Re: Backup power supply

bp@www.zefox.net wrote:

side, you could put it all downstream of the Pi power supply
and skip line detection entirely. That's a big advantage.
 

On second thought, this is probably wrong. If one connects
to a Pi a 5 volt supply in parallel with a blocking diode
and a 6 volt battery, the diode is still forward biased
and will drain the battery eventually. Some kind of switch
is needed, controlled by line detection.

Maybe a three terminal regulator set for 5 volts out powered
by a 9 (or higher) volt battery pack could be substituted.
Still, one has to consider the idle draw of the regulator.
TI's LM340 takes 6 mA roughly. Still, that's a smallish
supply for a Pi. You'll likely want something larger.

Apologies for the red herring!

bob prohaska