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 ?

Lucian
-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Russell McMahon
Sent: 23 martie 2004 04:19
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [EE]: 12V and 5V Power Supply

Rather than approaching the specification by successive piecewise
approximation, why not spell out what is REALLY required?

Explaining what you really as clearly and fully as possible is a great
help.
It doesn't matter if your English isn't perfect. Just do the best you
can to
describe your requirement.

What is the use that the outputs are being put to?
What are the actually required voltages (eg MUST it be 12 or could it be
anywhere from eg 10 to 14)
What are the load levels on each supply? (average, peak, minimum, other
characteristics?)

Is this in a vehicle?
What is the power source (main?, car alternator, ??? ...)
What battery is it that you are talking about charging for backup?
What must be backed up? (12v?, 5v?, both?, how long?
Is the backup battery voltage fixed?
How fast do you want it to recharge in?
Have you got a battery in mind to use or can it potentially be any
battery?
Is this a one off, small volume, large volume?

Is there anything else that is going to have to be explained before a
full
solution can be arrived at?



        Russell McMahon


> The problem is that getting a regulated 12V out of a 12V battery is a
> relatively "hard" thing, requiring a switching topology of some sort
> (and IIRC, switchers are easier when the output voltage ISN'T that
> close to the input voltage.)  If you could get by with 10V, you could
> use two simple linear regulators (depending on other circumstances, of
> course.)
>
> Are you allowed to use, say, 14V worth of battery?  Or is the 12V
> requirement loose enough that you can get by with an unregulated 12V
> direct from the battery, when the main power is off?

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