XPG direct drive in 3D Mag, what resistor do I need?

I've always used a driver in previous builds, but I'd like to keep this light around as a utility/loaner light, and therefore like to keep it inexpensive.

I have the emitter mounted directly to the heatsink (britelumens' deluxed) with thermal epoxy, and I'd like to drive it around 1200mA if possible.

I'm not familiar with the rate of battery sag WRT current, can someone recommend a resistor? This will probably only ever run on 3D alkalines...

Thanks!

This could be helpful, along with this post.

1.2 amps might be a bit much for alkalines.

The alkaline battery shootout could be useful to check D alkaline performance under an ~1A load.

Thanks everyone, very helpful information... Now for the building...

Ok everyone,

I installed a two way switch with two resistors. One resistor is 5ohm 2watt, and I measured 150ma at the tail. The other resistor is .5ohm 5 watt, and I measured 300ma at the tail.

I was hoping for a little more current from alkalines, but perhaps this is normal? Perhaps I have something wrong...

Thanks.

I tend to favor trying DD without a resistor first, measuring the tail current draw, and then determining my next step from there. It doesn't take much extra resistance to drop the input voltage too low. Sometimes the parasitic resistance in the flashlight system is already enough. It often depends on the batteries, the LED Vf-If curve, and how clean the flashlight host is.

Fresh alkalines?

Low resistance switch for the two-resistor option?

Clean contacts in the Mag to give low resistance?

Good soldering job on the Mag switch for your DD? Clean Mag switch contacts?

Is your DMM working reliably?

I'd probably take out the switch that toggles between your two resistors. Test you DD setup with just the 0.5 ohm resistor. I would assume that you'd get close to 1A current draw at the tail.

Thanks for the tips. I took the switch apart and hard wired a few spots, and I also put in some fresh cells. Took another current reading and it was over what my meter could read. (thats good enough for me)

Thanks!