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As will become obvious, I am new to Eagle and PCB layout. I set out a simple design, starting with a power jack. The power jack seems to clearly have a "+" terminal labelled, so I connected that to the positive input of my 5V step-down converter.

schematic

On the board layout, I should have noticed that what I thought was the positive terminal was labelled GND (but alas, I did not notice):

board

However, the board passed ERC.

erc

Did I misinterpret the "+" symbol on the schematic view? I would appreciate any insight into my error.

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    \$\begingroup\$ ERC doesn't know about things like that. Of more concern is those tiny traces you are using for power, I hope the current is minimal (even if so, the traces should be much wider). \$\endgroup\$ – Dean Franks Mar 2 '18 at 3:38
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    \$\begingroup\$ Connector pins are arbitrary. Even if the ERC for both symbols was done right, both jack pins would be "power output" and both converter pins would be "power input". Some designs with barrel jacks use the middle for (-) and the outside for (+). It's a free world. \$\endgroup\$ – uint128_t Mar 2 '18 at 4:06
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    \$\begingroup\$ It is your call to decide whether you want center negat6 or center positive. You don't want any program to get in the way of that \$\endgroup\$ – PlasmaHH Mar 2 '18 at 9:04
  • \$\begingroup\$ @Dean Franks - while the device current is very low ~100mA (it's an esp8266 controlling a couple of led), thanks for the advice on the traces. I'll beef them up in "rev 01" \$\endgroup\$ – Roberto Mar 2 '18 at 11:17
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The power jack seems to clearly have a "+" terminal labelled

It's not a "+" terminal, it's the symbol's 'origin' crosshairs. If you look closely you should see one on most symbols, though often it is partially obscured by other elements in the drawing.

Here are some examples (taken from Sparkfun's tutorial Using EAGLE: Schematic):-

enter image description here

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