Hey Guys
Thanks for all the info. I went through the FET datasheet and I wasn't aware of the internal diode.
@ Raschemmel
You seem to be a few steps ahead of me:
At some point , maybe now , maybe later , maybe never, you might wonder "if the +5V from USBVCC1 can pass freely through the internal diode, then how can the comparator block the voltage when power is plugged into VIN ?
The answer is that when power is plugged into VIN there is 5V coming from IC1 5v regulator on the CATHODE of T1 diode. If the diode has 5V on it's CATHODE and you apply 5V to it's ANODE , WHAT HAPPENS ? Answer, NOTHING , because the diode has the same voltage on both terminals and no current flows through the diode, thereby blocking USBVCC1 by not conducting. (thanks Larry)
Thanks for that explanation as well, it is much appreciated. Thanks to both of you gentlemen I now know a bit more about FETs and am very thankful for it.
Dirk