Hello all,
Thanks to all for all your feedback! While I'm glad that I've heard a good number of folks who have successfully got their driver working well, I also hear some challenges and problems faced by some other members. I'd like to apologize that the experience hasn't been as smooth as it could have been, and I really appreciate the feedback.
In all my prototypes, I've tried to make the performance of the driver as good as possible while keeping within some specific electrical requirements I set for myself, as well as additional constraints such as layout and cost. I wasn't expecting the product to be for sale like it has been, so I think I could have made the design a little more user-friendly for people who did not have the equipment for micro-soldering, as well as to have a more detailed notice about a fair amount of DIY required. For example, I hand solder all the components on the driver myself, so soldering on the wires for the Aux board is easy in comparison for me, but I understand not everyone has the same setup as I have! I do appreciate the patience that everyone has.
The key to soldering this in my opinion is to use very fine soldering wire, needle nose tweezers, and some sort of magnification if possible. If it helps, here are some of my suggestions for basic tools which will help in any sort of soldering:
- Precision tweezers - any of them works even the really cheap ones: https://www.amazon.com/Hakko-3-SA-Non-Magnetic-Microelectronics-Applications/dp/B00FZPEWI6/
- Very fine solder wire - this is very important - personally I use a 1lb reel of Kester but any brand will work, you want 0.3mm or thinner: https://www.amazon.com/Kaisi-Solder-Sn63Pb37-fluxed-1-76oz/dp/B07S7H1WP6/
- For mistakes - solderwick, I use a large reel of Chemtronics since I go through a lot, but a narrow, fine-braid one works very well for flashlights and small projects: https://www.amazon.com/MG-Chemicals-Desoldering-Length-Yellow/dp/B008O9W78C/
- TIP - e.g. if one of those 0402 resistors pops out, use the solderwick to clean all the pads first so they are flat with no more solder domes on top. Then apply a small amount of solder on one pad, tack the new resistor on with the tweezers and solder tip, then solder the other pad down.
- Flux - It says no-clean but I always clean the boards after. Kester 186 works well too: https://www.amazon.com/Kester83-1000-0951-Soldering-Flux-Low-Solids-No-Clean/dp/B004X4KOWS/
On to the boards: I was finally able to get a few production Lume1 boards directly from Neal soldered up on my test bench, and I've been able to test the basic functions.
I'm glad to say that so far, everything actually looks good on my bench for the two drivers I've tested.
Component inspection looks good at first glance (I.e. it looks like all ICs are genuine), but I've also requested for a detailed BOM from Lumintop because they did replace some components despite the specific BOM I sent over (e.g. the sense resistor is the same value as the one I specified, but not the exact one I chose). So I'll give an update on that. However, the main components and large passives such as the big caps which are important, seem to check out OK at the moment.
Next, I was also able to test all main functionality and I did not see anything amiss. Specifically, some users seemed to have issues with the lowest modes, while others seemed to have no issues.
As contactcr correctly points out, at the lowest setting for the Lume1, there is in fact a small 'pwm' due to pulse frequency modulation of the buck converter. However, the performance I measured on my oscilloscope was in fact better than expected, with a current modulation frequency of about 2kHz only at the lowest possible ramp value (measured with a 100MHz Keysight current probe), but with very consistent levels (i.e. brightness is consistent so there should not be any flickering like a candle). I'll post more scope shots later on. Keep in mind that this is not actually PWM, and it should appear visibly better than an actual PWM at the same frequency. This modulation frequency very quickly increases before leveling out and should not be noticeable to the eye without moving the flashlight around quickly, and it certainly should not appear to be anywhere remotely like candle mode.
In addition, Kapsyd mentioned something about flux on the surface of the boards, and calipsoii talked about sensitivity of some part of the circuit - the general idea is correct. At the lowest current levels, the feedback the circuit is receiving is very small, on the order of tens of uV. This is a very sensitive analog signal and care was specifically taken in the design to ensure best physical routing around these sensitive lines. However, it is possible that any additional dirt / flux / contaminant on the surface of the PCB can contribute to leakage current and messing with this signal line at the low current levels, which can cause 'flickering' issues. The easiest way to see this in action is to touch the feedback components with a greasy finger when the driver is running at lowest mode, and you may see the output flickering! Even probing the circuit with a scope needs to be done with care since the scope probe adds significant capacitance to parts of the feedback circuitry.
That said, from the samples I got from Neal, the boards look to be very cleanly soldered, and I haven't been able to replicate this issue in my production driver boards, or even the pre-production ones from Neal.
Regardless, the components most sensitive to this are the op-amp (the smallest 6-pin component on the board, near the edge of the PCB surrounded by a bunch of resistors and caps), as well as those resistors and capacitors. The specific op-amp chosen has very high precision, very low input current and very low input off-set, so it's critical that this component is used and not swapped out. A small amount of contaminant in a humid environment can also introduce leakage currents.
For cleaning, I like to use a stiff small brush and isopropyl alcohol to clean boards. Something like this with the bristles cut down work really well, or a small stiff paintbrush: https://www.homedepot.com/p/Lincoln-Electric-Flux-Brush-KH589/202939867. I hope that could help alleviate the problems.
Meanwhile, I'll continue to validate the driver boards I have and I'll post more updates over the next few days (will be out during the weekend but back after).
Thanks to all again!