So you think that you cooked it due to poor heat transfer? Did you flow the LED yourself?
It was the highest configured Mtn boost option. I guess 4.x @ 6v? I cant remember if i did the reflow or not. It was in a mini c8 (jaxman m8). I had been trying to get more heat xfer to body applying paste to pill threads and stuff and testing turbo on second highest heat stepdown in firmware.
It did survive several turbo cycles before this but ultimately ended up losing 2 dies somehow… idk
I have a shaved one that i used much harder with a 7-8amp buck driver in a Z1 and it never died. Apples to oranges though…
The brass itself is probly not what killed your dies. In most cases the thermal path either between the LED and MCPCB (most often) or between the MCPCB and the pill is what causes this (when your using a regulated type driver especially). Often on budget lights the pill surface is not perfectly flat, so a trick you can use is to reflow solder the mcpcb to the pill for a better thermal path vs epoxy, you still want to keep the layer pretty thin though. The mountain driver max is 4.2A which should be sustainable until the time you start to burn your hand with the light if all the thermal paths are correct.
Hope this helps 
I guess my point in mentioning the brass was that I could never get the host hot enough to burn my hand before it stepped down. The driver itself would overheat before the heat path could do anything useful. Agree with all your comments though, cheap host requires a lot of tricks to make it work optimally.
Hello loneoceans, for pogramming your atmel microcontrol, I see on your webpage you use your 'pogoprog' programmer. Is this somewhere I can get this from? Do you have oshpark link like your gxb driver?
I think this is good for programming small board. thank you.
Happy new year to blf forum! I have got parts and waiting for pcb for gxb172. I will try to build one and see how good it perform!
I have all the parts needed and the pcb’s. Doing a lot of prep work to hopefully make soldering and reflowing these small components easier.
I enlarged and printed off all relevant pics and list on the GXB172 ,so I didn’t have to keep scrolling on loneoceans page to reference parts and their locations.
After OSHpark sent the email stating the pcb’s were being sent to Fab, they came in the mail a few days later.
Good to know we have both crawled down the same hole. I might need some guidance. 
Maybe we will both come out shinning our new GXB172 boost driver flashlights. 
Also have a copy of GXB172 and the parts for the 6V BoM, and will be embarking on a similar quest soonish.
I’m also interested in the schematic and source code, as I’d like to know how the opamp/current sense works in detail (esp with the DAC output, which I’m assuming is used to change the current output?).
I have looked but have never come up with anything. Hoping he might publish this at some point, it would be of great help.
I got the coil side completely populated and reflowed. Working on the spring side now.
Hoping I don’t have problems trying to program the 841 with avrdude. From what I have read it’s not supported and needs a patch in the config file.
I may end up having to buying a new programmer.
I explained it a few times already, but in other threads (and PMs). Can’t find it right now though.
You should be able to find out yourself what it does, but if not, the OpAmp essentially acts as an inverting integrator. With it comes some drawbacks in controlling the current, but it’s the only simple solution that I can think of (which is “independent” from the LED voltage).
And please don’t call an RC network a “DAC”, its a filter.
moderator007, I just want you to know we are applying the kid’s birthday party rule set to this.
If you make one for yourself you have to make one for everyone in the thread, it’s only fair…
I’m no professional at this, just a hobby. I’m just hopping I can actually make it work.
May the soldering gods please shine on me. 
0402 size is no joke, they are small that’s for sure. Not much bigger than a grain of salt. :person_facepalming:
I just want to comment on the XHP50.2. I don’t think it’s bad heat path that kills them. I had 4 out of 7 fail in my 7xC8 light, all came on 20mm noctigons from MTN, running 5A or so each for just a few bursts. Unfortunately they just seem fragile and/or bad process control in their production.
Gotcha. I’ll take a look!
I’ve gone so far as trying to diagram the circuit, starting with an opamp in a basic low-side current sense config, then attempting to hook in a “control” signal to see how that affected the output, along with tying it with the nominal voltage set network. Futzing with various resistor values from the BoM, I did manage to get something that output 0.5V, but seems a bit overall magic to me.
Hah, my apologies. I’m not a EE so I’m mostly winging the terminology here. It’s a low pass RC filter on a PWM output from the ’841, then, yeah?
You should be able to run ```avrdude -p ?``` and it will tell you what chips it’s configured to support. I’m running whatever is in the latest version of Ubuntu, and was able to get the given .hex file programmed after only populating the bottom side. Command line looked something like this:
I’m using a USBTiny ISP as the programmer. No mucking about with fuses, just use the defaults (as he notes in his instructions, IIRC).
If you have an Arduino environment setup, you can download support for the ’841. Look for ATTinyCore in the boards manager. Since we only have the hex file, I’m not sure that you can actually use Arduino to program the chip, but you should be able to compile an empty sketch, grab the avrdude command from the log output, and try to use that to program the hex file Lone provided.
