DonkeyBoy aims to build something, and he seems to be flirting with an S11 host. Kaidomain's ∅22mm H2-C driver (also available in AliExpress' LHT Flashlight Store) fits nicely, being an efficient fully regulated boost driver intended for XHP35 (4S) emitters, which can be easily modified to drive 2S emitters. For detailed information check this thread: Buck and Boost Drivers, Testing, Modding, and Discussion (Pic Heavy). The details for H2-C modding are in Jensen567's Entry for the 5th Annual Old Lumens Scratch Build Contest - Machine Made Category, but I doubt the driver strictly requires output voltage divider modification to drive 2S emitters when driven by 1S battery input. I mean, I think it should straight work with XHP50/XHP70 emitters.
In any case, an XHP35 build will hardly be less efficient. On the other hand, with an XHP35 the flashlight will be throwier. And with an XHP35 HI, a lot throwier. 
For big throw, I'd go with this host: http://kaidomain.com/Flashlight-DIY-and-Tools/diy-flashlight-shell-or-flashlight-host/New-DIY-K6-LED-Flashlight-Host-170mm-x-63mm-Black
Note: use BLF coupon to attain 5% discount at Kaidomain.
Give thanks if you like. O:)
:-)
Thu, 06/20/2019 - 19:21