I've got an Leonardo board, it seems to be official, in that it has the Arduino branding and Made In Italy on the bottom.

However I cannot get anything to run with external power supply. Everything when powered via the USB from the computer runs fine. As soon as I use any sort of external power, and I've tried both the barrel jack with 4x AA batteries, 6x AA batteries, 9v square battery and a Shield with 2x 3.7v lithium batteries - it turns on but doesn't do anything - the USB device doesn't even appear to the computer at all unless I pull the external power out again.

Is this a faulty board? Do I need to change the jumper arrangement on the board so it'll run with external power?

  • I have just tested all combinations of power plugging and unplugging on my Leonardo. No problem with anything. – Majenko Jan 11 '19 at 14:13
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    And what "jumper arrangement" are you on about? There's no jumpers on a Leonardo. – Majenko Jan 11 '19 at 14:19
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    I think key to my sketch is that it's working over USB-HID - it's that connection which is never made when powering externally - it doesn't appear to the computer at all. As soon as I pull the external power it connects to the computer as a HID device. – Danny Shepherd Jan 11 '19 at 14:46
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    4xAA won't work as the VIN voltage must be greater than 6.6V to get precedence over the USB voltage. Did you check the polarity? The middle contact is positive, the surrounding contact is GND (negative). – Pylon Jan 11 '19 at 18:08
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    @Pylon While a voltage lower than 6.6V will cause the USB's P-fet to get turned on when it's not desirable. It won't affect the operation of the sketch at all. Worst case scenario the computer will disable the USB port completely to isolate the external power. And it would alert you to that fact (Windows would pop up a bubble, linux / OS X would put it in dmesg, etc). – Majenko Jan 11 '19 at 23:08

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