this should be a simple question, but i am having trouble finding the answer by myself… i have a circuit, basically it centers around an atmega328 chip (32 pin TQFP) and FT232 UART chip. most of the I/O’s are for data collection, and one controls an on board relay.
the board uses right around the max current i can harvest from the single USB connection, so i have an additional barrel jack and a +5v 1A wall wart (yes its regulated).
the question is - how do i correctly combine the two power sources?
1-should i leave the USB power pin (and ground, too, maybe?) unconnected and use the external +5v source?
2-should there be some sort of diode(s) somewhere to make sure im not pushing voltage back thru the USB circuit?
3-other?
leaving the USB power hooked up would make life easier for testing and setup (when all i need to power is the atmega chip), so that i could, say, flash firmware without the wall wart. so option 1 is less desirable.
thanks.
Isolate the 2 supply rails completely, except for the one line feeding the '328.
USB powers the '328 thru a diode, preferably one with a low Vf when the USB is plugged in, wall wart feeds the rest.
When the USB isn’t plugged in, the wall wart feeds the '328 thru a diode, preferably one with a small forward voltage drop (i.e. 1N5820 type and not a 1N4148 type).
Of course, it would really help to see a schematic…
As skimask says, you don’t.
You can either power everything from USB, power everything from the wall wart, or power some things from USB and other things from the wall wart. Don’t attempt to use both power supplies on the same net.
Don’t worry about powering some parts with one supply and some with the other supply. The two 5V supplies will be close enough that everything will work.
NogginBoink:
The two 5V supplies will be close enough that everything will work.
However, if there is a chance you could have conflicting output drivers, a series resistor helps.