CN3791 MPPT Solar Li-Ion Charger Module Hinky Circuit.

Last year, I paid about $3.66, with shipping, for this solar-powered MPPT lithium ion battery charging module on eBay to use with my small solar panels and scavenged 18650 batteries. It has some issues.

First off, the version I purchased/received is intended for 9v solar panels and I wanted to use it with a ~6v panel. This is set with a resistor divider. Careful study of photos from product listings showed that the divider was implemented using the same resistor value for the high segment of the divider, changing only the value of the lower segment’s resistor to change the setpoint.

The high segment had a value of 178KOhm and the low ranged from ~42KOhm for a 6v panel down to 12.6KOhm for an 18V panel. I didn’t have any SMD resistors of suitable value in my supplies, and I couldn’t find any I could scavenge on any surplus PCBs. I decided to use a trimpot instead. I had a variety on hand, and it would allow me to experiment on the optimal clamping voltage for the panel I had on hand, and an 18V panel I’d ordered. I chose a 200KOhm trim pot with the idea that approximating the total resistance of the existing divider would help preserve the stability of the control loop. If I were going to do it again, I’d probably choose a different configuration to minimize the impact of the pot’s temperature sensitivity. A simple choice would be ~20KOhm trimpot, configured as a variable resistor (short the wiper to one terminal) used it to replace the low segment, leaving the 178KOhm resistor in place.

After adding the potentiometer, I connected the battery and panel and adjusted the potentiometer until I maximized the charging current. I was a little surprised by how low the panel voltage was, and so I started poking around. The first thing I checked was the voltage drop across a P-Channel MOSFET on the panel input. I was surprised to find that it was 500mV, though knowing that, I wasn’t surprised the IC was noticeably warm. The panel was dissipating 1/10th of the panel voltage over the MOSFET!

Some of the photos on some of the product listings showed a simpler circuit, without anything in the panel input current path. My guess is that the MOSFET and accompanying resistor and diode were added in a revision in order to protect the circuit in case the panel polarity was accidentally reversed, and/or to block leakage of charge from battery through panel at night. A schottky diode would accomplish the same thing more simply, but with a voltage drop of ~300mV. Properly implemented, a MOSFET based “ideal diode” would have an effective resistance of ≥ 50mOhm, and a voltage drop of ≥ 50mV at the ~1A max current my panel could deliver.

I’m not completely sure how the circuit was intended to work, but clearly, it wasn’t doing the job. I wondered if it would work properly if I was using the module with a 9V manual, as intended, but that didn’t seem possible, either. The panel + was connected to the MOSFET’s source, the rest of the circuit to the drain, and the gate was connected to the drain via a resistor and diode. By my reasoning:

  • that the gate would ≅the potential of the drain
  • the voltage drop from source to drain should be as close to 0V as possible in order to maintain the efficiency of the curcuit
  • therefore, Vgs would/should approximate 0V
  • but it won’t because the Vgs threshold for the MOSFET was ~2V!

I wasn’t sure how to fix the circuit, but I was sure that the gate needed to be pulled down to a lower voltage, so I cut the trace connecting the resistor the drain and connected it to ground instead. It worked well enough that the voltage drop over the input MOSFET went from 0.5V to a trivial number. I’m pretty sure though that I didn’t fix the protection function.

I’ve since received another version of the module which has revised the input circuit. The diode and parallel resistor connecting the gate and drain are still used, but there as another resistor which connects to the charging indication pin on the CN3791, and in so doing. This pin is open drain. When the battery is charging, it is pulled low, lighting the charge indicator LED AND pulling the input MOSFET gate low. Vgs ≅ -Vpanel ≅ Vs ≅-6V, turning the MOSFET fully on.

Thinking through this further… if the battery is charged and the panel is illuminated the gate will approximate the potential of the input MOSFET drain and, since the only load on the panel is the quiescent current of the module, then Vsd ≅ 0V ≅ Vgs and so the MOSFET will be off, save any current through the body diode.

