Is Voltage Back feeding via USB OK?

Thread Starter

DBBCircuits

Joined Apr 22, 2015
10
I have an Adafruit 5V Buck power board that I will be using to power my Adafruit Feather. I will supply 12V to the Buck board and then use the 5V output to power a sensor and to power the Feather which is connected to the 5V from the Buck through the USB connection on the board. I need to download the program for the Feather from my computer over the USB port. I expect that I should not connect the USB from my computer when the 12V is connected to the Buck because that would be back feeding the Buck with the the 5V from the USB port. BUT, the question is: If I turn off the 12V connection to the Buck, can I use the USB 5V to power the board (but not the sensor)? I'm still back feeding the Buck but since the buck is not active it may be alright??? What do you think?
 

Thread Starter

DBBCircuits

Joined Apr 22, 2015
10
I think it may be alright, but I'd need to see the buck power board schematic to be sure.
I'm afraid I'm not qualified to get the schematic. The only place I can find it is on Github and that doesn't help me. You are probably more familiar or have the tools to read it. Here's the location of the schematic.

Github schematic


Sorry for the naivete but I'm frustrated after spending an hour trying to get it!
 

crutschow

Joined Mar 14, 2008
27,736
I was not able to read the schematic file.

You could add a P-MOSFET circuit to act as a near ideal diode and block any reverse current (LTspice simulation below):
The circuit consists of M1, M2, and R2, and blocks any current from the output to the Buck input as long as the buck is turned off before the USB is connected (If the Buck is not turned off before the USB is connected than the circuit won't block any current).

The simulation shows the load current being supplied by the Buck converter (blue trace) for the first second when the Buck voltage (green trace) is present, and supplied by the USB (purple trace) when the switch is on for the USB (red trace) at the 2 second point.

The P-MOSFET (M1) can be just about any logic-level type with a low enough ON-resistance for your purposes.

1626971148021.png
 

Thread Starter

DBBCircuits

Joined Apr 22, 2015
10
Thank you for your detailed reply - I appreciate your effort! Rather than getting into that level of fix, I think I will simply cut the 5V lead going from the USB connector to the board and always have a 12V battery around when I'm programing/loading the Feather.
 
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