I am trying to get a AL8861 LED driver hooked up. Right now I've connected it to a single LED (this one) to test (see circuit below). I eventually intend to connect it to a DAC5578 Digital to Analog converter to communicate via I2C. For now, I'd like to test if the lighting circuit works.

UPDATE: I found the primary issue - my board had an error with GND not being connected to the driver for that channel. Now I have light, but not very bright. Connecting VSET to GND or 3v3 doesn't seem to do anything. CH1+ reads 2.54v and CH1- is 0v. Still stumped...

AL8861 Wiring Diagram


I've connected the components and supplied 24v. As soon as I plugged it in, the LED flashed and then went dark. I've also tried connecting VSET to ground, and it begins to glow slowly but remains very dim. I tried connecting VSET to +3v and +5v (probably a bad idea) and it flashed again. Now when I connect VSET to ground I get no glow.

I get some weird results when I check the test pads voltages. Here are the readings:

  • (+24v) 24.30v
  • (VSET) 23.93v
  • (CH1+) 24.30v
  • (CH1-) 24.27v

I thought VSET would float. When I connect it to ground, it has ~60mA of current running through it.

Is my circuit wrong in some way? I appreciate any help. Thank you!

Edit: Here's a picture of the circuit board, if that helps. I haven't yet added the indicator LED / resistor: PCB

  • 1
    \$\begingroup\$ Can you explain why you added C72? \$\endgroup\$ Feb 20, 2019 at 18:03
  • \$\begingroup\$ This is a small section of this PCB I'm using / modding from MIT OpenAg. I wasn't the original designer, so I don't know why that component is there. I do see that capacitor in the wiring diagram on page 3 from an AL8861 user guide (C4), so I assume it's ok? \$\endgroup\$
    – dmayman
    Feb 20, 2019 at 19:56

1 Answer 1


I figured it out – it was two things:

Something in the board must have been soldered improperly, because I reflowed everything and tested all connections, plugged it back in and it was suddenly working as expected.

The LED isn't shining as bright as I would have expected because of the RS2 resistor value. The formula in the datasheet is Current = .1/R. Right now my RS2 is 6.34ohms, which is 15mA. To run this much higher, I will need a resistor value between .1 and .15 ohms (I didn't even know these existed).

Hope this helps anyone else out there with this chip!


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