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# How many volts do I need to power a white 3mm LED that has a 240 ohm 5% resistor attached to it? Answered

I sacrificed a string of battery powered Christmas lights hoping to be able to use the LEDs in other projects.  The whole string contains 18 LEDs, every 4th of which blinks and is powered by 4 AA batteries.

The resistors attached to the LEDs are very small and look like they have red, yellow, brown and gold bands, which is how I came up with 240 ohms, but I'm not positive I'm seeing the colors correctly.  The yellow may be white.  Is there such a thing as a 290 ohm resistor?

I'm determined not to let good LEDs go to waste, but I don't want to just try random batteries and hope for the best.  I want to use the right combination to get the most life out of both the LEDs and the batteries.  Thanks in advance for your help.

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## 7 Replies

steveastrouk (author)2009-12-05

Looks like they're designed to run off 6V. Are they all wired individually in  parallel ?

Steve

alwaystomboy (author)2009-12-05
mathews (author)2009-12-06

From the images, they look as they are wired in parallel, and so will be running on 6v. And since:

i = V / R
i = 6 / 240
i = 0.025A = 25mA, which is about right for an LED.

dataphool (author)2009-12-06

I would suggest, since you intend to use these separately, to hook an led up with one battery, that is 1.5 volts.  The led won't light, probably.  Add another battery (in series) for 3.0 volts.  That may not work, but then try 3 cells for 4.5 volts.  Keep adding batteries one at a time, until the led toasts.  That will be too much.

cyberpageman (author)2009-12-05

Agree with steveastrouk; how are they wired?  If the R is 240 ohms, at 6v that will pass 25 ma, which is about right for 1-2 LEDs, but not enough for 18.  Could the colors be red-yellow-black, for 25 ohms?

If you want to use the LEDs in other projects, the question to answer is what resistance will you need if you use, for example, 4 LEDs?   Assume the voltage is fixed at, say, 6v.  There is an LED resistor and wiring calculator at http://ledcalculator.net/

For it you need to know the LED current and voltage.  LEDs typically require 20 ma and and LED voltage of 2 v (4 for blue and white).  Passing less current is okay--the LED will be less bright.  If you pass much more, the LED will burn out.  Note: the LED voltage is not the same as the supply voltage, which can be anywhere from 2-24v or more.

Search "LEDs explained" for further info.

steveastrouk (author)2009-12-06

Great answer. Gets my vote !

cyberpageman (author)2009-12-05

They look like parallel wiring.  You can real all about it at http://www.theledlight.com/ledcircuits.html

Don't be afraid to try hooking one or more LEDs with a resistor to the batteries. Reverse the connection if the LED doesn't light.