Issues regarding powering a microcontroller using a 7805 and a USB port

[ivar]

I want to power my microcontroller board using a 7805 (which receives input power via a DC adapter) and a USB port. Assuming only one of these power sources is active at a time, can i just tie the output of the usb and the 7805 together without using a jumper to select the appropriate power? If I tie the outputs together and use the USB, the OUTPUT pin of the 7805 will be subject to 5V. Is that a problem?

 

[ivar]

Re: Dual Power supply

Update: When I apply a 12V DC between the output and ground pins of the 7805, it draws about 50mA! I guess that means i need to isolate the outputs....

 

[zula]

Re: Dual Power supply

you can use a diode on the usb's power line if you are suspect.

 

[ivar]

Re: Dual Power supply

How would that help?The Vusb will still be available across the 7805's output pin right? Besides, I cannot afford a 0.3 volt drop across the diode (I need a full 5 V supply). Anyway, I'm now using a SDPT across the output to select the source

 

[rohscomplaint]

Re: Dual Power supply

I think you can try LT4413 to solve this problem. Even to power source are active at one time, it will still work fine.