I've got a constant current MeanWell LED driver that has 3-in-1 dimming (100k potentiometer, 0-10V signal or 10V PWM), and would like to control the dimming with a microcontroller.
I found a few suggestions online, the simplest one is feeding a PWM signal to an optocoupler connected to directly to driver's DIM+ and DIM- (without any external power supply).
This works because apparently MeanWell dimming circuit looks like this (courtesy of https://www.eevblog.com/forum/projects/mean-wells-2-in-1-dimming/msg1835957/#msg1835957)
However I have a couple of additional requirements
1) I need to limit the maximum brightness to about 75%. Just limiting duty cycle is not enough - I want the driver to never output more than 75% of rated current even in case of software bug/controller failure.
2) I need dimming to be 0% when microcontroller is powered off.
If I was using the single optocoupler schematics, I could use 75K resistor in series with opto output, but requirement (2) means I have to invert the output somehow with a BJT/MOSFET.
Could you please suggest a schematic and help with components choice?
PWM frequency doesn't need to be high (100-200 Hz is enough). The driver I'm using is ELG-100-C350B https://www.meanwell.com/Upload/PDF/ELG-100-C/ELG-100-C-SPEC.PDF
PS. Just realised that 75K resistor won't work with opto as it needs to effectively switch from "short" to "75K"