Comments: FemtoBuck Constant Current LED Driver Hookup Guide V12

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  • What is the limiting factor for the circuit's max current (1A with 0.1 Ohm 1/8W resistor)? Driving up to 1.0A is ok, but I would like to drive up to 1.2A.

  • Is R_set for the current setting the same as the "Current Set Resistors" underneath the "Current Level Set Jumper" in the pictures? So to change the current I need to replace both of the resistors with another pair?

    • You don't need to change both resistors, only the one closest to the edge of the board. It might be easier if you remove both of them prior to replacing that one, however.

  • -------------------- Tech Support Tips/Troubleshooting/Common Issues --------------------

    9V Power Supply w/ 5x 3W LEDs?

    The power supply depends on how you wire the high power LEDs [ https://www.sparkfun.com/products/13105 ] to the FemtoBuck [ https://www.sparkfun.com/products/13716 ] . The tutorial will have more information on wiring the LEDs with the FemtoBuck [ https://learn.sparkfun.com/tutorials/femtobuck-constant-current-led-driver-hookup-guide-v12 ] . Assuming that you are using a 9V power supply from our catalog, it might be better to not have more than 2x high power LEDs wired in series with each FemtoBuck. The tutorial states that you would need to add the forward voltages of the LEDs together with an additional 2V of head room. Testing with a benchtop power supply, it looks like two high power LEDs wired in series using the default setting (no solder jumper on the "Current Level Set Jumper") would pull about 0.222A w/ 9V.

    The total will be about 9.6V so this 9V power supply would work [ 9V/650mA – https://www.sparkfun.com/products/298 ] with the female barrel jack adapter [ https://www.sparkfun.com/products/10288 ] . You could wire two setups (2x high power LEDs in series with one FemtoBuck) in parallel to the power supply. The 5th high power LED and FemtoBuck can probably be powered in parallel with the same power supply. The total pull from the power supply will probably be around 0.563A. If it is too much strain on the power supply, maybe you can add an additional 9V power supply for one of the Femtobuck setups.

    Testing two Femtobucks (with no solder jumper) each with two 3W LEDs in series, there was no issue powering the setup with a 9V/650mA power supply. I was also able to power the Arduino. The system was pulling about 0.48A. The wall adapter gets warm but it’s well within the recommended rating and does not shut off. As soon as a solder jumper is added, the power supply shuts off after a few minutes because it is not able to handle the load.

    12V Power Supply w/ 3x 3W LEDs

    w/ Solder Jumper Set

    The Red 3W LEDs look like 3x LEDs (2.8V forward voltage * 3 + 2V head room => 10.4V) can be added in a series with a 12V power supply. The other 3W LEDs should require a total of 13.4V due to the 3.8V forward voltage and head room requirements. At most, I would wire 2x 3W LEDs in series with the Femtobuck. With the solder jumper, the maximum the Femtobuck would theoretically allow is around 660mA at 12V as stated in the hookup guide.

    I was actually able to get 3x 3W Cool White LEDs [ https://www.sparkfun.com/products/13105 ] to light up with both a 9V/650mA and 12V wall adapter. Solder was added to the solder jumper. The Femtobuck and 3x 3W LEDs was pulling about 0.26mA @ 9.1V and 0.58A @ 12.32V. While it is not ideal, it appears that it is possible to power 3x LEDs in series with a 12V power supply. You just might not be turning on the LEDs to the maximum using the Femtobuck. Try testing it out with a multimeter set to its current setting to see what current your system actually pulls from your power supply.

    w/out Solder Jumper

    With one Femtobuck and 3x 3W Cool White LEDs, it was pulling about 0.21mA @12V.


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