Femtobuck 1A

Components & Accessories LEDs
1

I need two femtobucks converted to 1A, any images/examples of what a proper modification looks like?

This thread says to add a third resistor but the hookup guide says to replace the resistors.

2

The default is replacement; you’d replace the 0.3 Ohm ones with 0.1 Ohm and be good to go…the linked thread is in the uncommon case that you do not plan to replace the resistors and instead add a third in parallel. Either should yield the desired result

You can solder the 3rd 0.3 resistor in parallel https://cdn.sparkfun.com/assets/4/a/3/d/0/femtobuckv13.pdf

https://cdn.sparkfun.com/assets/learn_tutorials/4/3/0/13716-02.png or replace the ‘current sense resistors’ pictured with 0.1 :slight_smile:

FWIW I think replacement is simpler

3

Thanks Russell. So just remove both resistors and put one 0.1 on one of the pads?

4

Yep - The left one is R1, the one you’re after…in theory you could just swap it only but I imagine that could be tricky with the other one in the way

5

Hey Russell…I went to change out the resistor today and pulled two pads off. Any tips to make the change easier or not really feasible to do by hand? I was using a really small tip to get to the ends but dont have tweezer tips to heat both sides.

EDIT: I actually was able to scrape off some mask right behind the pad and placed the .1 ohm resistor. Its measuring .08-1.1ohm from VIN to the far side of the resistor so I think I was able to save it.

6

Nice! That was going to be my suggestion…scrape off some solder mask with something slightly rough (dremel, etc) and wire/tap into the trace

Good work! Those are very small and fiddly

7

Hey Russell.

I need to make another 1A board and lost one pad on each of the other two resistor pads. R1’s pads are fine. Do you think this is ok before I go through wiring everything up?

Thanks again for the help!

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8

Unfortunately no…the pad is gone entirely, you’d need to sand/grind the solder mask down next to it and solder to the bare trace which is much easier said than done

I’d recommend grabbing a replacement

9

Thanks Russell. I figured that might be the case.

I did solder on the .1 resistor anyway and I do get .1 resistance across the AL8860’s pin4 (set) and pin 5 (vin). Are those the correct pins to test the resistor replacement? I will try to replace the resistor again on another board anyway, I have extras.

10

If you only have one in-place and the set jumpers soldered, yes (it’ll be 0.05 with both 0.1 resistors working)

11

Does the set jumper need to be soldered with only one .1 resistor?

I did replace on another board at work where we have a proper rework setup.

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12

Hey Russell.

Just getting clarification on your last reply. Does a single .1 resistor need the jumper closed?

You also said “both .01 resistors working”, but I thought only one .1 was needed?

13

It’s confusing because in this case you arfen’t using it in a standard way…Check out the schematic for clarity https://cdn.sparkfun.com/assets/4/a/3/d/0/femtobuckv13.pdf (there are 2 paths, only R2 goes through the set jumper if closed…it doesn’t do anything otherwise. Relevant areas are R1, R2 and SJ2). It’s just a classic parallel circuit, everything simply follows general ohm’s law rules 1 / R1 + R2, etc for calculating the circuit resistance (and subsequently the power).

14

Thanks Russell.

I understand now. Looking at the schematic again, out of the box it’s .3, jumper closed it’s .15 and replace both with one lower resistor it’s .1