If the panel is dark and the battery is charged then Vd of the input MOSFET will, at most, be at battery voltage (Vbatt), Vs will be ~0v, Vg will ≅ Vd, Vgs ≅ Vd and the input MOSFET will be off.

If the panel is reversed Vs will be below GND and well below Vg ≅ Vd ≅ Vbatt so Vgs will be Vbatt + Vpanel, and the MOSFET will be off. Note: This means that reverse polarity with an ~18V nominal panel would exceed the Vgs maximum of 20V for the TPC8107 MOSFET used at the input.

If I get around to it I’ll draw a schematic and add it to this post.

VG-10ish Osram “White Flat” Mod.

Last week I received some OSRAM KW CSLNM1.TG emitters (aka the “White Flat”). These are dome-less, small die (1x1mm) LEDs. Today I installed the first one in a VG-10 style flashlight. The results are impressive.

I don’t have good beam shots, yet, but I can say that the hotspot is slight larger than the hotspot from a Convoy L2 with an XPL-HI emitter. Not bad for a light with a 23mm reflector.

Update: I measured the hotspots of this light, and my L2. The diameter of this light’s hotspot is about 1.25x that of the L2, so it has 1.5x larger area.

Poor quality beamshot. Also the automatic HDR on newer iPhones is crazy.

I’m currently running it off a 3 amp buck driver, so it’s isn’t putting out as many lumens as the XP-L HI in the L2. As a result, the intensity is lower. I plan on upgrading the driver soon to push it up to ~5A or so, at which point the output and throw should rival that of the L2.

These emitters have a 3x3mm package. They work pretty well on a standard XP footprint (3.5×3.5mm), but they don’t work so well with XP centering rings. I ended up using a modified XP centering ring. I placed it on a drop of UV set resin and then lifted out slowly so a thick film was left across the opening. I then “popped” that film so that the resin clung to the edges of the whole. I hardened it with UV light, then opened the hole up with a small triangular file until I had a centered opening that fit the emitter. It came out pretty well and the focus isn’t bad either, though I’ll probably try and adjust it further when I open the light up again for a driver upgrade.

I have two more of these emitters. Next step is to put one of them in a C8. I’m not sure what to do with the last one. I’ll probably put in in the L2.

Stripped to conceal poor removal of ugly “Forfar” logo. I like it. Protip: UV set resin makes a nice & precise resist.

Solder paste stencil for Noctigon 4XP 33mm MCPCB / Emisar D4S

I wanted a solder paste stencil to make it quick and easy to reflow emitters onto  Noctigon 4XP 33mm MCPCB used in the Emisar D4S flashlight. I made a gerber file that I used to cut a stencil out of polyester sheet using a craft vinyl cutter.

Most flashlight hobbyists don’t bother using a solder paste stencil when reflowing emitters. They just slop some solder paste onto the MCPCB thin it out a bit, put the emitter on and then heat the MCPCB until the solder paste melts. Then they wiggle the emitter around to make sure the solder is evenly distributed. Finally, they give the emitter a “bonk” to eject excess solder and let it cool.

I’ve used the slop & bonk method before with decent results, but its a little to fussy and inconsistent for my tastes. It’s workable for single emitter boards, but with triples and quads, trying to manage the inconsistency leads to more fussiness which leads to the emitters being heated longer, which can reduce their efficacy.

Solder paste stencils reduce variability and fussiness. They make it easy to apply a precise amount of paste evenly, which makes it practical to reflow emitters with little or no manipulation during the molten solder phase.

I’m sharing the gerber file in case anyone else has access to a suitable cutting plotter and wants to make their own. I used the open source Gerber2Graphtec software to convert the gerber into a file that I can send to Silhouette Cameo 2 cutter. This software only works with Graphtec-based plotters, like the Silhouette family.

ThorFire C8 2×2 “Frankenquad” Mod

I’ve been looking for an inexpensive way to build a compact, high-output (>2500lm) flashlight. This post documents a successful result.

There are two straightforward routes to a high-output flashlight. The first is to use a high-output emitters like the Cree XHP70.2. The second is to use multiple (often three or four) lower output emitters.

