I am powering an Arduino Nano rev 3.0 device with an external USB power adapter (1000 mA). When I connect it to power input, board acts weird, and the servo goes bananas. If I use the same adapter on a USB port, board acts fine. I would like to leave the USB port free and use an external power supply. Why wouldn't board work from same power supply and what could I do to stabilize it?
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3 Answers
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It seems that your external USB power supply is manufactured with more than enough to power a few servos (1000 mA). Try to use another power supply (rated to 5 V @ +300 mA).
If you don't have another power supply, make sure the output is around 5 V, and put a big capacitor (200 uF or more) between the 5 V and GND on your Arduino. I suspect that the power supply might just have a half-wave bridge rectifier, which might be insufficient to drive the servo motors.
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As far as I understand, you can only draw 100mA from the USB until the connected device negotiates over the USB protocol to receive more.
https://electronics.stackexchange.com/questions/5498/how-to-get-more-than-100ma-from-a-usb-port
