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Dual Source Power supply - USB and/or external question

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DirtyLude

Well-Known Member
Will this work?

I have a PCB that requires both 5v and 3.3v operation. It has a 3.3v and a 5v linear regulator on it (type LD1117). I can power the board from an external 9v source, but I also want to be able to power it from USB, if the USB is connected to the jack.

This means that the power to the board could come from the 9v source only, the USB only, or both could be connected. I have come up with this schematic and I wanted to make sure this will work. If the Power line is connected, I get power to both the regulators. If the USB is connected, I get 5volt power on the 5volt rail and 5v (minus the diode drop) to the regulators.

If this doesn't work, do you have any other suggestions?

 

DirtyLude

Well-Known Member
Thanks. The board has low current requirements. 200ma is far above the max that it will need.
 

DirtyLude

Well-Known Member
Did you not read the original description? Yes, I want to be able to have both connected. There's a diode right in the schematic shown, to protect the 5v line from the 9v input.
 

ecerfoglio

New Member
You'll get a small drop but a 1N5817 from VBUS to block current from entering the USB port should do the trick.
Did you not read the original description? Yes, I want to be able to have both connected. There's a diode right in the schematic shown, to protect the 5v line from the 9v input.
Yes, you protected the USB port from the 9V but not from the regulated 5V.

Also, verify if your 5V regulator is "happy" with 5V applied to its output.
 

DirtyLude

Well-Known Member
Yes, you protected the USB port from the 9V but not from the regulated 5V.

Also, verify if your 5V regulator is "happy" with 5V applied to its output.
There would be my question. Do I need to protect the USB input from the 5V regulator output? The 5V regulator will not be outputting 5V due to it's own voltage drop plus the diodes voltage drop on the supply line, so it's not going to go higher than the USB input. Problem is that it will not have 5 volt at both input and output, it's going to have less voltage at the input due to the diode drop, then the output.

I will test the regulator to see if it has any problem with voltage at both ends and slightly lower at the input.
 

blueroomelectronics

Well-Known Member
Well the diode will only drop about 0.2V and depending on what your running that may or may no be a problem.
A tiny 5V NC reed relay could be use to disconnect the 9V battery when USB is connected.
 

DirtyLude

Well-Known Member
Well the diode will only drop about 0.2V and depending on what your running that may or may no be a problem.
The diode would protect the USB from over voltage back flowing into it. The way everything is setup, that can't happen, so I'm not certain that it would be useful.

Don't know if it will be a help but here is a link to the latest Arduino board that does a automatic switch between external power and USB power if both are connected.

http://www.electro-tech-online.com/custompdfs/2009/01/arduino-duemilanove-schematic-1.pdf

Lefty
Thanks, that is useful. They are using a FET to switch supplies and an op-amp to switch the FET. In that case, though I think it would require me to run the 3.3v regulator off of the 5V line, rather than the power line. It means the 5V regulator would have to handle all the power of the 5V circuit but also the power of the 3.3V circuit.
 

DirtyLude

Well-Known Member
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