How do I figure out what kind of resistor I need to go from 12volts to 1.7 volts and 2.8 volts??

Theres a formula to do this and i dont know what it is...

i think 330 ohms will take 12 volts to 1.5, but not sure if thats even right.

need this for alarm leds on my car.

thanks for any help!

 

i'm not sure...but i think we would need to know the ampere of your electrical system.

 

the formula is E=IxR. E is voltage. I is amperage. R is resistance.
I=E/R
R=E/I

Hope this helps you.

 

i knew you had to know the ampere

 

If you have an alpine sec150r like mine, it can do 3 blue leds or 5 red leds.

 

I'm not sure why you used E as voltage. Usually E stands for kinetic energy but you formulas are correct. You will need to figure out the current first as already mentioned. Then it's just simple math.

330 ohm resistor will take 12V to 1.5V if the current is 4.5mA. If this is going in a car remember that while the car is running the voltage is usually around 14V.



[Modified by nsxxtreme, 9:07 PM 12/29/2002]

 

Everyone use E when they use Ohm's law, so I use it. If this is wrong, please correct me.

"Ignorance is not bliss."

 

Thanks for all the help.

Now, next question, what wat resistor? 1/4, 1/2 or 1 watt?

also, someone else told me just to divide the volts (12) by the amprage (say 20), so that 12/20, or 12/.02=600 ohms, meaning a 600 ohm resistor is needed...is this correct?

but then, someone else said use volts minus voltage of LED divided by amperage,
in other words 12 - 1.7 = 10.3, then divide by amperage, or 10.3/.02=515, or a 515 ohm resistor...both are close but which is correct?

 

It does not hurt anything as long as you know what E means which you do. You could use any letters you wanted as long as you know what you are doing. The norm is to use V for voltage. E is usually used for kinetic energy.
http://www.grc.nasa.gov/WWW/K-12/Sam...w/ohmslaw.html


[Modified by nsxxtreme, 9:52 PM 12/28/2002]

 

Thanks for the clarification!!

 

What about the watt's? 1/4, 1/2 or 1 watt??

another stupid question, does it make any difference which way the resistor is placed in line as far as direction?? Towards ppower source or toward the LED??

Thanks

 

http://wild-bohemian.com/electronics/led-rest.htm

 

That's what I though he wanted. He kept saying 20 A this is why I deleted my last post. You can figure out the wattage by P=I^2*R


So your 20A is actually 20mA or .02A


[Modified by nsxxtreme, 11:39 AM 12/30/2002]

 

E stands for "electromotive force" (aka voltage).

 

once again, thanks for all the help, you guys have been great!