I assume the BLF15DD V3.0 OSHPark listing here: https://oshpark.com/shared_projects/lQUEtZcJ, has the wrong value for the R4 resistor? Says 100 ohm, should be 130 ohm? Can Mattaus correct that?
So the "Revision 2" has no limiting resistor while the V3.0 does. This is a little confusing between V3.0 and Rev 2 - different nomenclature - I'm not even sure if the rev # pertains, because one version has an option while the other doesn't. I wish rev #'s could be put on the silk screening somewhere, because I have mutiple board rev's all over the place... Are there any recomendations for R3 values if we want to limit current? Is anyone doing that? There's no BLF15DD discussion thread, so no source of info on this board.
The limiting resistor is just a resistor that goes between the FET and ground. Knocks the current down a little or a lot, depending on... everything. The idea was stolen from the generic little drivers like these:
R300 is the limiting resistor, 0.3ohm. Most of the time you'll just replace it with a jumper anyway. It's still direct drive, meaning the output is determined by the input voltage and the LED's Vf, the resistor just reduces current across the board. You could do the same by using a PWM value of less than 100% (and scaling the other modes down to match).
The value for the gate resistor at R4 should be the minimum value that lets the mode changing happen like it's supposed to, whatever that number turns out to be (that goes for all these drivers, no matter which size or version). 130 is just a conservative starting point mostly guaranteed to work. If it works with a smaller value, you should use a smaller value. If it works with none at all, you should replace it with a jumper. Using a higher value than the minimum required makes the FET's life much harder (slows down the switching speed, and generates unnecessary heat while the FET is in that not-on-but-also-not-off inbetween region).
Thanx comfy - planning on using a couple of these on those AA size cheap zoomies (https://budgetlightforum.com/t/-/21936), so don't want too crazy of amps.
Tom just an FYI, if you don’t want to use a limit resistor on that version of 15dd you must use a jumper otherwise the FET’s source pin has no connection to ground.
Personally I find the newer 15’s to be much better overall, I “limit” them by setting lower PWM levels, never liked the idea of the limit (sense) resistor.
Well, PWM's have their issues - so I'd rather keep 100% usable. I'm having a really tough time programming this board. The resistors to the right of the MCU are too close. Finally got one programmed by filing down the sides of the clip, but the other just won't program no matter what I do, and even the clip looks perfectly seated. Boy these are a PIA... I think I read others struggling with programming because of clearances - not sure if it was the BLF15DD or not. I'll have to remove the resistors and see if it will work then. Might as well try something because the board will be useless with no firmware anyway...
Hi Tom, you might want to pick up a Nanjg programming board and preflash the driver before you solder it in. The smaller drivers were too crammed for space to allow for clip clearance. The board will save wear and tear on your clip making it last longer for the drivers it will fit.
Thanx Scott! Actually I got the BLF15DD boards working! I did end up removing the 2 resistors, then I could program it. The other board I build up and could program. Think for me, I'd be ok with doing the parts reflow in 2 steps - seems to work ok for me. Actually I think I got those OSHPark boards - but didn't get a setup to use them.
My $3.xx "slim" mini CREE AA zoomie is sure zoom'n with a BLF15DD and a XP-L V6 on a 16mm Noctigon (sanded down)!
CC, I checked out your thread. Good stuff and hopefully we can all learn more about these drivers. Would you like me to put a link in the FET board header? You might want it linked in the various FET discussion threads as well. Be nice to verify that the signal bounce is the cause of the erratic behavior and not something intrinsic to each different FET.
I am only able to guess right now about the weird bounce being the cause of erratic behavior, still have lots more configurations to look at before the fog will begin to clear. Specifically, what does the waveform look like on the SRK-DD boards, which have never had issues running correctly without the extra resistors. The overall trace length on the V2-up 17/20DD is longer than the SRK even when the 17/20's resistor is replaced with a jumper. I also have some of the first version 17DDs with the super-short trace and without the resistor pads, that would be an interesting comparison too. And then there's all the hardware variations and how they work with different firmware versions... like does the waveform look the same for fast-PWM luxdrv and fast-PWM STAR clicky and fast-PWM STAR momentary?
See post #1045 for current status - new version that no longer requires an additional capacitor, but instead just relocates the single original cap
ATTENTION
Gate/pulldown resistors no longer needed on the 15/17/20DD drivers (and though the SRK-DD technically works as-is without the resistors, it likely has this same voltage spike issue, it just happens to not be as severe, so this same fix needs to be applied on that one as well).
In the normal build of these drivers, there's a HUGE voltage spike at the MCU's Vcc pin #8 when the FET turns off. Up to 6v or higher... and if it goes too high, the MCU's overvoltage protection kicks in and it shuts down until power is removed. It also puts 6v onto the FET's gate, but that's a result of the voltage spike issue, not the cause. This is what it looks like on a driver with the gate resistor bypassed with a piece of wire (blue trace is the signal at the FET's gate, yellow is the battery voltage (the signal looks the same when measured either before the diode at B+, or after the diode at MCU's Vcc):
This is what happens if you place another 10uF/16v capacitor in parallel with the polarity protection diode, no other changes:
That's not perfect but I wouldn't expect it to be, and as long as it stays in the less-than-6v safe range it doesn't matter. Problem solved.
See post #1045 for current status - new version that no longer requires an additional capacitor, but instead just relocates the single original cap
Would a resistor in series help? That capacitor sure does smooth out the pulse to the capacitor, would a larger capacitor smooth it further (or were you staying with the off time cap for commonality)
The off-time cap is 1uF, the 10uF is the same part as used as the main cap at C1. I don't think any value cap will totally eliminate the oscillation even if big enough to no longer fit inside a flashlight - and as long as it isn't a spike high enough to trip the attiny's overvoltage protection I really don't care. There are lots of hardware combos that work fine with less-than-perfect waveforms on various parts of the circuit. It just has to work, not look perfect.