Ok, finished my GXB172 build this weekend and can confirm it works! Overall a decent and fun DIY experience. Pix (complete album):
Fully assembled and operational. Fiat lux!
Halfway done
Fully assembled, top
Close up shot of the driver mounted in the pill. I had to dremel down the inside of the retaining ring, as C1*5* was shorting out on it.
XHP70.2 4000k P2 bin LED for testing
Beam shot, taken during daytime.
Assembled flashlight with the two other GXB172 PCB copies from OSHpark.
Thermal image of light just before it turned off from thermal shutdown (or voltage, hard to tell if it’s 4 or 5 blinks). Outer shell is 64C, which would make the 75C threshold for thermal shutdown likely since it’s going to be hotter inside the brass pill.
Notes for fewllow attempters:
1. Miso en place. I sorted and labeled all my components in to two bins, one for the top and one for the bottom. The resistors are mostly unlabeled, so if something goings flying, you may want some tweezer component testers to make sure you are putting the right value in the right place.
2. Get some magnification, some good tweezers, plenty of toothpicks (for solder paste application), a LOT of light, and plenty of patience.
3. Took me about an hour or so to set all the components on the bottom side. Top side went quicker, though I don’t remember the exact time it took.
4. If you are using a Convoy S2+ as your host, the XAL7070 inductor will fit. However, watch the clearance on the bottom side! C1*5* was shorting itself out on the brass retaining ring for me, so I had to dremel it down until the ring no longer made contact with both sides of that cap. (Contacting some parts is fine, as long as the side is already connected to groud. C16 is sideways, so it winds up shorting out entirely (I did not power it to find out what would happen with it shorted).
5. The ramp-in took some getting used to; sometimes was hard to tell if it switched modes or not. However, I wound up figuring out a way to tell what mode I was in because…
6. There is audible noise for me at the 250mA and 1000mA power levels. I’m using the low/50/250/1000/4200 set of modes, J1 bridged, J2 open. The noise is fairly high frequency but definitely audible (I double checked with my roommate). Strangely, it is NOT present at the low or 50mA levels, nor is it present on the turbo level (due to the ramping turn-on, the noise will start when you switch to turbo mode, then get higher and higher frequency until it is no longer audible).
7. In the S2+, it switches off from thermal limits and does not seem to reduce output to keep within limits. Not sure if this is a bug or something wrong with my build, or working as intended.
Definitely would like to take a look at some source code, to see what can be modified and how to adjust things like power limits (I want a version that tops out at 3A output, so I can do some versions with LH351D’s in 3S and 4S configs).
EDIT: Corrected capacitor name that was causing the shorting problems — C15, not C16 as I originally had (C16 is on the top side of the board).
Really good writeup and thanks for putting in the effort to test it and document. I think this is what this project needs to go further. Beyond my scope but maybe not once it’s more refined or adapted to 20-22mm
Well done and documented schizobovine.
Just a bit beyond my abilities as well.
schizobovine, wow this is very good job on your build! I am inspired to finish mine.
For the C16 problem, I notice that loneoceans also previously had to add a ring of wire around to avoid this. He also did this in his gxb17 original driver.
I am amaze you use XHP70 in such a small flashlight! This is very impressive!!
For the audible noise, I read in the loneoceans website that he use ultrasonic mode below 23khz to avoid audible sound. base on datasheet of mp3431, the mode pin (10) when floating should be automatically in ultrasonic mode. If low is high or low (in range), then it can become audible. My guess is maybe if you can hear it, then either pin 10 is shorted somewhere or there is some residue on pcb / leakage? Base on loneoceans layout picture it looks like pin 10 is not connected to anything so this can be a place to check?
Is there way for you to measure power / light output of the driver?
I am curious of how bright the high mode is, and how low the lowest mode is.
I like your idea of using brass disc for spring. May i know when you get brass disc from?
Did you use hot air or oven to reflow the pcb components?
thank you for details of your build! You must be second person in this world to have such a powerful driver and probably first in world with XHP70 led in such a light! I hope we can have more of them soon!
Current/voltage at LED and current at tail would be interesting to know.
If using a brass button on the driver like schizobovine you will want to thin the ring otherwise your batteries will be way too loose. S2+ host has a lot of extra room in it as it is.
I think the 18350 tube is a bit tighter fit though if I recall. Age old S2+ problems I suppose.
User kiriba-ru sells a really nice copper single pill with a custom thin ring (pictured bottom right) that I highly recommend for this type of build:
very good find contactcr. This look like it will be perfect for such application, with better thermal property of copper and thin ring. schizobovine, does the flashlight get very hot quickly?
This looks like perfect match for the copper pill: https://budgetlightforum.com/t/-/51666
contactcr, it looks like there is a gxb202 and it works at 12V, but I have not seen the files for it yet.