Probably not practical… it would have to dissipate a LOT of power. If you are dropping 2 volts at 2.5A that would be 5 watts.
Electronics noob here to trash your thread!
Why would a Zener be better than a cheapo black barrel silicon diode, or a silicon diode in one of those heatsinkable packages that you see FETs in? Do you just need something that will drop a few volts, may be inefficient, but may be easiest.
Can we discuss this more please guys, what other options are there to physically limit the red channel (so the PWM settings in the PCM can be equal)?
Remember I need it to fit on a 17mm board!
You know what they say, always go full retard.....I think.
hehe
Don’t know enough about FET’s to be be technically accurate enough for the discussion
I wonder if you could calibrate different gauge wires, as in on the red one, use a 30 ga wire instead of a 22 ga and see if it restricts current flow enough not to overdrive the red led
In a fet driver with a led that can not have the full current the question is just: where do you want to burn the power, and perhaps a resistor (near the edge of the board) is a better place than a fet or a wire?
I’ve asked comfy to weigh in on it, I’m thinking about the possibility of purposely picking out a bad fet, that is one with higher RDS (on), to lower the current by adding resistance in there instead of needing another resistor. Same idea as with the different wire size, it shouldnt take a dedicated resistor to add resistance into the circuit if its determined thats all it would take to adjust for the lower vf.
I have the one light running now with everything equal and while red is brighter cause its being overdriven it is running otherwise on evenly powered channels, this one will be alot more power tho
If it needs one where should the limit resistor go? In line with the FET’s source (GND) or in the red LED’s + path (or both)?
I’m now using a 14 pin QFN PIC chip and will be running 4 channels (R G B and W) with the option to add one more for UV so space is very very precious on this 17mm single board driver, I still intent to have a batt+ contact on the bottom for direct installs.
Don’t put it on the FET ground… the FET may not turn on.
The RGB stove strobe is the bees knees! The LA county is fielding a light that is nicknamed the “Puke Light”. It’s a giant clunker of an RGB strobe. Each channel varies frequency randomly and independently from the others. Make a triple XPG2 like that and you could save lives. Make a quad with the fourth channel for white and that would be awesome. Keep it up.
Well guy’s I’ve finalized the plans for the 17mm driver and have a rough draft up, it still needs several things (notably the programming and SW+ pads and t and bstop rings at the edges to expose the GND rings (they’re there, just not exposed cause I forgot) and other stuff I’m sure but here it is for now, single sided! Nothing on the bottom and still has all hookups from either side (for piggybacking applications)
Making this list more for myself, feel free to add to it…
To do
•tstop and bstop top and bottom GND rings
•Add programming (and SW+) pads (SW- is GND)
-SW+/MCLR = pin3
-CLK = pin 11
-DAT = pin 12
-add pads inline with pin 13 (GND) trace and pin 16 (VDD) trace.
•Add 1:30 and 3 o’clock GND ring via’s
•Add pad (via if possible) pin 2 for UV output (output is NON PWM)
•If possible make all programming pad’s via’s so it can be programmed from the batt side while installed in light
•Find way to make via’s without solder stop on one side (or both) so its covered.
Remember QNF packages have NO EXPOSED LEADS out the sides so any pin needing used MUST have an actual pad.
Hat’s off for trying this ambitious project. To be clear, led’s will be direct drive, one die at a time, no color mixing?
I’m guessing that the fet’s will end up on the backside again to allow for more vias from the micro; like for a 5 pin (0.05” O.C.) SIP programming connecter. Also a smaller B+ pad for a post or upside down spring. There is a 14 pin Pic with 4 hardware PWM’s (16f1503).
The channels are individually controllable, just one or any number can be on at any time and each can be at any level, with enough programming it’ll be able to make about any color.
I really want to keep the FET’s on the front, I like the clean back look but will be adding the pad’s/via’s for programming, instead of a premade header/connector I plan to use another identical non-built board with header pins soldered to it (with wires direct from the programmer) to make the connection for ISP without having to even remove the driver from the pill (that’s one of my main goals anyway)
Thanks man, this is actually the easy part, it’s the C coding that’s gonna be the real test for me… I need to use this specific MCU cause at first I’ll be running FW provided by 3ttereve till I can write my own. His UI uses a momentary switch however I plan to write mine to work with just a rear tail clicky cutting power for both on/off and mode switching so I have more host options. (I’m actually building this 17mm baby FET version specifically for a supfire A6 with a quad MCPCB and optic and W,R,G and B emitters so it has to be able to run without a momentary switch).
Another application for the rear clicky only FW is to eventually make a triple S series light with no white, only RGB, I want to make that my EDC!
People suggesting heatsinking for whatever method you use to drop the excess voltage for the red channel are on the right track. You’re talking about quite a lot of heat. It seems like dropping it using a power diode next to the edge may be the way to go.
On covering vias (“tented” vias), see post #426 in the Knucklehead Buck Driver Discussion Thread. I generally crank the limit value up higher than what I’d use and then manually set the Stop anywhere I want it.
The power supply cap should be on the micro side of the reverse polarity diode. SW+ should have a 0.1uf cap in parallel to the switch for debouncing (assuming the MCLR pin is made a digital input, and the internal weak pullup is enabled).
By way of explanation, please allow me to point you here: FETs and gate resistors - scope images.
Debouncing the momentary switch pin may not be a bad idea, but I don’t think we’ve been doing it on other designs.
1) I once built a board for a linear driver and had my supply cap on the led side, instead of the micro side, of the reverse polarity diode. That driver didn’t work properly until the cap was placed on the micro side. I can only assume that the micro was being starved of power in that configuration, causing a brownout or reset? I realize that this was not a fet driver, but any micro manufacturer usually suggests that a bypass cap be placed as near to the micro supply pins as possible.
Can’t argue with the comfychairs’s scope pics. There was no reverse polarity diode on my 2014 contest DD fet driver, so no boost voltage to worry about. No gate drive resistor either, and seems to work just fine.
2) The DD fet driver of my 2014 contest build had the problem of skipping modes, because of the noisy momentary switch. Using a 0.1uf cap in parallel (along with the internal weak pullup) took care of that problem as a “hardware” solution, pretty common practice, highly recommended. It can also be done in software too, but I’m lazy in that way.
I don’t disagree with anything you are saying. FWIW I’ve been advocating moving the decoupling cap as close as possible to the MCU while keeping it on the BAT+ side of the protection diode. OTOH if we left the decoupling cap in it’s normal place (right next to the MCU with nothing in between) and just used a really large cap across BAT+/GND we could maybe smooth the problem out that way too. Bottom line is that right now comfychair seems to be getting away with just using that single cap as described, at least with a single FET / LED running at moderate currents (AFAIK).
Got some time this evening to get on some stuff, here it is now, still need the CLK and DAT via’s
Now that I can see it let me point out the issues-
no bstop on the Batt+ pad then in the 3 large traces from the center pad to the outer arc for the LED+ via’s there will be a spot at 8mm out that’s exposed for an 8mm spring to make contact with the 3 traces, the pad is 5mm for either a 105c spring or a brass button.
I rearranged stuff to leave room at the top for the possibility of adding a bank of limit resistors. remember the old version of the 15dd has a limiting resistor on the FET’s GND leg that you had to jumper if you wanted full current, is that not the same principal I’m needing here?