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Hi

I have an Arduino Mega 2560. The description states that the power source is selected automatically.

I would like to know how this is done.
If the Mega is connected to a powered USB port (5V) and a 9V battery block on VIN, which is it going to use?
If it uses the USB supply as first choice this setup would make an easy battery backed system.
If not would the reverse work, have a regulated battery on the USB port an the VIN connected to mains.

Cheers
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Hi

I have an Arduino Mega 2560. The description states that the power source is selected automatically.

I would like to know how this is done.
If the Mega is connected to a powered USB port (5V) and a 9V battery block on VIN, which is it going to use?

If both external power and USB power are avalible, the auto-voltage selection feature will select the external power.

If it uses the USB supply as first choice this setup would make an easy battery backed system.
If not would the reverse work, have a regulated battery on the USB port an the VIN connected to mains.

Yes, it works 'backwards' so supplying regulated +5vdc from a battery supply wired to the +5vdc and ground power pins of the USB connector or the +5vdc and ground shield connector pins would allow the external power connector to be normal mains power with automatic switching to the USB or +5vdc pin supplied power.

Cheers
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How do I disable the Mega2560 from powering up via USB? I am supplying external power VIN and only want to use that, as the current drain is too high for USB. The voltages are too low when USB is connected.
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How do I disable the Mega2560 from powering up via USB? I am supplying external power VIN and only want to use that, as the current drain is too high for USB. The voltages are too low when USB is connected.

As long as the Vin voltage is applied before plugging in the USB cable, then USB power will not be drawn. However Vin voltage goes only to the on-board +5vdc regulator so it will not supply all that more current then using USB power, perhaps 800ma with Vin  Vs 500ma for USB. Also the specific voltage of Vin can limit the current avalible from the on board regulator as if too high a voltage then heat dissapation of the regulator could become the limiting factor.

Lefty

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I need to disable the USB power on my Mega2560. The 8U2 prob. needs to see it which means disabling the comparator/USBVCC switch circuit.

My project has an on-off switch for external Vin and I don't want a sequence (turn on Vin, then plug in USB) to power things up, since 5V load is more than USB can provide.

I pulled the mosfet T2 (SOT-23) and it seems to work. A solder pad jumper would be nice instead.
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