A battery switch and a switching power supply

[myp71]

I have two questions #01 is there a circuit that when two aa batteries go dead will switch over to another set of 3volt batteries i'm going to use this cricuit on a solar charger for the batteries so if the solar panels do not get enough power during the day to charge the batts the circuit will switch over.
Next question is has anyone ever used a switching power supply out of a old computer? the question is there a way to hook up the 5 volt output to the 12volt output to get the amps it puts out on the 5volt output.<p>so what i'm trying to say is the output is 5 volts 18 amps then for the 12 volts output is only 5 amps so can i wire the 12 volt to the 5 volt to get the amps the 5 volt puts out but in 12 volts.<p>thanks for reading this all
Ryan

[fsdenis]

myp71:<p>I use a nautilus deep cycle 12v battery about the
size of a car battery as a high current power supply. This will give you at least two hours of
18 amp operation before the battery voltage drops to 11.5
volts. If you have 120vac 60hz (regular purchased
power from the electric company) you can charge
this 12v battery.<p>I know of no way to do the power supply finegling you are talking about.

[bruinbear714]

You cannot plug the 12V into the 5V and use it for extra amperage. Nor can you parallel two 12V power supplies together to get double the amperage unless you want a fireworks show.

[myp71]

that's what what I thought because the switching power supply shares the 12 volt and 5 volt ground so I just wanted to be sure.<p>
thanks you all for your help<p>If any one still has a circuit digram that will switch over when the batts go dead let me know.