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> Something with regulators with battery back-up...

Just some drowsy thoughts.............(it's 3am. Flip, not again !)

Lucian, this would work as a basic linear setup to get 12V and
5V. However, there is a limitation on the 12V side. Because of
the diode, you'll need to use a low dropout regulator. That's 1.1V
for the LM1084 at 3A. Even with a Schottky, the battery cannot
fall below about 13.4V, if it's floated at 13.8V (assuming a lead
battery, varies slightly with temperature). Which doesn't give you
much capacity without losing regulation

I have similar applications, which need only a few 10s of mA back-
up power, and use a SPDT (ie a toggle) relay. The relay energises
to close the normally-open contacts when mains power is available,
otherwise battery voltage flows through the normally-closed contacts.
At least that makes an extra 0.6V because there's no diode. I'm
wondering if the input to the charger can be isolated another way,
perhaps with a semiconductor or relay on the input

Because of the droput problems and currents that you need, I think
you may have to go to a higher V batteries, perhaps a 24V for 12V
and a 12V for 5V ?

Maybe someone can suggest a step-up switcher instead

A junkbox way to get 13.8V is with a jacked-up 7812, that is one
that has the reference pin lifted 1.8V above ground. Better would
be with a specialised charger/monitor so it's not over-charged or
discharged it too far.

The other thing to watch out for is the wattage dissipated by the 5V
regulator. It will be dropping around 9V. A more efficient way would
be to have an additional lower voltage tap on the transformer and
a 6V battery as well




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