Simple LED question

[colm_mcm]

Got some light thingys similar to torches, they have 3 LED's and operate from 3 * AAA 1.5v batteries.

Would I be correct in assuming that I could wire 2 of these in series into the 12v from the car and not have any problems?

[steelroe]

http://www.llamma.com/xbox360/mods/How% ... %20LED.htm

[colm_mcm]

erm...

still not very clear. the board has resistor on it already, and it runs from 3 1.5v batteries, (total 6V) I'm guessing that if I run 2 of these in series, there shouldn't be a problem, but I'm worried about the current from the car battery being too high.

[steelroe]

Better one here
http://www.theledlight.com/ledcircuits.html

[colm_mcm]

Thanks Rory, but that article deals only with voltage, they've deliberately left current out to keep it simple.

my main problem is that I don't know the values of any of the items I have, LEDs or resistors, and I can't find my multimeter.

thanks for the help so far though!

[steelroe]

They are most likely in parallel in the torch to save power. WHat colour are the resistors, are they all the same.

[steelroe]

This is very helpful
http://led.linear1.org/led.wiz
http://led.linear1.org/category/led-basics/

[colm_mcm]

They are most likely in parallel in the torch to save power. WHat colour are the resistors, are they all the same.

the LEDs are in parallel alright,

Looks like a 20 ohm resistor from the + end of the batteries, it does all 3 LEDs.

[karlf]

Did a little bit of this as part of my electrical apprenticeship. It wasn't car related so I wont even attempt to tell you what to do for fear I'd steer you in the wrong direction. (that and it was about 5 years ago I last worked on this sort of thing and the memory is a bit hazy). But I can tell you how to work out the resistor size if that helps.
Each colour band represents a number, the following is a table to show the value of each colour.
Black =0
Brown =1
Red =2
Orange =3
Yellow =4
Green =5
Blue =6
Violet =7
Grey =8
White =9

There are 4 or 5 bands on a resistor (depending on the type). The easiest way to work it out is to look for a gold or silver band and start at the band at the opposite end of the resistor.
The first 2 bands are simple they are just representations of numbers. The third band is a multiplier. This is to indicate how many 0's come after the first 2 numbers.
The silver or gold band represents the tolerance of the resistor. (ie the resistor could be + or - on the value stated) Silver means + or - 10% gold + or - 5%.
The fifth band(if there is one) is to show the quality of the resistor.
So for example green red red gold is 5200 or 5.2k Ohms at + or - 5%
red orange black silver is 23 Ohms (as black =0 in the table there are no zeros after the 1st two numbers) at + or - 10%.

Hope this helps.
If your still unsure visit here http://www.doctronics.co.uk/resistor.htm#convert

[karlf]

I just read my notes on this and here's what I've come up with. The resistor will depend on two things-
1- the size of the supply voltage (the battery voltage)
2- the current rating of the LED
Once you have these use Ohms law.
V
---------
I R
Where V= voltage
I=current
R=resistance
Ohms law states that voltage is equal to current multiplied by resistance.
So if you have 2 of these figures its easy to work out the value of the third.
In your case you have the voltage figure (your battery voltage) and the current (the rated current of the LED, not sure if you'll know this but if not I'd suggest going to your local electronics supplier and get 2 new ones and they can tell you the rating. I wouldn't even guess as to what the two you have are rated for as there are several types rated at all different currents. They wont be that expensive anyway a couple of euro at most).
Divide the voltage by the current and this will give you the minimum size of resistor reqired for your circuit. Always go higher than the minimum value required. (ie if its 20 go 22 Ohms) I'd allow 10% of the value myself.
Here's an example for you to work from.
Voltage = 12v
Current = 0.5 amps
Divide 12 by 0.5 = 24 (this is the minimum value in ohms you need)
If your still not sure give me the figures and I'l work it out or visit here
http://www.the12volt.com/ohm/page2.asp#resistance
Use the middle option on the resistance finder as it is the formula that suits what you are trying to find.

What is it your trying to install? It might be easier to go to an electronic shop or car wholesaler and buy a pre-made LED set to suit your needs.
I got a set of strip LED's free in a car mag that I used for lighting the footwell. I'm guessing if they came free with a car mag they're not that expensive or they wouldn't give them away. They came ready to install just needed to be connected to a feed and an earth.