Using a resistor to drive 3Xmls

[m.pille.led]

Hi guys,
I am planing to repair a dive torch wich I had previously modified to a 3Xml drop-in. After a while in usage the driver burnt (my fault, I used it outsider the water and it overheated).
Anyways, I was thinking to use a simple resistor instead of a led driver, so I would also be able to use the step less dimmable funktion off the torch.

The battery pack is a 11cell 1,2v NiMh and the 3Xmls are in series (planing to drive them at 3.3v and around 2A )

(Vf - Vs) / Amp = Ohm

9.9v-13.2v = 3.3v/2A = 1,65Ohm

So I would like to ask you if I can use a 1.5-1.7Ohm 20-30watt capable resistor for this repair.

I hope you guys can understand it
Thanks a lot
Mark

 

[Rick NJ]

Hi guys,
I am planing to repair a dive torch wich I had previously modified to a 3Xml drop-in. After a while in usage the driver burnt (my fault, I used it outsider the water and it overheated).
Anyways, I was thinking to use a simple resistor instead of a led driver, so I would also be able to use the step less dimmable funktion off the torch.

The battery pack is a 11cell 1,2v NiMh and the 3Xmls are in series (planing to drive them at 3.3v and around 2A )

(Vf - Vs) / Amp = Ohm

9.9v-13.2v = 3.3v/2A = 1,65Ohm

So I would like to ask you if I can use a 1.5-1.7Ohm 20-30watt capable resistor for this repair.

I hope you guys can understand it
Thanks a lot
Mark

Your math looks good. Did you check the datasheet to make sure that your LEDs are going to be at 3.3V when drawing 2A? Your set up working will depend on that.

Also, you don't need 20-30watt. Your math shows 6.6watt at the resistor. So 10watt resistor is pretty good margin and 30 watt looks rather a bit overkill.

Lastly, since you are dropping 3.3V off 13.2V, why not use an LM350 or something like that. For a dollar or so, you get drop 3-ish volt while holding constant current. LM350 will take you to 3A. Now as it runs (as voltage on your battery sags), your LM350 will still feed the LEDs the same current as before - up to a point. Probably down to 11volt or so. I think the official drop out for the LM350 is 3v-ish, but you still get pretty good regulation up to about 1v drop. Certainly better than the resister.