want to insert a switch into a 12v line with an led in parrallel after the switch. The purpose of the led is ti show the line is live. I know I will need a resistor for the led so the 12v does not burn it out. The question is how large should the resistor be to ensure that the led does not burn out? Isnt a 100ohm resistor used infront of an led when the voltage source is 9v? What would I use for 12v?

Thanks for the help.

 

[horndude]

ohms law voltage=current * resistance

12 volts=80ma * X
X=150ohms

that will work but its not quite right, subtract 1.2-1.4 volts off, the LED drops that much on its own, try about 120ohms

 

[AND_YOU_ARE]

Thanks for the help horndude, I know V=IR, but where did you find 80mA? The current on my 12v rail is 36A. Why the .08A instead of 36A? I dont need it to be super bright, so I dont mind the resistor being to large, just so that the led does not burn out.

Thanks.

 

[horndude]

most led's dont use much, check its specs before you install it

superbright large LED's use like 110-200ma tops, tiny small ones only like 10-20ma

36A*12V=432watts, LED's use a few milliwatts worth of power

diodes also by nature drop some voltage when they are running, there's a threshold value that must be overcome, usually its a little over 1 volt

 

[AND_YOU_ARE]

alright thanks for your help, i think i will go with a 200ohm resisitor to be safe. Thanks again.