The downside of high-output emitters is a combination of expense and limited choices for both driver and emitter. The emitters themselves aren’t badly priced when considering power/$, but they typically require a 6v power-source, which requires either a high powered boost driver, or two batteries in series, requiring a larger host. In addition, there are fewer choices for CCT, tint, and high-CRI among high-power emitters.

Multi-emitter configurations are commonly configured with the emitters in parallel, meaning they can be powered with a single cell controlled by a wide-range of drivers. In addition, they can most of the 3v emitters on the market, in any combination your heart desires. The downside is that they generally require specialized MCPCBs, which must be pared with optics or reflectors with matching spacing. These reflectors and optics are shorter than the single reflectors most hosts are built for, so they either need custom spacers, or a specially built host.

My project has the advantage of working in a variety of single-emitter hosts with minimal modifications, while allowing the use of 3v drivers and a wide array of emitters.

Ingredients:

  • One Thorfire C8s
  • Four warm white XP-G2 of unknown flux binning…
  • Mounted in a 2×2 array on a direct thermal path copper MCPCB which electrically connected them in parallel
  • DCfix diffusion film
  • Lexel’s 17mm version of the TA v1 driver.

For my first pass, I used an orange peel reflector I had on hand in the hope that it would, on its own, blend the beam enough to eliminate a dark spot in the center caused by the gaps between the emitters.

When that didn’t work, I used some diffusion film, which worked really well, well enough that I decided to try the original reflector. The film was still enough to blend the beam enough to remove the dark spot in the center of the beam.

The end result is bright and quite floody. I don’t have a great way to measure the brightess or intesity, but I’d guess that its less bright than my BLF Q8 and brighter than the 3x Nichia 319a emiters I put into a Sofirn C8F

I still have more of the MCPCBs, I’d like to try another build, but I’m not sue what emitters to use. I want more power, and also higher CRI.

Update 2018-09-02:

I got a UNI-T UT201-E clamp meter last week and rigged up a modified tail board so I can measure the current draw of flashlights. I wanted to approximate the electrical characteristics of an actual flashlight, so I used a standard tail-switch board, with a bypassed tailspring. I attatched 13AWG wire so I have a loop I can use to measure the current with the clamp meeter. I also put a standard Omten 1288 switch in line, to reproduce switch resistance. With fully charged, high-drain cells, I got a peak of about 11A. Not bad, but I wanted more.

After studying datasheets and independent tests, I decided to use some 90 CRI Samsung LH351D emitters I bought recently. By my estimates, they’d peak at ~4A each (16A) total, and produce a peak of ~1200lm each, for a total of 4800 lumens, peak (out the front lumens will be lower). Not bad for a 90CRI light.

I have 5000K emitters (PN: SPHWHTL3DA0GF4RTS6) and 4000K emitters (PN unknown).  I decided to use two of each. I reflowed them on to an empty 2×2 emitter MCPCB that I’d lapped for better contact with the emitter shelf. Once I had the light back together, I did more tests.

XP-G2 (left), LH351D (right)

I’m happy to say, my estimates were pretty good. With a Sony VTC5A, the peak current draw was over 18A. With Samsung 30Q or LG HG2, the peak current was ~17A. I don’t have way to measure output, but I’d assume those numbers are on track too.

Not surprisingly, the light heats up very quickly on full power.

To give some sense of beam uniformity, I took a couple shots at different distances against blank surfaces. Don’t compare the color between shots, the surfaces are different shades of off-white, but the color uniformity in each shot should be useful.

Progress on Custom Triple LED MCPCB Project

I’ve been making slow, fitful, progress on an custom MCPCB assembly for my triple Luminus SST-40 flashlight build. I’ve had to back up once or twice, too, but I’m getting a lot closer. In retrospect though, I probably should have reworked things and rotated the SinkPad mcpcb’s so all the negative contacts were oriented on the outside, and the positive contacts on the inside (or viceversa). It would have made the electrical interconnects much easier.

