I'm building a circuit to test linear regulators up to 24V using avr ADC.
There is a voltage divider in output line to reduce voltage to 1/5 (4kOHM and 1kOHM); for example 25 to 5, 20.5 to 4.1 ,...
As the over voltage can damage MCU, I want to protect ADC from high voltages using a 100mA fuse (ADC shouldn't take much current. Should it?!) and a Zener diode to the ground.
I have two options:
- to put a 27V Zener diode before voltage divider.
- to put a 5.1V Zener diode after voltage divider.
If I use the first method, maximum voltage to ADC pin will be 5.4 and in the second method will be 5.1.
Which method is better and safer?
Note: Regulator has a separate power source and shared ground with MCU power supply.