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3w led and resistor help

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I plan on putting 2 3w led's in series and use your diagram. With that, I still only need the 1w 2 ohm resistor, correct????




Craig
 
I plan on putting 2 3w led's in series and use your diagram. With that, I still only need the 1w 2 ohm resistor, correct???

Craig

Yes, 1W 2 ohm, FYI: with a LM317, it always wants 1.25 volts from output to adj and works to maintain that. With 1.25v, 2 ohms, you get .625ma*1.25v=0.78125 watts in the resistor, regardless of applied voltage as long as it is enough, however, the power dissipated by the LM317 will equal the applied voltage14.2 minus the load(7.2)(2 LEDs) times the current(.625a), so if you have 2 LEDs, the LM317 will be dissipating ~ 7v*.625a = 4.375 W.
if you have 3 LEDs(3.6v*3)or(10.8v), the LM317 will be dissipating ~ 3.4v*.625a = 2.125 W. Thus less heat from the LM317, the heat dissipation is transferred to the LED. With 3 LEDs, If the voltage dips some, the LED brightness will be slightly less, but probably not noticeable. If you stay with 2, just up you heat sink a little. 1 4X2X1/4' piece of aluminum should be sufficient, but check it to how hot it gets after it's been on for a while. for dimming for tail lites, you'll have to experiment, possibly 2 2ohm resistors in series. -- got an idea for you-- see drawing
kinarfi
 

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Thanks, but what about my plan to mount these led's to a pcb? Will there be any
problems there (I know I am going to have to get a heatsink for them anyway)?????
Also, what you were saying is that instead of a 2 ohms resistor I would need close
to a 4 ohms resistor?


Craig
 
Thanks, but what about my plan to mount these led's to a pcb? Will there be any
problems there (I know I am going to have to get a heatsink for them anyway)?????

They DO get hot (very hot), so you WILL need to mount them on a generous heat sink, mounting them on a PCB will not be sufficient, they will suffer thermal breakdown.

If you have a piece of 3/16 aluminum big enough to act as a mounting plate for your application, you can drill and tap it to accept the M3 screws (make sure you use a fiber insulative washer!) to secure it to the plate, which will be your heat sink. A couple of drops of thermal paste will do you good. You will need a metric tap set, cheap enough at Harbor Freight, or your local hardware store (I hope!).

40 Piece Metric Carbon Steel Tap and Die Set

That one is carbon steel, so something soft like aluminum or copper should yield no problems with it. Mild steel and the like, you will need to go slow and easy and use cutting fluid/oil.

HTH,
Andrew
 
The fiber washers you want to use, are the same that usually come with the screw package in a new computer case and/or motherboard.

**broken link removed**

I got a 50 pack off ebay last week...

Andrew
 
They DO get hot (very hot), so you WILL need to mount them on a generous heat sink, mounting them on a PCB will not be sufficient, they will suffer thermal breakdown.

If you have a piece of 3/16 aluminum big enough to act as a mounting plate for your application, you can drill and tap it to accept the M3 screws (make sure you use a fiber insulative washer!) to secure it to the plate, which will be your heat sink. A couple of drops of thermal paste will do you good. You will need a metric tap set, cheap enough at Harbor Freight, or your local hardware store (I hope!).

40 Piece Metric Carbon Steel Tap and Die Set

That one is carbon steel, so something soft like aluminum or copper should yield no problems with it. Mild steel and the like, you will need to go slow and easy and use cutting fluid/oil.

HTH,
Andrew
When cutting, drilling, tapping aluminum, Alcohol works great, gives good clean cuts and doesn't gall. I use denatured alcohol all the time, not sure about rubbing or isopropyl alcohol.
 
Thanks, but what about my plan to mount these led's to a pcb? Will there be any
problems there (I know I am going to have to get a heatsink for them anyway)?????
Also, what you were saying is that instead of a 2 ohms resistor I would need close
to a 4 ohms resistor?
Craig
Since your using 3 watt LEDs, My thoughts are that you will run them dimly for you tail lites and then go full on for the brake lites, full bright would be the 2 ohm, the tail lites will have a higher value that will get shorted out by the brake or turn signal. not sure how you're gonna handle the turn signals, maybe use a relay to short the resistor for the tail light when brake light and or signal is supposed to be on.------?????????

You might consider 5mm White LED Super Bright Round Light Bulbs, they won't get hot, can be mounted on a PCB and use several of them instead of just a few. Another no fun idea is use led type bayonet replacements --- YUCK ---
Keep us informed.
Kinarfi
 
I think I may have confused you with what I am doing. I am making the "sequential/chaser" knight rider light in the front of the car. I am doing nothing with
the brake lights. I hope this doesn't effect the information that I have gotten so far.
Does it??
And with what I am doing, I am trying to design a unit that NO WIRES are hanging out of the back. I have
yet to see one that doesn't have this in their design.

craig
 
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8 sections/lights and they will turn on in sequence 1-8 so "technically" they won't all be
on at the same time.
Yes they will scan like knight rider's, but the old style (trans am version not the mustang version).
Exactly like the first pattern seen in the youtube video link you provided.

craig
 
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Quick question on the metal for the heatsink. Is what you suggested thick enough to put
2 led's on it??

for just 2 led or the whole string, is this the mounting surface for all and each 2 come on momentarily, can you post a drawing of what you intend to do and how. IMHO 1/4" min, unless finned like a heat sink.
 
1/4 cut to hold 2 led's for each section. If I do one whole 1/4 bar I run into the problem
of having a hole bunch of wiring out the back which is what I don't want.

Craig
 
ok, I really don't know how much heat sink you'll need, you'll just have to guess and test
 
OK, so I am ready to buy my components. I made a schematic (the first one I ever drew), and I want to make sure that it will work:
The caps are there to make the output led's slowly come on and off.
The bottom section is where it will connect to my control box.
 

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build one leg & try it
 
Hey people,

It's been a while since I had an opportunity to even think about this project but I have already
started purchasing items to build it (mainly the led's).
My next question is since I am trying to keep the space need for the pc board as small as possible, would I be able to use a 1w 2ohm resistor network? A 1w resistor is pretty big and
takes up a bit more space than I had intended.
Also, how close does the resistor have to be to the led?


Craig
 
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