1 ohm resistor?

[Triskal]

Thanks, Rob, for helping me understand resistors a little better.

Here's the project I'm working on.  I want to use a single Luxeon I Emitter LED:
http://www.lumileds.com/products/family.cfm?familyId=1
or a Luxeon III Emitter:
http://www.lumileds.com/products/family.cfm?familyId=7

My source voltage will be 3.6v.

The Luxeon I lists forward voltage as min: 2.79v; typ: 3.42v; max: 3.99v; and Forward Current: 350mA.
Your calculator says at 3.42v, I will need a 1/4w, 1ohm resistor.
The luxeonstar.com calculator:
http://www.luxeonstar.com/resistor-calculator.php
says I should use a 1/8w, 1ohm resistor.
Why are the results different, and does it matter?

The Luxeon III lists forward voltage as min: 3.06v; typ: 3.70v; max: 4.47v; and Forward Current: 1000mA at 3.9v (about 700mA at 3.6v).
Your calculator says at 3.6v, I will need a 1w, 1ohm resistor.
The luxeonstar.com calculator says I should use a 1/8w, 1ohm resistor.
Why are the results different, and does it matter?

Of course,  I want to get the brightest light possible, though heat may be a problem since it'll be enclosed in a metal container.  Also, since it's going to be in a metal container, I'm going to want to cover the resistor in a heat-shrink tube so it won't short if it should touch the side of the container.  So, will the resistor heat up too much?

Thanks!
SD

[justDIY]

Hello Triskal, welcome to the forums.

Sounds like you're building a flashlight?

I'm certainly no expert at flashlights, however, the number one issue with led flashlights is managing the heat.   It doesn't matter which luxeon you use, the lux 3 will overheat faster than the lux 1, but both will have problems sealed up like you describe.

The common design factor I see in many well made led flashlights involves a heavy slug of copper / aluminum which the led is mounted onto.   this slug is either part of the head or coupled with the head and/or body of the flashlight.   this allows heat to migrate from the led to the outside world.  high power led flashlight usually have a finned head, adding to the surface area.

the reason you're getting this business about 1 ohm, is you really need some fraction of resistance, less than 1 ohm, but it's safer to err on the side of caution and round up to 1 ohm.   You could even use something like 5 or 10 ohms, and see very little difference in brightness.  Getting the most light possible will come from managing heat, not driving the LED to its maximum ratings ... all other things being equal, a cool led puts out more light than a warm one.

99% of the resistors I've worked with are non conductive... I wouldn't worry about the body of the resistor shorting out inside your enclosure.   you'll want to insulate the leads of the resistor if it's just going to be hanging free.  or solder it down to a little bit of prototyping board, and then cover the leads with nail-polish or hot glue.  Resistors aren't effected much by heat, as long as you stay within their ratings, it doesn't really matter... so if you really want to enclose the entire resistor in insulation, there shouldn't be a problem.

you might want to take a look at the flashlight specific websites (such as candlepowerforums) ... the flashlight folks have a ton of great information on design and construction of flashlights ... one thing to keep in mind however, most of them seem to have a generous allowance when it comes to spending on portable light-sources 

[Rob]

Welcome to the boards.

I agree that we can be more helpful if we know more about the application.

But the difference between the 1/4W and 1/8W resistor is just its ability to dissipate power. You can always substitute a higher wattage resistor safely for one of the same ohm value. They're just more expensive once you get past 1/2W or so, and don't come in the same tolerances at higher wattages.

As a side note, that calculator they run is derived from my original javascript LED calculator. They added the tweak to recommend a resistor (and removed my copyright notice), but they don't allow any margin of safety. If the calculator determines the resistor would dissipate 249 mW, they would spec a 1/4W resistor. I use a safety factor of 60% in my new calc, so that you'd get a 1/2W resistor recommendation for any resistor that needs to dissipate more than 150 mW.

[Triskal]

Yes, you guessed it, I'm making a flashlight... of sorts.

Thanks for the candlepowerforums link.  I'll take a look at it.

I am, in fact, planning on using a heatsink for my LED.  This one:
http://www.aavidthermalloy.com/cgi-bin/bga_disp_led.pl?partnum=037801&size=23&height=18&pkg=Plastic&led=YES&length=23&nc=23.4&fc=7.39

I'm using a DIY battery pack with three C-cell 1.2v NiMH Batteries which, after I charged it, was giving me about 4v on my multimeter.  Does that change my resistor plans?

Looks like I'm going to have to get a few different resistors and decide what's going to work.

I'm also going to make good use of my thermometer for testing heat.  Is it okay to touch the bulb of the LED with the metal thermometer probe while the LED is hot?

