How can I switch from a 12v DC source to a 9v DC source? Something like an automatic transfer switch?
The main source is the 12v. If the 12v goes off, it should automatically change to the 9v. But once 12v comes back it should switch back to 12v.
Is there any device that does it automatically (hope so ) or do I have to do it using a μcontroller? Where should I look?

 

You just need 2 diodes ORed together. The 12V will win when it is there.

OK as long as you can handle the diode drops.

 

You just need 2 diodes ORed together. The 12V will win when it is there.

OK as long as you can handle the diode drops.

ok, thanks for your help.

What diodes should I use if I'm going to use 12v and 9v?

 

How much current is your device taking?

How much voltage loss out of the 9V supply can you tolerate? "Anything above a volt" is a great answer.

 

Your best bet is to select a Schottky diode. It will have a smaller voltage drop for the same current than it's silicon cousins.

Something like a 1N5818 should get you started

http://www.datasheetcatalog.org/datasheet/diodes/ds23001.pdf

They are good up to 1 amp continuous. If you need more current there are devices available. If you need SMT deveices you can get those as well.

 

Your best bet is to select a Schottky diode. It will have a smaller voltage drop for the same current than it's silicon cousins.

Something like a 1N5818 should get you started

http://www.datasheetcatalog.org/datasheet/diodes/ds23001.pdf

They are good up to 1 amp continuous. If you need more current there are devices available. If you need SMT deveices you can get those as well.

Usually it's the programmers who write the code before a specification is developed.

 

Usually it's the programmers who write the code before a specification is developed.

Ehhh...?!!?