Direct Drive Lights - Inefficient?

I am noticing a lot of direct drive lights (XM-L lights specifically) and am wondering how good of a setup this is. Consider the Ultrafire HD2010 which has been reported to reach about 5A. According to Match's test of the XM-L at various currents, the XM-L reaches a peak (at least with Match's nearly optimum heatsinking conditions - way better than a flashlight heatsink) at 4.4A. It seems to me there isn't much gain from going higher than about 3.5A.

So do the manufacturers of these drivers/lights just figure most batteries will not sustain or reach currents higher than that? Or is it cheaper to manufacture these types of drivers? Or is it done so that cheaper batteries will be able to put out some amount of light in high mode (ie. whatever they can handle - even if it's only 2.5A) instead of trying to pull 3.5A from a battery that can't deliver which might throw the driver into "wacky mode" (flashing, jumping modes, etc. . .)?

It seems to me these types of drivers are rather inefficent - just wasting your batteries without giving your much in return and also generating a lot of unnecessary heat! Wouldn't you be better off with something like a modified Nanjg 105C with 3.5A output level? This would hold "high mode" at 3.5A, keeping heat down (yes, still hot), and conserving a little bit of battery life.

So what do you think?


I haven't read where anyone has tested the driver efficiency in HD2010 or HD2011. I have two of the latter and plan to use those drivers in more suitably sized (in terms of heatsink mass) hosts - so I'll do some testing.

I'm not convinced that, even though they're 'direct drive' there aren't some losses. Now, 5A w/ 10% loss would be 4.5A, so your point is still valid...

...and I do agree, which is why I'll be using KD V2 w/ some extra chips (on the output side, since they won't fit on the battery side) to get to 3.5 ~ 3.7 A.

I do have a single xm-l light set up for 4.3A and I'm quite happy with it that way because it is significantly brighter than at 3 and 3.5A - at least for the first 10 seconds of a cold start - and that is how I use it...

Yeah, I realize that many people probably don't run lights on high for long periods of time. I'm not dead set against using such a driver either. I plan to buy an HD2010 in the near future and don't expect to mod it in any (major) way.

Just curious,

For efficiency, you want to underdrive the LED. The L Prize LED bulb has a bunch of LED's in it, but doesn't drive them anywhere close to what they can acheive. But it keeps the power consumption down that way. It makes me kind of wonder if we should be using triple XM-L's driven at 700mA each to get the most efficient flashlight, instead of running them at 3A each to get maximum output.

In the market currently, many (most of them are newbies) want to own a very bright flashlight, meaning high lumens output.

I think the reason why is that the manufacturer trying to capture those people who want to get a bright flashlight with a cheap price. therefore by having a direct drive, they can boost the output to the maximum lumens.

However, many may not know that by drawing such a high current will actually kill the batteries, especially a cheap batteries. Also, they don't know the consequence of this high current driver such as the heat produces, the run time, etc.

Yeah, I am sure many are caught by the high output. Even here on the forum people seem to go after the highest drive currents even though a lower current may produce more output. (It's that old "mine is bigger/better than yours" game.)


i'm afraid that they will not be able to use the high mode for long, due to the heat and the battery capacity. The emitter will have a shorter life.

yeah and make sure to bring extra batteries.

For starters the HD2010 is a good thermal design. Its a heavy, all aluminum host with threaded junctions at every point in the thermal path. None of the thermal path is brass, which is GOOD because Brass has a higher thermal resistance than Aluminum.

FWIW heres the driver used in the HD2010. I am guessing the LED in this light doesn't see full DMM-tailcap current (or anything close to that). Factoring in cumulative resistances through the spring, spring contacts and switch.

I've been wanting to get a second HD2010 light and mod it with a 8x7135 driver (or maybe modding that to 9x7135) AND doing some low resistance mods. So that would mean copper braid in the tailcap switch spring and a +B bus-bar (like I have done on my MCE dropins shown below). The idea is to draw less current through a lower resistance circuit, while delivering the same current to the LED as stock. Thinking ahead, the switch would become the current flow bottleneck at that point.

I'm all about efficiency! Let's drive 'em hard (but at an output level that makes sense and) AND conserve battery power! I like your idea Kramer!


Lumens/Amps are like horsepower you can never get enough! Doesn't matter if either is efficient but more is better(general population that is)!

anyone has the data for current - lumens of XML t6?

See the link to Match's test in the O.P. above.


Horsepower sells cars, as lumen ratings sell torches.

oldbobk and I seem to go back-and-forth on this issue every few months. He is a big proponent of max power turned down, but human nature leads me in the direction of lower-specced lights for those appropriate situations.

Back to the car analogy, boost can be addictive. "Just a little more...just a little more...POP !"

Exactly, sometimes less is more.


well, with true direct drive lights, you get all or nothing

but with these fet based? drivers, you don't have to go full blast.

but - i'm still wondering if pwm of direct drive, as opposed to pwm of a regulated current, is less efficient, at least in terms of emitter life

unless it was a skunk ;)

Anecdotally, my experience has been that Match's 3.5 amp thing is right on the money. On the Foyometer my 980L (nigh on to 5 amps) reads almost exactly the same as my E modified F15 that pulls 3.5 amps.


I made a direct drive XM-L 2D mag. I have two separate battery cases for it. One battery case holds a single 18650 and the light runs for 1.5 hours. One battery case holds 4 AA Eneloops and the light runs for about 30 minutes. The brightness on the 4 AA is roughly 70% brighter (ceiling bounce of 12 vs 7).

When I'm using it to "wow", I put the AA case in it. When I'm using it outside for long walks, it gets the 18650 case.

Would it be more efficient if I put a 2.8A driver in it and tried to run it at 8.4V? Probably not :)

yeah, the last one my dogs got into was all black. I saw it but let them out, figuring it was a cat that would jet.

I'm within city limits, but next to ~20 acres of wooded area so I see a lot (deer, foxes, skunks, possum, coons, etc). They probably are less common in the wilderness but the little bastards seem to like campsites and have no qualms about strolling right through a quiet one (if there are no dogs)