I need to measure bus voltage and guarantee my ADC is protected in a compact way. I came up with the following measurement circuit a long time ago in another design.
The LED characteristics: starts conducting at 2.7V with 30mA at 3.3V.
The bus being measured has motors on it and may rise above 40V if/when they are generating. The goal is to prevent ADC damage from unforeseen overvoltage. Typically the bus will be at 24V or 36V nominal. The system would take action to prevent excessive overvoltage over 40V.
I had tried a zener in place of the LED and it did not work. I don't remember why and I would not be surprised if, at the time, I either did not drop the divider resistance down enough to supply enough breakdown current, or even stuffed the part in backwards. I'm using 0402 1/10W resistors. Probably I was concerned with them dissipating the 5mA needed for zener breakdown. The 3mA for the LED circuit was a bit closer to what they are rated for.
I have severe space constraints. Is there a better way to do this?