Did I kill my Femtobuck?

Components & Accessories LEDs
1

Hi,

I was trying to drive 7 UV LEDs in series using a femtobuck. I had it hooked up to a 31.7 V (measured), 625 mA DC source, and it worked. Then I wanted to increase the current to 660mA. From the data sheet of the [AL8805, page 6, I assumed that the input voltage and current are enough for 7 LEDs at 660mA.

I disconnected the power and followed the instructions applying a “glob of solder” [like this. But now I get no current or voltage at the exit of the femtobuck, even after removing the solder glob again. What went wrong, most likely, so that I won’t kill the next one too? I wonder if I clumsily unsoldered one of the R300 resistors, or otherwise overheated the board.

Thanks,

Mirko](https://cdn.sparkfun.com/assets/learn_tutorials/4/3/0/FemtoBuck_Image.jpg)](https://cdn.sparkfun.com/datasheets/Components/LED/AL8805.pdf)

2

Hi Mirko,

I am not sure how closing that jumper would completely fry the FemtoBuck here. Can you take a few photos of your FemtoBuck and the circuit you have it in and attach them to your response? That will help us get a better idea of what went wrong. Also, if you could include any information/datasheet you have for the UV LEDs you were driving with the FemtoBuck, that would help as well.

3

Hi Mark,

thanks for replying. Here are a few pictures:

femtobuck-jumper-details.png
femtobuck-jumper-details.png528×607 467 KB

top: the femtobuck as I intend to mount it

center: details of the jumper after soldering, then removing the solder again

bottom: the femtobuck with only a single LED attached

Even in the case of just one LED, the LED doesn’t illuminate. I also tried this with the femtobuck connected to a known-good lab DC power supply - LED doesn’t light up.

I got the LED from here: https://www.amazon.com/gp/product/B00OFR96XG/ but have little in the way of specs. The description only states:

DC Forward Voltage: 3.4V - 3.8V DC Forward Current: 350mA-700mA(when 350mA,power is 1 Watt;if 700mA,power 3 watt) Luminous Intensity: 50~55mW

Eventually, the femtobuck is meant to drive 7 LEDs connected in series mounted on a heatsink:
LED-on-heatsink.jpg (Note that there is a wire on LED #7 that is attached, but not connected to anything else yet. It’s really just a loop of 7 LEDs.)

The femtobuck did light up the 7 LEDs initially, before I tried soldering the jumper. Just not any more. If I connect the LEDs directly to the lab power supply, they still work:

lab-power-supply-7LED-working-2.jpg

So with the LED’s still working, and working in the way I meant to connect them, and the femtobuck working initially, at 330 mA output, I got the idea that somehow I damaged it when I tried soldering the jumper. Although I don’t quite understand why, and am worried that I’ll kill the next one too.

Thanks again for taking a look.

Mirko

4

Do you have a picture of what it looked like when the jumper was soldered close? Looking at the schematic I agree with what the Sparkfun support tech said about it not frying the board unless you potentially applied a lot higher wattage than what you’re describing. Have you verified that the power supply does not have an initial spike in voltage when powered on? I love the way the power supply looks, it’s got that great analog vibe, but looking at the Tenma power supply units, I see that some other models have historically had issues with a voltage spike from the output terminals not being isolated from the power switch - nothing for the 72-6153 Tenma specifically, but it’s something I would investigate if I were in your shoes and something decided to fry itself for no good reason.

5

Kansukee, thanks for taking a look at this.

Unfortunately I don’t have a picture with the solder still on. I reflexively unsoldered it, thinking I had shorted something and unsoldering would fix it.

The power supply in the pictures is only the one I used to check if the LEDs still work. The femtobuck died when I used an old 32 V/625 mA wall-wart type power supply from an old HP printer. It worked too before soldering the jumper. I suppose next time I will first use the regulated power supply and gradually dial it up to the target voltage to test the circuit. Then I’ll connect a ~100 Ohm (>10 W) resistor to my wallwart and check with a scope if there is any initial spike. Unless that sounds like a really dumb idea (does it?)

6

Hi again schoenitz,

Sorry for the late reply. Kansukee has a good point here regarding a voltage spike at power-on. That would be a good place to check since you’re running it right at the limit of the input voltage. I would also double-check the wall-wart power supply you powered it with to make sure the voltage is not fluctuating and is sitting at a steady 36V. I think that may be the issue here. Another suggestion, assuming you have not tested yet, would be to probe the board for shorts to ground to see if you can identify what component failed.

7

Mark, thanks, and no big rush.

I did some homework, and first tested all visible contacts on the dead femtobuck for continuity to ground. On the picture below, everything with a yellow arrow is connected to ground. No other contacts have continuity to ground. Does that tell you anything?

fb-continuity-to-ground.PNG
fb-continuity-to-ground.PNG800×289 379 KB

I then connected a 70-Ohm resistor (a large 7x10-Ohm one, so it could handle the current) to the old 32-V power supply. I tapped into the R at 10 Ohm and connected that to a scope. Turning on the power supply takes about 6 ms, and at least on that time scale it doesn’t seem to produce power spikes or noticeable fluctuations.

