-The FET driver from the OP, and a lot of other FET-drivers use PWM for lower modes, the FET is used as an on/off switch.
-in very few FET-drivers (I know just the drivers sold by led4power) a different type of FET is used that can regulate the current controlled by a chip, so the lower modes have no PWM.
-a FET-driver developed here on BLF (contributions by RMM, wight, and others, a famous firmware for it written by Toykeeper), also uses a FET in combination with a 7135 current regulator. The 7135 is used for low modes, the FET for highest modes. Most modes have PWM, except the highest (full FET) and 350mA (full 7135).
I have been considering a similar mod to the one in the OP, but leaving one or more 7135’s on the board, making for a cheap dual channel fet driver using STAR firmware. Has anyone done this? I think it would just be a matter of some trace cuts and maybe an air wire or two.
I am thinking the R3 pads on the AK-47A would be ideal for the OTC cap if the existing traces to the MCU pins and the via were cut. It looks like R4 on the AK47C would be even more ideal for the OTC since it is right next to pin2, if it doesn’t already have a trace to it! Can’t tell from the photo. I suppose a really tiny cap would make jumping from pin 2 to ground easier on drivers where there isn’t exactly ideal placement for an otc.
Hmm. Offtime cap from pin 2 to pin 4, jumping over pin 3, would work fine too eh? Could be a simple solution when there’s no board space, and rather than cutting traces and stuff.
The OTC is useful with “offtime” firmware but is not necessary for the operation of the fet + 1 scheme. For E-switch / momentary firmwares you would connect the E-switch to pin 2 and not use an OTC.
For most dual PWM firmwares, it seems that Pin 6 is PWM and pin 5 is ALT PWM. This is the case for A6 and STAR variants by default. By default, alt PWM (pin 5) is the single 7135 pwm channel, and PWM (pin 6) is the fet channel. Be sure and define the mode levels based on which pin controls what.
TK-otc, A6, starry-offtime, STAR momentary, MTN momentary temp, are some of the dual PWM firmwares that would be good to look into. TK-otc is like A6 but easier to configure.
First attempt at reworking an AK-47 A. I see a hot air rework station in my future.
Cut the MCU trace from pin 3 and used the R3 pads for the offtime cap. Rotated an exacto blade in the via of star 3 to remove the potential to ground. The reset pin isn’t shorted to the cap although it looks like that in the picture.
Pin 5 controls the two remaining 7135’s; I jumped some solder to the PWM trace that used to connect to pin 6. The pin 6 trace was cut just after the PWM pad and before pin 5, and the 7135 chip right in front of it removed; that PWM pad is used to connect the wire leading to the fet gate.
Used a PSMN2R4-30MLD fet which is LFPAK33; it was what was in stock at MTN. It’s got the gate on the inconvenient side. I bent the gate leg up and ran a wire from it to the aforementioned PWM pad which connects to pin 6. Some super glue was applied to the top of the fet to pot the wire. The fet could be closer, diagonally, to the source pad by about .5mm I suppose, thought that pad might need trimmed in order not to short to drain.
Nice work!
First question , why 2x7135 ? Having one only wouldn’t be better ? (lower moon mode)
Also , did you modified the firmware ? I’m interested in the firmware you used , if you can share it of course!
I figure maybe 2x7135 will still offer a low enough moon mode—will have to test. A benefit of having two 7135’s is I could have a regulated 700mA mode for medium, or for a higher medium with some fet output in the mix, I believe the output will be “more regulated” than it otherwise would be. I’m not exactly sure how that plays out.
Any of the dual PWM (w/dual channel control) firmwares I mentioned a couple of posts back will work good. I’m using tk-otc. (Thanks ToyKeeper) I change lots of things but none are necessary to get it to work, I think. Still have testing to do.
Good thinking!
With the nice BLF-A6 driver around I forgot about this little project. I actually have this driver around and will try it, good option and cheaper than the BLF-A6 driver!
