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[TTC]

Fellas, im making a guage cluster and i need the LEDS for my blinker, highbeam and low fuel lights n shit.  What resistor or whatever to I need to run them at 12vs?

Thx.


http://led.linear1.org/led.wiz
http://soarercentral.com/sc-forum/messages/9739/238744.html

Incase someone else looks

[SgtB]

Did you ever get it figured out? 13v, 2v, 20ma,  1 = 560 ohm. That's a single led though. The same values would be used, but add more in the LED count. the first link is pretty good at configuring the series/ parallel pairs for the right values.

[patsmx5]

Uh.... LED's run a certain voltage all the time, and then you adjust CURRENT to dim/brighten them. They are rated up to a certain current. You need to build a current regulator for them.

[TTC]

Does that mean i have to make a current regulator for each and every led?

[crttaz]

Buy 12v LED's and be done......

[SgtB]

Uh.... LED's run a certain voltage all the time, and then you adjust CURRENT to dim/brighten them. They are rated up to a certain current. You need to build a current regulator for them.

A resistor is all you need. You just need to calculate you current limiting resistors value with the maximum voltage you expect to see in the cars electrical system. 

if you order cheap LEDs online, you usually can get a datasheet with forward voltage, and current sink. I used 2v and 20ma because it's middle of the road.

[MTZ]

Buy 12v LED's and be done......

+1 radioshack has em

[ryan89crx]

Buy 12v LED's and be done......

http://search.ebay.com/search/search.dll?from=R40&_trksid=p3907.m37.l1313&satitle=12v+led&category0=

[TTC]

WTF, i went to an electronics store and they didnt have 12v leds.

thx for the link

[SgtB]

12v LEDs don't exist. The ones you see are normal 3 volt LEDs with a resistor on one leg stuck in a housing. I'm not saying they're junk, just saying.

[patsmx5]

Does that mean i have to make a current regulator for each and every led?

Every LED has to have a current limiter, yes. You can put several LED's in series though and use one current limiter.

IE- Say you run 14.4V when the car's running, and your LED's require 3.1V, then 4 of those = 4 * 3.1 == 12.4V < 14.4V. And let's say they're rated at 20ma. Then you build a current limiter that limits current to say, something slightly less than 20ma. Say 19.2 or 18.8, etc. Whatever it comes out to be.

You'll lose some amount of voltage against the regulator though, I forget how much though. So really you might could only do 3 in series else they may begin to dim before you hit 12.4V theoretically. Been a while since I played with LEDs. The regulators I built before were just a LM3705 IIRC. It's a voltage regulator, with a resistor hooked between the base and something so that it limits current but voltage can vary. That way your LED's can be setup to run at full brightness, and they never dim. With resistors, you have to run a large enough resistor to limit current at the highest expected voltage. IE-you'll spec them for 16-18V because if you spec them to 12 or 13 and your car charges to 14.4 or 15.1, etc, it will fry them.

[SgtB]

You can swing the current a bit without affecting the brightness. I usually run mine under for safety. Brief spikes in voltage shouldn't fry the led, but may take a few hours off of its looong life. It all depends on the cars voltage regulator. I forget how stable hondas are voltage wise. If you're seeing long spikes then you would need a voltage regulator. I've been running resistors for ages with no problems.

[patsmx5]

You can swing the current a bit without affecting the brightness. I usually run mine under for safety. Brief spikes in voltage shouldn't fry the led, but may take a few hours off of its looong life. It all depends on the cars voltage regulator. I forget how stable hondas are voltage wise. If you're seeing long spikes then you would need a voltage regulator. I've been running resistors for ages with no problems.

A voltage spike with a coresponding current spike (ie-using resistors) will damage the LEDs. Most aftermarket LED crap you buy for cars is spec'd to like 18V so that they hold up. But if your car runs 15V when running and then 13 when KOEO, they dim. And even still, they'll never reach full brightness as they're spec'd to see full current at 18v.

[stealthiskey]

Put a big cap between power and ground to abosrb voltage spikes.  I like patsmx5's idea.

Does that mean i have to make a current regulator for each and every led?

Every LED has to have a current limiter, yes. You can put several LED's in series though and use one current limiter.

IE- Say you run 14.4V when the car's running, and your LED's require 3.1V, then 4 of those = 4 * 3.1 == 12.4V < 14.4V. And let's say they're rated at 20ma. Then you build a current limiter that limits current to say, something slightly less than 20ma. Say 19.2 or 18.8, etc. Whatever it comes out to be.

You'll lose some amount of voltage against the regulator though, I forget how much though. So really you might could only do 3 in series else they may begin to dim before you hit 12.4V theoretically. Been a while since I played with LEDs. The regulators I built before were just a LM3705 IIRC. It's a voltage regulator, with a resistor hooked between the base and something so that it limits current but voltage can vary. That way your LED's can be setup to run at full brightness, and they never dim. With resistors, you have to run a large enough resistor to limit current at the highest expected voltage. IE-you'll spec them for 16-18V because if you spec them to 12 or 13 and your car charges to 14.4 or 15.1, etc, it will fry them.

[jagojon3]

You can also use diodes to drop the voltage to the LED. I did this when I had some laying around and no resistors, ended up using 2 diodes from radio shack to drop the voltage by 1.4V total on a hobby battery charger I have.

[SgtB]

I was doing a little research on car PSU's and found this paper (pdf ):
http://www.eetchina.com/ARTICLES/2003MAY/PDF/2003MAY26_POW_AN04.PDF
It says the voltage can spike up to 120v on a load dump! The 78xx series  works, but the transients need to be addressed. Obviously a POW could be used (that's the whole point of the paper). This is off of the main battery though, the acc line will be much smoother on most cars, and the 78xx will work fine with the correct protection I believe. Slightly off topic. I just thought of this post when I read it.