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So far, I have connected plus to VIN and minus to GND, but I am curious what would happen if I do it the other way around. And I am not going to try in case I break someting.

So, what will happen?

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    Magic smokes will apear. If you have such a question, then DO NOT TRY THIS! Yes, you will definately burn your device
    – Martynas
    Feb 25, 2015 at 12:57
  • Well, good I did not try then :)
    – Mads Skjern
    Feb 25, 2015 at 12:58
  • FYI: If connecting through a power supply plug (eg on the Uno), no problem. arduino.stackexchange.com/a/896/3267
    – Mads Skjern
    Feb 25, 2015 at 12:59
  • So we are talking about specific device?
    – Martynas
    Feb 25, 2015 at 13:02
  • I am talking generally. But the Uno has a power plug (besides the VIN pin). And as described in the answer I linked to, the power plug has protection against reverse polarity. I thought that was worth mentioning.
    – Mads Skjern
    Feb 25, 2015 at 13:14

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Some systems have protection against people connecting things in reverse.
Some don't.

On unprotected systems you MAY destroy the powered system and/or you MAY destroy the power supply - both these depend on happenstance, system design and luck.

Possibly worse - you may damage the target system in an inobvious way so that it subsequently behaves incorrectly or damages other equipment.

Systems which are designed to be protected may not be protected in all cases. eg if you have a "shield" connected that accesses the Vin pin outside the protection diode the reverse polarity voltage may damage the shield AND current may travel via the shield circuitry back into the Arduino via other connections.

Advice on You tube may just sometimes very very occasionally be wrong. Or more often.

If this "Reference Schematic" is correct (as you'd hope it was) then Vin is connected to the "inside" of protection diode D1 - so D1 does not protect against reversed polarity. See lines and boxes in red.
Larger version of diagram here

D1 does protect against reversed polarity on "PWRIN".
How PWRIN relates to shield feeding I know not.

enter image description here

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  • And D1 refers to a diode on the diagram? (not to the pin D1)
    – Mads Skjern
    Feb 26, 2015 at 12:07
  • @MadsSkjern - D1 - yes, diode D1 as per text. In RED box at upper right here. I looked at a UNO. The power supply jack circuitry is used only by that board and not by attached shields so diode-D1 does protect them against reversed power supply. BUT reverse voltage on VIN line is NOT protected against. .
    – Russell McMahon
    Feb 27, 2015 at 0:03

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