Oshpark Projects

What's the minimum-cost oscilloscope that'll give us a picture of what's going on with the gate signal? Are the sound card-based DIY things good enough? Are the Hantek 2-channel USB scopes worth the going price of $60-70? We don't need lab-grade precision, just something that shows the waveform, right?

In a like-for-like comparison between the AOD510 & 70N02 - identical SRK-DD drivers/firmware, same cells, same 3XP/XPG2 board, I measure 41.5mV drop across the AOD510, and only 32.6mV across the 70N02. So the specs in the datasheets that show the 510 having lower Rds(on) don't hold true in this lower-voltage application.

I don’t know, but the soundcard option is free. I think you want your sample rate to be at least a few times as high as the freq of the waveform you want to look at.

Is it possible/convenient for you to re-run that test on 5.0-5.5v input to the ATtiny13A? If doing that turns the tables we’ll know more about the situation.

In terms of the ringing problem, I thought to go and look at what the East-92 schematic does to help with that. It seems that a diode is used across the LED. Maybe this is similar to what I’ve seen referred to as a “freewheeling diode” elsewhere - the Schottky diode in a non-synchronous buck driver. When the sudden collapse hits (FET turns off) there is a path for the stray electricity to get away through… Anyway that’s the only reason I can see for putting a diode there. “Post #185”: East-92 4amp 17mm driver in the East-92 4A driver thread. Of course that wouldn’t come up in the TI document about synchronous buck converters since it would never apply!

I can short across the protection diode to raise the voltage on both drivers, would that be enough? Not sure I want to risk trying to rig it up with 5.5v powering the control side and 4v powering the LED side.

The SRK-DD board already has the provision for a big schottky at D2, between LED- and B+. It's not used on these two examples, as neither is using the inductor. I can add the diode to both, but since these both work properly I don't think that would tell us anything. Adding one to the 17DD would be... awkward.

If it’s easy to do it won’t tell us nothing… But it’s not ideal. The protection diode should only be dropping 0.2v.

In theory if you had a scope you could see the difference on that when you add the big schottky. Without that, the only way for us to observe the effect would be on the 17DD.

Hang on... can I just jumper something like 6v onto the gate pin from 2x CR123s, using the normal 18650s to the LEDs? With the MCU 'off'?

For the purpose of measuring Vdrop at 100% duty cycle? Good idea, but that will exceed the absolute maximum rating for “Voltage on any Pin except RESET with respect to Ground”: –0.5V to VCC+0.5V

So now that I think about it, here’s what you can achieve with jumpers if you are so inclined:

  1. jumper the protection diode to pickup 0.2v
  2. then jumper VCC to the PWM pin to pickup another 0.7v-1.0v on top of that

And that’s not nothin’. :slight_smile:

Thinking about it a little more, let’s go for a bonus round. Instead of doing both jumpers, just stick your DMM on the protection diode and confirm that it’s dropping <0.5v. If so you are good to go on jumpering directly from BAT+ over onto the PWM pin / gate pin.

I did see your project but did not realise that the board could be used like this :-) Sooo, if you replace the bottom by a big +pad and a fat -ring, the one-sided 17mmDD board would be almost there, but the reverse polarity protection would be lost because there is no space for the diode? But reverse polarity protection is a very useful feature (I do insert batteries the wrong way every now and then), I am not sure I would like the board without it.

(always complaining )

That board has only 7135’s on the bottom. D1 is on top between the mcu and the FET.

Ok, sorry, I switched on my brain and dived a bit into the board, now I see: the protection diode is there. So do I understand correctly that the place where the trace starts that the MCU uses for the voltage measurement for LVP is different from the 105C, namely on the other side of the diode, and this must be compensated either by the firmware or otherwise by using a different R1? Can this also be solved by cutting that trace and make a bridge from the trace to the other side of the diode?

Yes, you understand correctly.

That change in the trace layout was to allow the voltage divider to see the full battery voltage even after a Zener mod so that lvp could be made to work again with just a change in R1. It means that the 19.1k value for R1 isn’t quite right for single cell operation though.

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.

1,000

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...

PWM only limits the average current, not the peak current that occurs with each PWM pulse. If the pulse currents are too high, you could fry an LED.