I ended up with at least two interconnect circuits built and ready for integration with the MCPCBs, and a few more that I either decided weren’t worth finishing for one reason or another, or screwed up near the end.

This is the first one I finished. I wasn’t happy with it for some reason, I think because I thought the insulated conductors were too thick, above the tops of the MCPCBs. I should have just stripped the heat shrink insulation and relied on epoxy or something, but I didn’t

This is what I ended up with. I need to solder copper strips to them Then I’ll dip them in epoxy to insulate them, solder the strips to the MCPCBs and pot the whole thing in high temperature epoxy so I can reflow the emitters.

Then I have to figure out what to do about making a spacer to fit the assembly to the UltraFire F13 host I’m planning to use this in.

On the upside, if I do this again, I have a much simpler idea in mind. I just need to draft a PCB design that will do double-duty providing the interconnects, and positioning of the sinkpads.

Custom Triple LED MCPCB Technique

I’m trying to build a triple emitter flashlight, but not just any triple emitter flashlight. I am not taking the path of least resistance, which has presented some challenges. But first, these are some of the salient details of what I’m trying to build:

  • SST-40 Emitters
    • Same 5050 footprint as the common XM-L & XM-L2 emitters.
    • Better beam, less tint shift than an XM-L2 when used with a reflector,
    • More efficient than an XM-L, XM-L2 or XP-L.
    • Lower forward voltage than XM-L, XM-L2 and XP-L
    • Only available in cold-white (6500K or 7000K), but the tint is pretty neutral
    • Taken together, these characteristics make for an emitter than can be pushed to almost 2300lm @ 7.5A/27W off a single Lithium ion battery.
  • Triple reflector: The simple path to a triple-emitter build is to use a triple-optic, but the available options don’t really give the spot+spill beam pattern I’m after.
  • 26650 battery: Sofirn sells a great reflector-based triple emitter C8F host for just ~$14, MCPCB included. I have one. There are two problems with it. First, the provided MCPCB is for 3535 form-factor emitters. Second, it can drain any cell in no time, but 18650s have ~2/3rds the capacity of a 26650.

I have a reflector, and a host I can fit it to without much trouble. What I don’t have, and haven’t been able to find, is an MCPCB I can mount 5050 emitters to in such a way that they’ll line up with the reflector. I think the Noctigon XP32 has the right spacing for the reflector, but it only takes 3535 emitters. I thought I might have found a 32mm MaxToch PCB that would work, but it doesn’t.

My fallback plan is to solder some 11mm Sinkpad MCPCBs to a copper disc. The difficulty is getting them positioned properly and then keeping them in position while soldering them down. I’ve explored various ideas as to how to manage this feat, and I tried one yesterday.

I started by making a template in inkscape. It was a little tedious. Inkscape isn’t really made for technical drawings, so I had to double check dimensions and adjust alignment and sizes repeatedly. Once I had something I thought would work, I printed it onto some index card stock. Next, I smeared some non-corrosive silicone adhesive on the template, and positioned the little MCPCBs face down on the template. I pushed them down firmly, scraped away the glue around them with a toothpick so I could get a better view of the template, and then adjusted them until they were aligned as best I could.

A few hours later, after the glue had set, I double-checked the alignment, then I cut most of the paper away, to clear the way for my soldering iron.

I prepped the copper disc and the exposed metal bottoms of the MCPCBs by tinning them. I used lead free solder because I wanted something with a higher melting point than the eutectic sn63pb37 solder paste I use to reflow the emitters on to the MCPCB. I had trouble getting a nice even, thin, layer on the copper disc, so I filed and sanded the solder surface down before continuing.

I put a thin coat of rosin flux on the tinned surfaces of the copper disc and MCPCBs, and centered the MCPCB cluster on the disc. Then I heated the disc with my soldering iron. Once it was hot enough to melt solder, I fed solder wire to each MCPCB while keeping the copper disc hot with my soldering iron. Once they were all nicely flooded with solder, I tweaked the alignment with the center of the disc. Removed the heat, and pressed down evenly with a block of wood to get a nice close fit between the copper surfaces. Once it cooled down, I removed the block.

