selfbuilt's review has measured lumens, and I trust his measured estimated lumens more than the manufacturers. His # is 2,275 lumens, here: http://www.candlepowerforums.com/vb/showthread.php?421264. This makes more sense, plus that big 88 mm reflector - big reflectors do seem to squeeze out more lumens, like the old TN31. Maybe between the size, SMO, and good AR lens... dunno.
I actually didn’t even want to talk about XHP35’s in this thread, I wanted to design a buck driver to run them lol.
I honestly do not know what they are capable of, I never claimed I did. I simply said that if acebeam could get 2600 lumens out of them, then I think we can as well. I still do think we can match whatever acebeam did, there is no magic involved in it afterall.
I do think that is best left to another thread though.
Good review there, I would say that does sound reasonable.
Just for the sake of keeping everyone happy I took the 35’s out of the OP, I would not have mentioned them if I had known people would focus on that instead of the driver to use them lol.
I only used them as an example because they must have 4s and I wanted an example that used 4s.
So, back to the buck driver design
I have been reading everything buck driver all morning and they are really fairly simple, just a lot of components working together.
I think that by using the SIR400/800 FET we will have a pretty good FET efficiency, they have already proved themselves and the conditions will not be much different then how we use the FET now. Except it will have a full 5V and tons of currant feeding it.
The buck converter itself has already proved itself and looks good.
We need to pick an inductor but that should not be that hard, lots of options that will fit in a Q8.
Might possibly add a large cap to smooth out some of the ripple as well if space allows.
Then just a matter of picking out the parts.
Now for the fun part, reverse engineering the schematic….
If you stick with the freewheeling diode in MTN MAX, you will probably need to use 2 parallel to each other as they are only rated for 10amps. That or find a diode that can handle more current. It is important they are well heat sinked too. When the Buck converter cuts battery power to the LED’s, all the current supplied to the LED’s flows through the freewheeling diode.
So you have determined that this buck driver will have enough voltage overhead with 4S cells to drive 4S (3v) emitters? I would think that a buck driver in such a situation will provide limited High/Turbo mode performance due to battery voltage sag as current increases, increased Vf of the emitters, and losses in the buck conversion. I hope I’m wrong though.
I agree. This is why we don’t normally use buck drivers for 1S lights either, isn’t it? For 12v application and 4S, maybe we need a higher voltage linear regulator?
I don’t know any specific numbers, but I think the fact that it is 4s will help the voltage overhead issue. For example, say a single cell with buck driver driving a single 3V LED at 4A has 0.2V overhead. Now a 4s battery driving 4 3V emitters in series at the same current has 0.8V overhead.
How are you doing this? if you were trying to do it based on pictures of the MTN Max, just a tip (you may have already noticed) the lp boards are 2-layer, but the hp boards are 4-layer.
Well actually I just talked to Richard about this project and it might not be something that is best not taken on at this time as I only have a few more days to work on it and it sounds like it could be bigger then that.
Far as overhead voltage I think it would be fine, the losses in the buck circuit should be made up for by less losses in the rest of the circuit due to less current flowing through it. Might not be quite as good at 100% as an FET but should be far better everywhere else. For an XHP I think it would be better as the hard part with them is not given them too much current at 4s voltages.
A properly designed buck driver I think would be superior to an FET, the issue is proper design. Apparently according to Richard buck drivers involve magic to get right.
I am waiting to hear back from him but I might have spoken too early on this project, I let the relative ease of the FET drivers go to my head lol.
Yeah I was wondering your experience level but didn’t want to offend. A buck circuit is out of my league. I know Richard worked a very long time on the Mtn-max, and other members have tried and failed before. Some of that I’m sure was because they were trying to get it inside 17mm though.
It depends on the 12V used. XHP’s appear to sit around 14-15Vf which gives a reasonable amount of overhead. The issue is popping LED’s due to too much current with an FET. I have a dead xhp50 and xhp70 that can attest to this.
Yeah, no offense taken. I simply have a reasonably good grasp on electronics. I am by no means an expert in the field.
When I get something in my head to try, I tend to go after it, particularly if I can learn something from the experience. I have already learned a lot of about the inner workings of buck converters I never knew before that was worth the time invested if nothing else.
Mostly though I just hate knowing that something can be done better then I am currently doing it, it really itches me something silly and sooner or later I have to scratch it.
