I am planning on doing battery backup on a system containing of a GSM Module controlled by a Mega8 board , and a Mega32 board.
I need the system to send me an SMS when the AC power is lost.
I expect to feed it 24v via PDS-cable , and then use a National Switcher (those TO-220 ones) on each board , to regulate down to 5v.
I would then use the 12v battery as backup power , and have 2 Schottkey diodes to do the input selection , stolen from the "Butterfly" schematics
| Switcher Vcc Input | | 24v --------|>---O---<|------- 12v Bat
I have 4 questions ...
1: Would the diode trick work here ??
2: In order to have power in the time where the 24v is lost , and until the 12v battery kicks in , i was thinking about a Panasonic supercap on the 5V side of the regulator , is that an overkill ?? , would a normal 470uF be sufficient on the M32 board.
I allready have 2x 470uF on the M8 board as the GSM module draw current in pulses.
3: When i switch on the system would the Supercap kill my switcher (max 1.5A i think) or is the internal resistance high enough to prevent this.
4: The National Switcher would like low ESR caps what would happen if i put a Supercap on the 5v side , along with the 470uF low esr.
Well a lot of questions , but you guys usually have some useful guidelines.