What is the Circuit for a Diode OR Gate with a 12V Battery Backup?

[dobry_den]

Hi! http://www.phys.uAlberta.ca/~gingrich/phys395/notes/node71.html" 's an electrical network for a diode OR gate. It says there that "[t]he 12 V battery does nothing until the power fails; then it takes over without interruption."

I just wonder what electrical circuits (closed loops) are there. I attached an image of how I think the loop looks when the 15V power supply works. But what about the battery loop in this case?

 

[dobry_den]

In Loop1 (see the attached images), the orientation of the diode makes it impossible for the current to flow. But in the Loop2, the orientation of the same diode is fine for the current to flow - so what stops it from flowing?

 

[berkeman]

When the 15V supply is on, the 2nd diode is reverse biased (with about 14.3V on its cathode, and 12V on its anode). When the 15V supply is turned off, then the second diode conducts with 12V on its anode and about 11.3V on its cathode. This puts the first diode into reverse bias (off), with 11.3V on its cathode and 0V on its anode. Make sense?

 

[dobry_den]

Yep, it makes in a way..thanks. But still, I have another question. Could the electric circuit be redrawn in the way I did it in the attached image? The resistor is there to incorporate the resistance of the Earth. Or would you propose any changes?

Any help is appreciated, thanks in advance.