Resistor? Voltage Regulator? Power Driver? (For Luxeon K2 Star LED?)

Green Lantern

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Edit: I think this might be posted in the wrong section. Will a moderator move this thread to the flashlight electronics: batteries included section? Thanks!

I am currently in the process of building a custom flashlight. I will post all the schematics in the homemade & modified lights section, along with pictures of the completed flashlight and beamshots, (duh :)) when I complete the flashlight.

In this flashlight I am going to use 2 3V CR123A batteries, and a heatsunk Luxeon K2 Star (130 Lumens). The specifications of this LED are as follows:

Max Continuous Current 1500 mA
Forward Voltage: 3.85 Vf

More info here: http://www.luxeonstar.com/item.php?id=2213&link_str=1431&partno=5027-PW14

Do I need a resistor for this LED? I know ohms law, but when ever I try to make the calculation I get a ridiculously small number for a resistor. I have tried online resistor calculators, and they recommend I use a resistor with a high wattage.

What size resistor do I need to use? Should I use a voltage regulator? A power driver? How does this all work?

Thanks! :laughing:
 
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TorchBoy

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Do I need a resistor for this LED? I know ohms law, but when ever I try to make the calculation I get a ridiculously small number for a resistor. I have tried online resistor calculators, and they recommend I use a resistor with a high wattage.
I plugged some values into http://ledcalc.com/

Supply Voltage
6 VOLTS
Voltage Drop Across LED
3.85 VOLTS
Desired LED Current
1500 MILLIAMPS
How many leds connected
1

It suggested a 1.5 ohm, 3 watt resistor (although I'd personally go for a 5 watt resistor). That seems very reasonable, not ridiculous at all.

At 1.5 amps the voltage will very quickly drop to about 2.5V per cell according to Tom's test, so efficiency of the supply will be about 77%, but the current drawn with a 1.5 ohm resistor and 5V battery will drop to only about half that at the original 6V.
 
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Green Lantern

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It suggested a 1.5 ohm, 3 watt resistor (although I'd personally go for a 5 watt resistor). That seems very reasonable, not ridiculous at all.

Thanks! I took your advise and went with the 5 watt resistor. Found a nice deal and ordered them here on eBay. I can't wait to get it built! :)
 

Green Lantern

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There are also other ways of doing it which would give you a more even current, such as using a linear regulator board like http://www.kaidomain.com/WEBUI/ProductDetail.aspx?TranID=3256 (which also has three modes) or http://www.kaidomain.com/WEBUI/ProductDetail.aspx?TranID=3257 (19 modes in three groups).

633264720145892500_med2.JPG


That looks very intimidating... I think I'll stick will a good old resistor. :laughing:

I noticed when reading the data sheet for the LED that it does not recommend using the full capacity of 1.5A because it reduces the LED's lifespan. Since I am using a 5 watt resistor instead of a 3 watt resistor is this okay? Or is okay to use the max current in the first place?
 

TorchBoy

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A 5W resistor is designed to cope with 5W of power dissipation, so you won't have to heatsink it at 3 (or 3.375) watts, but it will almost certainly get too hot to touch. I once tried using one on the front of my plastic caving helmet, and it was mounted straight onto the plastic. In use, I got a whiff of hot plastic every so often.

With fresh CR123 cells at 3V each and a 1.5 ohm resistor you will get 1.4+ amps, which is close enough to the maximum current to not be worried about it. After a minute or so the cells will level out to ~2.5V each under load, and give only about 750mA. If you're happy running it at "only" that much current, then fine. Otherwise use a driver of some sort. That board is very easy to use.
 
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