I'm trying to replace a fan controller board with something a bit more custom based off an ESP32 module. The board has 36V input from the rest of the system and I'm trying to figure out the best way to get this down to the 5-16V range that my devel board can handle and I'm stuck.
To get something quick and dirty up and running, I attempted a 1k+500R resistive divider and tapped the middle (approx 12V) to my board. But that does not work - it drops the voltage over the 500R to 5V and the board doesn't power on. I'm not sure I understand why this wouldn't work. I also tried tossing in an LM7805 between the 12V point and my devel board but that didn't solve anything.
I've reverse-engineered the old board and the power supply for the 5V rail looks like this:
On the board, D1-D3 looks like this:
I assume that D1 is a reverse polarity protection diode (the text on it says "GM" and "45") and that D2 & D3 are zener diodes to drop the incoming 36V to something that the LS78L05 can handle safely. Does that sound like a correct assumption? D5 is an ST part marked "ER" and "Z406" but I've been unable to find datasheet for it.
Can anyone give me some pointers on what I should do here? Do I need to replicate the D1-D3 to lower the voltage before my voltage regulator or should I look at some other solution?