Well after talking to Richard I think the best idea is to wait to work on this until I have a lot more time to mess with it.
He did give some good suggestions such as using the LM3409 instead of the MAX1620 buck converter.
I do still think that a buck driver is the best option for 4s voltages but for now I am going to focus on polishing up the FET 46mm version in this thread: Texas Avenger "TA" Driver series - Triple channel + Bistro or Narsil + Clicky or E-switch - The Ultimate open source driver!
It should work fine for the XHP50/70. I think a quad XHP50 Q8 with an FET driver will still be something to behold. Just don’t think that an XHP35 will get along well with the FET.
So thanks for your time, sorry to get ya’ll hopes up. I just think my limited time is better spent on other projects at this point.
I am real curious where the possibility of a triple channel Q8 driver goes.
Nothing wrong with diving in and learning as you go. Gives me a chance to learn with you. Your buck concept is definitely worth exploring and I hope it can eventually be brought to reality.
I certainly appreciate all the stuff you’ve been up here to TA. The hobby has already benefited greatly from your contributions. Sure hoping Tom E adds the third channel to his FW. I have lots of momentary lights that I would like put your latest DD driver in.
Yeah, I am not giving up on it entirely, just going to finish up some other projects first before I dive into this one. I can live without an XHP35 SRK for a little longer….I really can….
I see china lights with what looks like super simple buck drivers in SRK and even when modded to 15A+ they still work fine, so I know it is not impossible by any means, Just something that would take more time and prototypes then I can afford right now. At $15 a prototype it could get expensive since it is unlikely that the first try would nail it according to RMM.
Thanks for the kind words, I have got a lot out of this site so I figured the least I could do is be open source with some driver designs. I am just glad that others find them useful.
If anyone can think of another size for the TA driver series please do let me know, in that thread naturally.
Because of the prior conversation regarding XHP-35 emitters, I thought I’d add this…
This afternoon I modified an LD-2 driver and put it in a Convoy L2 with 2 cell tube, 4 of the 26350 half cells to an XHP-35 E4 1D emitter on a Cutter 20mm star stacked on a second copper star. Results? The L-2 XHP-35 E4 1D, 3.13A at 2942.85 lumens with the half cells at 4.21V!
That’s the most amps and most lumens I’ve seen one make! New PB! Where’s my Gold Medal? 
Seriously though, it’s quite intense and it’s not even an HI, dome on HD. 170.25Kcd for some 824M throw. Learn something new every day… forget several old things in a matter of minutes so it’s a backwards curve. 
Edit: In retrospect the dome on is probably why it’s making 2900 lumens, all my other built lights using the XHP-35 are HI variants.
Very interesting, although this makes a lot more sense based on what cree rates them. This is right in line with what I expected them to make actually.
That said you might be onto something on the Dome possibly being to blame. Although it is the same bin it is possible that it still produced more lumens. Odd it is this much more though.
Has anyone ever tried a V2 XP-L dome on vs a hi version to see if they line up lumen wise? I think they would as my HI’s line up with ~28% loss in lumens you would expect vs the HD (in fact less then that in some cases).
Remember that in the XP-L, Cree automatically deducts in the binning for the lumens losses… Hence an XP-L V6 becomes an XP-L HI V2. 
Edit: It’s in that range anyway. I can remember building an FET X6 with XM-L2 U4 1C that made 1811 lumens, de-domed it and saw some 1500 and ugly tint. So I pulled the de-domed and went full dome again for max power. So they’ll all lose some lumens and take a tint hit when losing the dome.
They do this for the XHP35 as well, officially in the data sheet the HI version only exists up to D2 or D4 bin vs the HD E4.
Are there some F4 bin HD XHP35’s floating around by chance?
It is rather strange that the ratings would be this far off. I might just have to risk an LED, maybe just test it to the official rating of 1.05 amps to see what it does.
Well I am not sure what to make of it. I just tested a 3c E4 XHP35 HI and it made ~1900 lumens @ 2 amps and was not gaining a lot with more current. I am guessing that it would have maxed out around 2300-2400 @ 2.75 amps or so.
The odd part is it took twice the current to reach the rated lumens. That is not like Cree, they are usually very close to spec.
On the plus side it is still almost twice the lumens of an XP-L HI and should net similar throw figures.
I need to order an HD version to test side by side.