Thanks again!
SD

[justDIY]

sure, you can touch the bulb ... just don't press on it too hard, it's just thin plastic filled with liquid silicone

the bulb is not what gets hot however ... you want to measure the temperature as close to the die as possible ... lumileds recommends drilling a small hole next to led and inserting a temperature probe there ... however, just measuring the temperature of the star itself will give you a rough idea

[Triskal]

Thanks, you guys, for your advice. 

I ordered some Lux 1 and Lux 3 emitters, not stars.  The optic I found only works with the emitters.  Fortunately, the emitters were cheaper.

I tested out the emitters and burned out all three of my lux 1 emitters within a minute of plugging them in.  I tried a 1ohm, 1.5 ohm, and 2.2 ohm resistor, but all for naught.  They all fried. 

The lux 3 emitter worked great.  I ended up using the 1w / 1ohm resistor, which I jammed into the fins of my heatsink to help dissipate the heat from the resistor.  I used heat-shrink tubing to cover the wires and the solder-points.

The heatsink is connected to my metal casing, which also helps dissipate the heat of the entire unit.

Now, I want to consider the Lux 3 emitter in red.  The red ones have slightly different specs than the other colors:
http://www.lumileds.com/pdfs/DS45.PDF

They have a forward voltage of 2.95v at a forward current of 1480mA.  I'm worried that since the voltage is closer to the lux 1 voltage, that these will just fry as well.  Do I need to bump up the resistance?  Or increase/decrease the wattage? 

It seems that no matter what I put into your resistor calculator for 4v source and 2.95v forward, it says that the "power dissipated in your resistors is a concern."  Well, almost... I put in a forward current of 500mA and it says that 2.2-ohm / 1-watt resistor will do it.  If I put that resistor into the mix, will it really drop the forward current to 500mA, or will it still fry?

Thanks again!
Shawn

[justDIY]

for leds over 350mA, I strongly recommend a switching type regulator.   The power losses in linear regulation (resistor) at those currents are just enormous, often times the power loss exceeds the power output.

if you run the lux 3 emitter without a heatsink roughly 12 sq in (3.5" x 3.5") it will quickly fry.   How large is the heatsink you're using, and does it have active cooling?   using something small like a 486 cpu heatsink with a fan would work well.

[Triskal]

My heatsink is about 1"sq (see my previous post).  It has no fan, but it does attach to the metal casing which probably helps dissipate the heat.

What's a "switching type regulator" and where can I get one, and will it fit in a 1.2" diameter tube (like a flashlight)?

Like I said, my current setup with the lux 3 and the 1ohm/1w resistor seems to work fine.  To be honest, I'm not necessarily looking for ultimate power effencicy here.  If it gives me a couple hours of light, that's really all I'm looking for.  I just don't want the bulb to fry when I turn it on.

Shawn

[minster9]

I'm in the same boat as Triskal,sort of. I want to install a Luxeon III into a portable worklamp powered by a wallwart putting out [regulated] 4.5vdc @ 800ma.so I would have plenty of room in the lamp head for a large heat sink. I'm talking about a magnetic base 60 watt bulb capacity gooseneck unit. Could I get away with just a resistor? The Luxeon is a "U-bin" supposedly guaranteed for 100 lumens & handle up to 1500 ma. Would this led fry the wallwart,or should I just build a power supply? Company I bought led from wants 40 bucks for plug-in driver! Great site,still looking around,though. Thanks!

[Rob]

Welcome to the boards. You can drive the Lux3 at any current you want, but 1400mA is aobve the rating for the wall wart you have, so as you suspect, bad things will happen.

You may want to look this over:
http://led.linear1.org/a-cheap-current-regulated-luxeon-star-driver-design/

Swap out the LM317 from a LM350 for higher current handling while still being low cost. Then feed it with a low-cost, high-current wall wart. (or you can build your own linear supply)

[cadstarsucks]

My heatsink is about 1"sq (see my previous post).  It has no fan, but it does attach to the metal casing which probably helps dissipate the heat.

What's a "switching type regulator" and where can I get one, and will it fit in a 1.2" diameter tube (like a flashlight)?

Like I said, my current setup with the lux 3 and the 1ohm/1w resistor seems to work fine.  To be honest, I'm not necessarily looking for ultimate power effencicy here.  If it gives me a couple hours of light, that's really all I'm looking for.  I just don't want the bulb to fry when I turn it on.

Shawn

Shawn,

The problem with a 1 ohm resister is that your source is NOT 3.6V it is 3.6V nominal.  Fresh NiMH batteries are 1.4V each for a 0.6V higher for a 600mA current difference when they are fresh.  At the other end you probably do not care about the extra 20% of life you are losing.

The "current regulators" you see here require 1-3V to regulate which puts them right out.

You need a really fancy switcher to actually do what you are trying to do...