Measuring what resistor I need for LEDs

[dparm]

Just ordered up a set of high-end Luxeon-powered LEDs for the brake lamps. I will likely need load equalizers/resistors wired in-line to prevent a bulb-out condition. Supposedly these are 7W (14 x 0.5w Luxeons). The stock 7506 is 21W, 1.75A, 12V according to Sylvania.

How do I know what size to get? I'm not much of an electrician so forgive me for the easy question. I remember Ohm's law to some extent...

Do I need a "15-25ohm load equalizer" for each side? It claims to add 6-10W of load.

[ArthurPE]

they should be in parallel with the LED correct?
let's use 13 V (1/2 between running of ~14 and off of ~12)
the LED's will draw P/V ~ I ~ 7/13 ~ 0.54 A
effective R ~ V/I ~ 13/(0.54) ~ 24 ohm
the oem draws 21/13 ~ 1.6 A
so R ~ 13/1.6 ~ 8 ohm

you need an equivilent of 8 ohm
so you need something in parallel with the 24 to equal 8
(math steps skipped :D ) you need a 12 ohm
it needs to handle
13^2/12 ~ 14 W

12 ohm, 20W or as close as you can get

total W ~ 7 + 14 ~ 21 same as oem bulb

[dparm]

"For turn signal led bulbs, the equalizer must be spliced across the - ground wire and the + turn signal wire feeding each led bulb."

Same thing would apply for brake lamps though.

They sell 100ohm, "15-25ohm", 6ohm, and 3ohm. Those add 1-2W, 6-10W, 25W, and 50W of load, respectively.

[ArthurPE]

that is in parallel, not series (series = cutting hot an inserting inline)
across - to + (in parallel with the bulb)
try the 15-25? (how can an R have that range?)
get the 15 if available

"For turn signal led bulbs, the equalizer must be spliced across the - ground wire and the + turn signal wire feeding each led bulb."

Same thing would apply for brake lamps though.

They sell 100ohm, "15-25ohm", 6ohm, and 3ohm. Those add 1-2W, 6-10W, 25W, and 50W of load, respectively.

[ArthurPE]

radio shack http://www.radioshack.com/product/in...ductId=2062291
but it's only 10W
but it needs to disappate the heat
where's the link?

12.1 ohm/20W alum cooling fins, perfect https://www.surplussales.com/cgi-bin/cart.pl
(RWF) RH25-12.1 12.1 ohm 1% 20w Aluminum Body Resistor 3.50

[jfunkey]

I was going to say all of that but ArthurPE beat me to is.. ha ha LOL.. great info AthurP.

[B6JoeS4]

I sense an Electrical Engineer

[dparm]

The ones on the site I got the LEDs from are little heat sinks. It says they get hot and you shouldn't mount them to plastic.

[ArthurPE]

The ones on the site I got the LEDs from are little heat sinks. It says they get hot and you shouldn't mount them to plastic.

yep, same as the second link I posted...but without the clips, which you can buy at any auto store
they need to be in air to cool, and not in contact with plastic/cloth, etc....they get hot

1/2 way down http://www.surplussales.com/Resistors/Res-WireWnd.html
Aluminum Body Dale RH Style Resistors
Has built-in heat sink & small size / wattage
Models include: RH, NH, RER, ERH, RE
(RWF) RH25-12.1

[Retsujou]

The ones on the site I got the LEDs from are little heat sinks. It says they get hot and you shouldn't mount them to plastic.

Those are the ones that keep the LED in my headlights happy :)