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Author Topic: Disable USB power so when connected via USB without Ext power, Arduino is off  (Read 906 times)
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Hi,

I'm building a little control project that communicates with a PC via USB but uses (and needs) an external power supply to function, however I only want the board to be powered from the external supply - never from USB. If only USB is connected, I need it to be 'off' until the external power comes on.

It's using the Ladyada Motor Control shield, however due to some voltage leaks on the L293D chips (I think) causes the motors to be powered, from the USB supply (even with the power jumper disconnected) all be it without enough current to turn, but causing odd behaviour.

The Arduino automatically uses the external supply when it's connected, but I need to prevent it powering up without external power.

I'd rather it just didn't power up or draw any power from USB, but as a compromise, if I could detect the lack of external power in software and loop until external power becomes available I can be sure the motors don't get switched on.

Is there any way to disable the USB as a power source yet keep the data, or failing that, what's the best way to get an input based on the external power without just sticking a relay on it?

Thanks!
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What you ask is data-only USB cable or powerless USB cable.

A data-only USB cable is a cable that only contains the + and the - data leads. It does not contain the +5V power lead. lt SHOULD contain the ground lead. This type of USB cable is what is known as "Data-Only". You CANNOT just assume since your USB device is plugged into a power source that you do not need the power lead on your USB cable! That is just plain wrong.

The application is high-end audio DAC and very pricey.

You need DIY the cable.
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Quote
never from USB
Remove the USB polyfuse.
« Last Edit: June 17, 2013, 12:04:04 am by LarryD » Logged

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You CANNOT just assume since your USB device is plugged into a power source that you do not need the power lead on your USB cable! That is just plain wrong.

Yes, you're right - I wasn't planning on doing that and am aware the ground would be required. I would rather not rely on a custom USB cable though.

Quote
never from USB
Remove the USB polyfuse.

That's just what I was hoping for, could you describe where that is on the board so I know what to remove?

Thanks for your help.
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...
That's just what I was hoping for, could you describe where that is on the board so I know what to remove?
...

You didn't tell us which Arduino you have, You treat us as fortune teller?
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Yes, we need to know the Arduino board you have.
Show us a picture of it.
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That's just what I was hoping for, could you describe where that is on the board so I know what to remove?
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It's generally the very large flat component (compared to the other 2-lead SMD components) located near the USB connector.
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Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

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Sorry folks, it's an Uno.

Is it the golden one marked 501V?
« Last Edit: June 17, 2013, 02:18:13 pm by Sam2D » Logged

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Sorry folks, it's an Uno.

It's that squarish component just below the USB connector and closest to the left card edge in the top left picture.

http://arduino.cc/en/Main/ArduinoBoardUno

Lefty
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Thanks so much for your help, appreciated.
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yes, 501V, or 501L
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Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

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