> Ok, the source I need is for a security system I'm designing. if it
> works well, it could be produced in high volumes.
> The system is powered from the wall outlet to a 13.8V step-down
> transformer, which feeds the regulators. This would be simple if I
> didn't needed a battery backup. From the sensors point of view, the 12V
> could be between 9V and 16V, but I have also some comparators which work
> at 12V and if the voltage drops bellow, the result isn't accurate
> anymore.
> When the power is off, the battery must backup both the 12V and the 5V.
> I thought of using 2 regulators, and when the power fails, a relay to
> connect the battery to the 12V directly and to the 5V regulator.
> Is this suitable ? I would also need a charging circuit for a lead-acid
> battery, can you indicate me a good circuit ?

That's getting clearer. If the comparators are all that's making you need
12v then I would seriously consider looking closely at their circuitry and
seeing how you can reduce the voltage. Unless you are doing something VERY
high speed indeed you are unlikely to really require 12v. if this is a slow
speed (a few Hz to 10's of Hz switching rate) then it should be easy (famous
last words).

The reason I'm so keen to reduce the 12v rail is

- 12v gel cells are the norm in security systems. Cheapish, ruggedish,
lowish cost, highly available, well understood by security system users.

- Once you get below 12v you can charge the battery from the 13.8v supply
and also use the supply or battery to drive the regulators.

- Other batteries have better energy density but are generally dearer.

You could consider floating the battery at slightly above 12v and using that
as the 12v output voltage.
A 13.8v  AC supply will easily charge a 12v battery.
(13.8 x 1414 =~ 19v.)
OR was that 13.8V DC output?
If so, why?
(Lead acid batteries are usually floated at about 13.6v but gel and auto
cells are slightly different).

But first - tell us about your comparator circuit(s):

What are the input signals and their sources?
What are the outputs?
How fast do the signals change?
What is the purpose of the comparators?
What IC are you using?
What goes wrong with less than 12v supply?
Why?

It's quite possible that an LM324 or LM358 running on 5 volts will do what
you want.

Also, what do you use the 12v rail for apart from the comparators?
You mention sensors which need 9 to 16v. (PIR?)
Wouldn't it be nice to be able to run everything on 5 volts :-) ?

Also: How long does the battery backup need to last?
What is the expected current drain in backup mode?




        Russell McMahon

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