direct drive luxeons in series with resistor?

Hi, I want to setup a simple direct drive circuit with a luxeon TX1J (3W emitter) in series with a resistor. According to the Lumiled charts the J-binned emitter takes forward voltage 3.27-3.51 V.

What forward voltage value should I use when calculating resistor values? The min, max or somewhere in between? /ubbthreads/images/graemlins/blush.gif

There is a bit of tolerance in the Luxeon. Assuming you are using 3 x 1.5V batteries, the resistance you need is ~1.2K for 700mA into the LED. You can safely use 1K. Just make sure the emitter is heatsinked properly.

**EDIT. 1.2 Ohms! /ubbthreads/images/graemlins/blush.gif

I never saw the symbol "K" used to refer to single ohms - but I assume that's what you mean (1.2 ohms), as opposed to 1.2 thousand ohms, right?

1.2 Ohms usually is pronounced as 1R2

But what forward voltage value would be best to use when calculating the proper resistance, if I wish to do the calculations myself? /ubbthreads/images/graemlins/confused.gif

If you dont know the exact Vf, I ussualy use the middle, between the min and max.

heh, what am I thinking,,,,, HERE !
LED_Pro

let me know if you have any problems

-John

Use both the maximum and the minimum, select a resistor value, and recalculate the maximum and minimum current using the voltage extremes (3.27 & 3.51). See if either result bothers you enough to change the resistor value. Then, measure Vf under actual use and see if reality is anywhere close to the magic world of theoretical math.

I think you'll find that there are too many unknown parameters to really tie this down. Many solid state components have as high as a 20% variation, and mostly that isn't enough to bother changing anything in the design.

I've built two modified mag 3-cell flashlights with LuxIIIs from the same bin (TWOK, IIRC). One drew 950ma, the other drew 1150ma. The difference is in the batteries. The 3C light has a higher current draw than the 3D light primarily because the battery internal resistance is less (IMHO), although the differences in the switches and the heatsinks could have some effect.

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I've built two modified mag 3-cell flashlights with LuxIIIs from the same bin (TWOK, IIRC). One drew 950ma, the other drew 1150ma. The difference is in the batteries. The 3C light has a higher current draw than the 3D light primarily because the battery internal resistance is less (IMHO), although the differences in the switches and the heatsinks could have some effect.


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Alkaline C-cells have higher internal resistance than D-cells. Varying switch resistance, and differences in Vf would be the main contributing factors. All things being equal, C-cells should produce less current.

thanks, I was leaning towards that too. It'd be a bit tough since I have no previous experience of how much current is "comfortable" but I guess I'll start at 200mA and go upwards while checking the whole thing.

Any suggestions on a suitable tool for measuring the temperature in the setup?

Thanks! I'll check it out when I'm in Windows sometime or try it out under Wine. /ubbthreads/images/graemlins/happy14.gif

I've been using the luxeonstar resistor calculator to double check my calculations lately and I've found it being easy and working quite well. /ubbthreads/images/graemlins/thumbsup.gif

I've been wondering about something regarding resistors...

If I calculate my resistor type to say 3.23W (e.g 4W) is there any practical drawbacks if I choose a 5W, 10W or 20W resistor? /ubbthreads/images/graemlins/thinking.gif

Just size considerations. A 10W resistor is going to be larger than a 5W, likewise for a 20W vs. 10W. They won't make the circuit behave any differently.

You should use at least a 5W resistor if you plan on dissipating 3.3W in it.

Interesting points in regard to the alkaline batteries. But I normally use NiMH batteries, and my 3C mag is definitely drawing significantly more current than the 3D. I built both of these with identical components (as much as possible). I think the main differences are the switch and that the d cell flashlight has longer total circuit length. Also the D cell, with a larger hotlips and body, may run cooler. IIRC, the current goes up a little as the light warms up. I agree with evan about oversizing the resistors, they get pretty hot when you run them close to their power rating.