Input protection with Schottky - what to do about this problem

[blapcb]

I am using the attached circuit to condition external signals for connection to a microcontroller. The Vcc is 3.3v. But, what happens is, that the voltage at the junction of the two schottkys is actually Vcc + the forward drop on the diodes which in some cases comes very close to the limit of 3.6v which the microcontroller is comfortable with. What should I change or which diodes should I select in order to have the input at Vcc? Thanks.

 

[millwood]

first of all, you should check your mcu as it is likely to have clamp diodes on the pins already.

2ndly, it is hard to imagine real-world conditions under which what you described will happen.

 

[Kurenai_ryu]

mmm... at 24VCC input.. i would change the lower diode with a zener diode... maybe 3v 500mW just to make sure...