Excuse me, my circuit skills just got rusty.
Can anyone please give me a clue what would the voltage rating be at the purple node? appreciated!

R.

 

Here's a datasheet for the BAV99; a dual diode:
http://www.fairchildsemi.com/ds/BA/BAV99.pdf
It's being used as a clamp to limit the voltage excursions above and below ground.

The AUTO_IN - whatever that is - limits the current through either diode in the BAV99 in case the signal goes above 5v or below ground. The clamp at low current values (1mA) is the positive supply +~.7v to ground - ~0.7v

 

Do you mean voltage rating or range?

Presumably input AUTO_IN can have a somewhat unspecified range (within limits). This bit of circuitry seems to be designed to ensure the volatge at the node in question does not go outside the nominal range of 0 to 5V.

The two diodes are orientated & biased to achieve that outcome.

 

Depending on the input level, either diode can see up to 5V + one diode drop, so probably not over 6V.

BAV99 is rated much higher than that. http://www.fairchildsemi.com/ds/BA/BAV99.pdf

 

That voltage at that point, if we assume the fvd of diode 0.7V, will not be less than -0.7V and greater than +4.3V

 

That voltage at that point, if we assume the fvd of diode 0.7V, will not be less than -0.7V and greater than +4.3V

Greater than 5V+Vf ..... The node has to climb over 5V to get the top diode forward biased.

 

An important point, that last. When adding diodes like this e.g. to protect an IC, it may be difficult to keep to safe voltages, which are often just a few hundred mV beyond the rails. Using Schottky devices may help, as may putting series elements between the big diodes and the device - the latter strictly restrain the current rather than the voltage.

 

Greater than 5V+Vf ..... The node has to climb over 5V to get the top diode forward biased.

Correct. Yes it must be 5+Vd . Didn't notice at once