# Arduino board Power Polarity

I don't know if the power plug should have + at the inner contact or the outer.

• take an ohmmeter and measure between something clearly marked as ground on the board (if anything). Perhaps large mounting holes if they have a metal ring. – old_timer Feb 25 '11 at 16:01
• please add your "solution" as an answer. Also, quoting some of that content in your answer or summarizing it would not only make the your answer more valuable in the event that Arduino changed their URL structure or removed that page, it would make it quicker and easier for readers to decide if they wanted to click that link. – Kevin Vermeer May 16 '11 at 17:56
• @Kevin Vermeer: I added an answer below – powtac May 26 '11 at 9:06

positive is in the center of the plug. This is a pretty typical way to do it, because having positive on the outside is just inviting shorts if you pull the plug, and then drop it on something that's grounded.

A good documentation on the official Arduino Wiki: arduino.cc/playground/Learning/WhatAdapter

+ is the center contact
- is the outer contact

         .---.
-  ----<  O----  +
---´
`
• DC should be between 9V and 12V (20V max)
• It's a 2.1mm power plug