XP-G mag mod question?

Ok, so I finally did my very first mag mod all by my-big-self. Now I feel a little more grown up as a flashaholic.

I bought a two mode driver, heat sink and XP-G led and just modified the switch blah blah blah.

It works brilliantly and on high it's pulling 660ma and 160 ma on low running 3D nimh (that's at the tailcap).

My question is this: What sort of power would be getting to the led at those current levels? The seller (DX) of the driver only gave a voltage input.

First calculate the amount of power in watts drawn from the battery:
3D NiMH = 3.6 volts (for more accurate readings you can measure the batteries, I'm just using a rough figure here)
Current draw at tailcap = 0.66 amps

3.6 x 0.66 = 2.376 watts input power

ASSUMING the converter is 90% efficient, and this is rarely the case,

Output power = 2.376W x 0.9 = 2.134W

You can stop here if all you need is output power.

If you know the voltage bin of your XP-G LED, you can use it. Otherwise I will just use 3.5V for rough calculations.

Output amperage of the LED = 2.134W output power / 3.5V = 0.61A to the LED


Congratulations on your first mod. Try the ROP for real firepower.

So I'm guessing you don't want to unsolder the lead from the star and don't own a DC clamp meter. You can make a pretty good estimate by developing a multiplier constant. That's 0.85*(Vbatt/VLED). The voltage of the battery (under load) divided by the LED voltage * 85% as an estimate of regulator efficiency. You can measure the voltage of the LED without unsoldering it. Not every regulator has the positive lead common to the input and output sides. You might have to run a thin wire down the side of your Mag to get an under load battery reading.

An Excel formula of =A1/B1*0.85*0.66 made it easier to change the Vf of the LED while it got in the right range. XP-G's have a Vf of ≈ 3.2 volts in the 700 ma range according to Cree's pdf. If you had an average LED Vf, a guesstimated battery voltage of 4.0 under load, that would be a somewhat disappointing LED drive current of 700 ma, approximately.

You didn't specify which DX regulator. What's the drive current supposed to be? If it supports the extra input voltage, put an extra NiMH at the tailcap while you are measuring battery drain. If it improves you can use 4*Sub-C's in a sleeve of 1" schedule 40 PVC. They are together about 8 mm shorter than a stack of 3*D cells.

Jash,

Congrats on your first build! Its great fun to make M@Gs that can really throw the lumens.

I just finished a K2 mag and had similar questions, I'm trying to get 1.4 amps to the K2, using a shiningbeam driver(sku 1218) 2.8v-6v, two cr123 show 1.4 amps at the tailcap so KevinL's formula would finish with 2.07 amps to the led, and a single 18650 would be .828 amps with .80 amps showing at the tailcap, this is with a listed Vf of 3.65, does this seem correct? thanks for the input