Breadboard Arduino Competing power supplies

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1

I have an atmega328p setup on a breadboard.

My 5v FTDI board can power the micro while I program it. When I'm not programming, I want to power my breadboard with batteries and a 3.3v regulator to conserve power. To be safe, I disconnect the 3.3v supply before connecting the 5v FTDI programmer.

What happens if I connected both power sources at once? Would either be damaged? Would the outcome be different if I removed the 3.3v regulator and had 4.5v of batteries connected at the same time as the FTDI board?

I realize I could just buy a 3.3v FTDI board, but I am curious about what happens when power supplies are competing like this.

2

I want to power my breadboard with batteries and a 3.3v regulator to conserve power

Then running it at 16MHz is running it out of spec.

Would either be damaged?

Yes possibly.

Would the outcome be different if I removed the 3.3v regulator and had 4.5v of batteries connected at the same time as the FTDI board?

Yes possibly different damage. It depends on the specific regulators you are using but connecting two power sources together is generally not a good idea. Sometimes the damage only shows up later.

3

jordain:
What happens if I connected both power sources at once? Would either be damaged? Would the outcome be different if I removed the 3.3v regulator and had 4.5v of batteries connected at the same time as the FTDI board?

You'd be connecting 5V to your batteries. Not good.

jordain:
I realize I could just buy a 3.3v FTDI board, but I am curious about what happens when power supplies are competing like this.

Power moves from high voltage to low voltage. Basically you're sending power backwards up the low voltage line. What happens will depend on the circuit but if there's any possibility of it happening you should put a diode in there to stop the flow.

4

Grumpy_Mike:
Then running it at 16MHz is running it out of spec.

Thanks, I was not aware of that. Luckily I am using the internal oscillator at 8 MHz.

fungus:
Power moves from high voltage to low voltage. Basically you're sending power backwards up the low voltage line. What happens will depend on the circuit but if there's any possibility of it happening you should put a diode in there to stop the flow.

I will try adding a decent Schottky diode to protect from current flowing into my batteries.