# How to arrange input protection components?

I want to protect the input of my 5V micro-controller from two types of disasters, I am detecting a low on the input (0V):

The input is from a switch. When the switch is on it provides a ground.

1. Over-voltage protection on the input (the input can range between 5V to 24V DC). Hence the over-voltage protection must be clamped to 24V DC MAX.

2. The minimum voltage must be greater than -0.3V. The device states that the input voltage must not be less than -0.3V. The device is not a common device, hence part numbers or datasheets are not available. I just have the specs for the device

Hence I have the below schematic.

D1 is a Zener diode that a Zener voltage of 5.1V and D2 is a Schottky diode with a forward voltage drop of 0.2V, which means any negative voltage is clipped at -0.2V.

I have not specced any components because there is no stock currently for the diodes

I want to know which placement of components is better, placement 1 or placement 2?

Placement 1

simulate this circuit – Schematic created using CircuitLab

Placement 2

simulate this circuit

• Commented Nov 28, 2021 at 20:31
• Question lacks detail. Explain why -0.3 V. Over-voltage range is 5 V to...what? No component part numbers. Site already has existing answer, as @jsotola explained. Input protection circuits are well defined and information is readily available on the internet. VTC for those reasons. Commented Nov 29, 2021 at 18:11
• You should specify your input requirements better. For example, if D2 is intended as a protection from reverse connection, then calculations for -0.3V MCU limit are pointless. 24V connected in reverse would result in 58 Watts from R3-D2... for as long as they last Commented Nov 29, 2021 at 21:30
• I am not asking about schematics. I am suggesting to specify requirements better for yourself. In my example, I questioned the statement "any negative voltage is clipped at -0.2V". Any? what about -24V? and for how long? "over-voltage protection must be clamped to 24V DC MAX" what does it even mean? You clamp it to 5.1V, not 24. "The input is from a switch" - how is that switch connected if you expect input "from 5 to 24V" or a ground? If you expect it to switch between voltage and ground, why do you need pull-up? if you define your requirements better you may see the answers yourself Commented Nov 30, 2021 at 9:46
• So, why then you connect switch to 5V? Isn't ground enough? High voltage SPST with pull-down or ground SPST with pull-up make more sense to me. And where 24V comes from? You want to protect "my micro controller" but then mention "the device". What device? Don't see it on the schematics. If "the input can range between 5V to 24V DC" where negative voltage comes from? If you trying to protect from ESD then why 24V, why not 24kV? Commented Nov 30, 2021 at 16:46