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LED array - resistors getting warm and too much voltagte to LEDs

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iamsuperbleeder

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So I've been on a mission to make an LED 3rd brake light for my 2006 Toyota Tacoma, partly because I just can't really ever shake off the need to tinker with things :p and partly because all of the aftermarket ones I've found online are WAY overpriced for what they are, and they just look cheesy in my opinion.

My first design was made in haste, and was a complete flunk. Technically it worked, but it was a bad design from the beginning. Basically it was 30 LEDs all wired together in parallel. They lit, but obviously that many LEDs in a parallel circuit is just a bad idea to begin with. Needless to say, running them all off a single resistor got that resistor EXTREMELY hot VERY quickly...

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After doing a little more research, it seems that making the circuit into 10 parallel circuits of 3 LEDs in series is the best idea. So I started over and that's exactly what I did, and put everything on a single board instead of two split boards like the original design.

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My issue NOW is that A) the 10 220ohm 1/4watt resistors get a little warm after about 20 seconds of power applied, and B) I think I used the wrong ohm resistors to begin with... I originally thought the LEDs were rated for 3v, but I misread and they're actually only rated for 1.9v... I should have used 470ohm 1/4watt resitors...

So if there's 3v getting to the LEDs that are only rated for 1.9v, how long can I expect them to last? Also, is this what's possibly heating up the resistors? I realize that heat is a byproduct of electrical resistance and it's to be expected, it just seems they start to get warm rather quickly.
 
another thought. aside from removing all of the resistors and putting 470ohm resistors in their place, is there any chance of putting a single resistor in line on the input power to bring the voltage down? if so, how would I go about calculating this resistor's ohm and watt rating?
 
LEDs behave as a constant voltage load. The LEDs aren't getting 3V each. They are getting the nominal 1.9 volts, but the current through the LEDs is higher than your design target.

What I would suggest is to rewire your array to be 5 LEDs is series. Then calculate a new resistance.

5 (LEDs) x 1.9 (volts per LED) = 9.5V

14V (nominal when the engine is on) - 9.5V = 5.5 Volts

5.5 Volts / 0.02 (LED current) = 270 ohms

Resistor power = 5.5^2 / 270 = 120mW per resistor.
 
that would be ideal, but then I'm looking at desoldering all 30 LEDs and redesigning an entire circuit board. not hard, but EXTREMELY time consuming and I run the risk of damaging the LEDs, putting them through another 2 heat cycles. I'd much rather just replace all 10 resistors with higher ohm ones, even better if I could just put a since, higher wattage resistor, in line before the entire circuit

according to my calculations, assuming the entire array draws 200ma (20ma per series circuit X's 10), leaving the current 220ohm 1/4watt resistors soldered in place, a 10v input would give me the desired 1.9v @ 20ma at each LED. that 10v could be achieved using a 22ohm 2watt or more resistor on the input

unless I'm mistaken
 
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Put a second resistor in series with the first on each bunch of LEDs - the power will then be dissipated between the two resistors as well.
 
The LEDs are taking 37mA. This is too high and the 220R resistors will get too hot.

Simply changing the 10 resistors for 390R will fix the problem and the losses in each resistor will be 160mW.
 
Well I picked up some 470ohm 1/2watt resistors this afternoon on my way home and I'm just going to remove the 220's and put the 470's in their place. I had the light already installed on the truck, and actually drove into town yesterday using it, but it survived so that's good; no burnt out LEDs; wasn't too concerned if I burnt out the resistors, I can get those locally, but the LEDs I had to order online, and they cost a little more than the resistors, lol. I took it off this morning before I went to work to avoid any damage. I have a spare stock light in case I royally screw this one up; perks of working where I do ;), and just installed it back onto the truck for now. Hope to swap the resistors tomorrow and see how it works out.
 
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Keep it simple, put some 1 N4004 diodes in series with the feed. each diode drops 0.7 Volts, 5 or 6 diodes can easily be wrapped in a heatshrink sleeve and disguised in the wiring. No mods on the board needed.
 
Higher wattage resistors are WIN

But you don't want to over-drive the LEDs, or else you'll fry them. It doesn't matter what wattage resistor you have.

Once you get the current right, if the resistors are still getting hot, THAT'S when you'll need to get some higher wattage ones.
 
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