The first time I tried, the alignment of the MCPCBs was off center a bit, so I heated things up and tried again. This time the results were a little better. As you can see, the paper charred a bit, but I think it still did its job of keeping the three MCPCBs positioned relative to each other during the process of soldering them to the copper disc.

I peeled the paper and silicone adhesive off with tweezers, and then rubbed the remaining adhesive off with a cloth. This is the result. I’m pretty happy with it. Or I would be, if, after all that, everything was aligned properly with the reflector. It’s not.

I’m not sure what, exactly, went wrong. I think I had the spacing right on the template. I think the main source of error was probably in getting the MCPCBs aligned to the template while glueing them down to it.

I think that I’m just going to try reheating the assembly again and adjusting things until they line up better. If that works well enough, I’m just going to call it good and finish building the light.

If I can’t rework it well enough, I’ll probably desolder everything, clean it up, and use some thermal epoxy. It won’t be as good as the solder, probably, but it should be a very thin layer, so hopefully the thermal transfer will still be pretty good.

Update:

I reheated the assembly and adjusted the position of each MCPCB. I checked their positioning using the reflector before moving on to the next one, then one last time before removing the heat and letting the solder cool. It’s pretty good, but most importantly, it’s good enough. Now I have to figure out how to do the electrical hookups.

Amutorch AX1 Quick Look

I just received a newly-released flashlight from Banggood. It’s called the AX1 and it’s made by Amutorch.

It has a thoughtful, attractive design, and the overall execution is great. But the example I received has a few flaws that really detract from the experience.

First though, some quick observations:

  • Unusual, attractive, well executed, two-color design.
  • Attractive body form, with thoughtful, unconventional, functional details.
  • Large, 22mm, reflector for a 18650 tube light without significant added exterior bulk.  I measured the outer diameter as 25mm, which is just ~1mm larger than a Convoy s2+. The inner diameter of S2+ reflector, though is ~17mm, 5mm smaller than the AX1s. As another point of reference, the Zanflare F1 has an ~20mm reflector, while the OD of the bezel is about 27mm, and the max OD of the whole light is closer to 30mm.
  • Good action on the reverse-clicky switch.
  • Modes seem good. The AX1’s low is pretty close to the low on a Convoy S2+ with an 8×7150 3/5 mode driver. The high/turbo is similar to other ~3A ~1000 lumen lights. There is strobe, but it is pretty well hidden. It has mode memory.
  • The beam pattern is quite nice. Much less floody than a standard S2+ with an XM-L2 emitter and an OP reflector. At 10-15′, the hotspot and spill are much closer to an S2+ I modded with an XP-L HI and a SMO reflector, but the AX1 has nicer smoother, corona with less tint shift, though not as nice as it would be with an OP reflector.
  • I’d call the color temperature neutral white and without an objectionable tint.

Now, for the big problem with the AX6 I received: The surface texture on the blue head and tail pieces is visibly, glaringly, uneven. I had some trouble photographing it, but it is quite apparent when you have the light in hand. When I first saw it, I thought/hoped that it was some surface schmutz. It isn’t, it’s the metal.

Note the machine marks on part of the piece. This is one of multiple areas where the surface texture is inconsistent/in-complete.

It looks like it was supposed to receive some sort of uniform finish, like bead or sand blasting, but some areas were barely touched, so the machining marks are obvious. It doesn’t seem very even over the length of the head either, nor is it consistent around the axis. The problem is most obvious on the head of my light, but the tailcap also has similar but more subtle problems.

In addition, there were a few nicks on the body. The photo above shows the largest of them. These are unfortunate, but the truth is with a weeks use, I probably won’t be able to distinguish them from other wear and tear. The inconsistent finish is going to be obvious for a long, long time.

So, bottom line, this is a nice flashlight for $20-25 if the surface finish is as it should be. Mine isn’t. So I’ll be asking them to replace the faulty parts, if not the whole flashlight.