[no subject]

4x 1.2v
Rechargable         PDA
AAA

-5v

Common
-----------------+------------------+


Assumptions



I'd like the unit to:

Power the PDA when plugged in to USB port.

Recharge the (emergency) batteries when the PDA is unplugged

Act as emergency charger / external power supply when USB is not present /
plugged in.



Useful but not essential:

Illuminate LED to indicate power is coming from USB / recharging



So in my diagram I've used



R1 To limit the current from the USB port to under 500ma.

R2 Is this needed? Thinking was to limit the current from batteries when
USB
was not present.



D1 is to prevent reverse flow back to the PC.

D2 stops the PDA charging the rechargeable (emergency) batteries.



My Questions?



Will this work or have I got it very wrong?

What values do I need for the resistors?

What part numbers will the diodes be?

Do I need a current limiting resistor on the LED?

What happens if everything is connected (USB Supply / Batteries and PDA)?




Any advice would be gratefully received.



Thanks in advance



Mark