LED Trailer Lights

[GaryBDavis]

I'm building a new welding trailer. Nothing fancy - except the LED lights.

I ended up buying Optronics brand. That's what they were pushing at the local trailer parts place. The oval tail/brake were $28 for the 48 diode kit (grommet and wiring lead). The 2 inch clearance lights were $6.66 with four diodes each.

I would have prefered to get Peterson as they've are popular, but they didn't have any I liked. The Optronics come with a lifetime warranty. Hopefully, I won't crack the housings when I'm backing it into the barn.

 

[Berniep]

<font color="blue"> I bought some no-name aftermarket LED taillight bulbs that were direct replacements for the 3157's, and they burned out after six months. </font>

Sounds like they forgot to build in or at least send the resistor that goes inline with a led to make it work on a 12v system. Led usually like about 3.5 to 5 volts and more makes them burn out very quickly.

 

[frederic]

Most LED's are current sensative. A good design has a current regulator and a small resistor in series with each "string" of LED's just in case one of the LEDs burn out. This prevents the other strings from sucking too much current until you can replace the burned out LED.

 

[Mike_C]

Frederic,
I found the link to the schematic on your web site. One thing I do not see is a part number for the LED's themselves. Realizing that there are hundreds of types of LED's out there with various sizes and intensities, is there a particular LED that you feel is best suited for this application? I think I'm going to make my own LED tail lights for my truck.

 

[Mike_C]

Although it would take more parts and real-estate, wouldn't a better design be to have a current limiting resistor for each LED so that if one burned out, NONE of the others would go out?

 

[frederic]

Mike -

You can use any high brightness red LED that's in the 8000-1000 mcd range, that requires 20ma nominally and provides a 2V drop.

I used Kingbright LED's simply because I snagged a huge pile of them on Ebay and this facilitated my life-long desire to be cheap.

Here are the ones I used:
http://www.us.kingbright.com/product.asp?catalog%5Fname=LED&category%5Fname=X%2Dbrite+LED&product%5Fid=W7113SEC%2FH

http://www.mouser.com sells them. I believe http://www.digikey.com does as well.

With the LED's in a parallel/series arrangement, you need less current to drive them all. Also, each LED in series provides a 2V drop (or so) for that string. Since you have 12V more or less, this is useful as it makes the circuit less complicated.

Lets say you have 40 LED's in one taillight. In a series- parallel arrangement you'd have 10 strings of four, so you'd need a little over 200ma to drive the taillight (20ma * 10). If you ran them all in parallel, with a series resistor, you'd have 40 * 20ma, for 800ma total draw. 600ma is not a whole lot, however you're getting closer to the 1a limit of the voltage regulator (operating as a current regulator), therefore it will get hotter, and require a heatsink. Excessive heat (and a heat sink) inside a plastic taillight may or may not be a problem, depending on the volume of air contained, and if the taillight has a metal backing that the regulator can be attached to (in an insolated fashion of course) to transfer heat out of the regulator.

You could use a larger regulator too I guess if you want to run them all in parallel, instead of a heat sink. Then you have to change the schematic a bit to compensate for the lack of voltage drop across the strings.

LED's last forever if you don't provide too much current - and this circuit does just that - so the series-parallel arrangement is "good enough" to last decades. I'm sure your truck will be recycled into toasters long before the LEDs burn out.

 

[slowrev]

Good info, thanks. I am just a bit leery of series arrangements though. Kind of like christmas tree lights...which one opened up the circuit ?
Ben

 

[frederic]

Fair concern.

However, xmas tree bulbs are good for about 200 hours. LED's are easily good for 40000 of lit time. Triple that figure with half the nominal current.

How many hours are your taillights on? ;-)

 

[slowrev]

Actually my tail lights are on when the engine is running. I run my headlights all the time and the running lights with them.
Ben

 

[frederic]

Okay. Lets do some math then!

Let's say you're driving your truck 4 hours a day. Two hour commute each way to/from work. There are 200 workdays in a year, so that's 800 hours the LED's would be lit.

Out of 40,000 hours minimum life ;-)

There are only 8760 hours in a year, so even if you ran your truck and left the LED's on 24/7, they'd last many, many years.