IMG_20190514_104456.jpg
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On a shorter time scale (note 1 us/div), and before the main rise of the voltage/current, there is some reproducible MHz frequency noise though.

IMG_20190514_102051.jpg
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Connecting a capacitor across the power supply (tried a few from pF ceramic to uF electrolyte) didn’t seem to have much effect on this. Does this type of noise look like it could have damaged the femtobuck?

Would it be a stupid idea to connect LED- to PGND and both to actual earth?

Now that the new femtobuck has come in the mail (alright!), my options seem to be:

  • try soldering the jumper again and cross my fingers (risky)

  • look for a different power supply (may take a while for >30 V DC and >600 mA)

  • give up and use the circuit at 330 mA (less UV light)

Thanks again for all your help,

Mirko

8

Hi Mirko.

Everything you’re seeing with continuity to ground should be connected to ground so there’s no worries there.

Connecting a capacitor across the power supply (tried a few from pF ceramic to uF electrolyte) didn’t seem to have much effect on this. Does this type of noise look like it could have damaged the femtobuck?

It's possible but hard to say for sure. If the noise riding on top of your DC power exceeds the FemtoBucks maximum voltage (36 volts) that can absolutely blow the board.

Would it be a stupid idea to connect LED- to PGND and both to actual earth?

Not stupid, but it won't really make any difference. Personally, I wouldn't bother.

Now that the new femtobuck has come in the mail (alright!), my options seem to be:

  • try soldering the jumper again and cross my fingers (risky)

  • look for a different power supply (may take a while for >30 V DC and >600 mA)

  • give up and use the circuit at 330 mA (less UV light)

This is what I’d do:

  • - Make sure all your LEDs are wired in series.
  • - Go ahead and solder the jumper. There's really no risk to running the board at 660mA vs 330mA.
  • - Go with a 24 volt power supply. We carry [[this supply](https://www.sparkfun.com/products/14100) but it's massive overkill for your project. 24 volt power supplies are pretty common and you should be able to pick them up pretty inexpensively. (Try [[this](https://www.mpja.com/24-Volt-Adapter-Power-Supply-5A-Megmeet/productinfo/32885+PS/) or [[this](https://www.allelectronics.com/item/ps-2420/24-vdc-2-amp-power-supply/1.html), either should work.)
    • [/list]

    You might run into a situation where your LEDs forward voltage is too high to light all of them off of one FemtoBuck at 24 volts. If that’s the case, run half the LEDs on one FemtoBuck and the other half on another FemtoBuck and then power both with the same 24 volt power supply.

    Since you’re not using a microcontroller to dim the LEDs, you need to create a small voltage (roughly 2.5 volt) on the CTRL pin to force the driver on. I’ve used a 2.2K and 10K resistor to do that. See the photos below.

    8 LEDs

    8LEDs.jpg
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    Two sets of 4 LEDs

    4LEDs.jpg
    4LEDs.jpg1102×379 103 KB

    Closeup of the connections

    closeup.jpg
    closeup.jpg906×511 62.6 KB
    ](https://www.allelectronics.com/item/ps-2420/24-vdc-2-amp-power-supply/1.html)](https://www.mpja.com/24-Volt-Adapter-Power-Supply-5A-Megmeet/productinfo/32885+PS/)](Mean Well Switching Power Supply - 24VDC, 14.6A - TOL-14100 - SparkFun Electronics)

    9

    Chris,

    thanks so much for the detailed reply, it helps a lot.

    I have now tried out my circuit with the new femtobuck and 7 of my LEDs. So far I have used a lab power supply instead of what I want to use eventually. I did solder the jumper, and it successfully lights up the LEDs at 600-700 mA.

    The lab power supply lets me adjust voltage continuously, and I have noticed that as I turn it up from about 28 V to above 31 V, there is a transition where it suddenly draws less current - as if it turns on. I’ll follow your suggestion w/r/t the small voltage on CTRL to see if that helps with this.

    Finally, 24 V won’t be enough to light the 7 UV LEDs I have, they want ~3.8 V forward voltage each to draw ~6-700 mA. But I hadn’t considered two femtobucks, and that does indeed look like the best way out. Old 20-24 V power supplies seem to be easier to find than 30+ V ones.

    Thanks again, also to the other folks who replied. Sometimes it even helps just writing things down.

    Mirko

    10

    Hi Chris,

    Can you elaborate on the math round the voltage divider?

    I calculated 24/12,2k = 1.967 mA.

    Voltage drop on the 2,2k resistor is then 1.967*2.2=4,33 V the rest is dropping over the 10k resistor (24-4,33=20,67 V).

    How did you calculate if the CTRL-pin needs only round 2,5V volts?

    Or can the ctrl-pin work also with 4,3V?

    By the way: need the ctrl-pin ALWAYS to be driven with a small voltage for the femtobuck to work? Even when no dimming capacities are needed?

    Thanks,

    Ron