Hi folks, thanks for the very good comments that you have all made.
I did consider putting an input protection diode but in this case I am relying on the diode within the the lm7805, the input diode on the USB and input and output caps. The circuit simply will take on the surge current when supplying it through the USB. Given that the voltage at the output is going to be less than 4.8V, the cap at the output will start charging and forward biasing the internal 7805 diode that will start charging the input cap once the input cap is charged, the internal diode of the 7805 will only carry the the cap's leakage current (very little).
The surge current transient is handle nicely through the 7805. It is not a fully protected if heavy loads are taken at the input. But with no load there it is more than happy with the input caps.
This board has been powered through the USB only for many nights without a single failure. A different story would be if you tried to power a load at the input. That would kill the 7805.
I did consider putting an input protection diode but in this case I am relying on the diode within the the lm7805, the input diode on the USB and input and output caps. The circuit simply will take on the surge current when supplying it through the USB. Given that the voltage at the output is going to be less than 4.8V, the cap at the output will start charging and forward biasing the internal 7805 diode that will start charging the input cap once the input cap is charged, the internal diode of the 7805 will only carry the the cap's leakage current (very little).
The surge current transient is handle nicely through the 7805. It is not a fully protected if heavy loads are taken at the input. But with no load there it is more than happy with the input caps.
This board has been powered through the USB only for many nights without a single failure. A different story would be if you tried to power a load at the input. That would kill the 7805.