I always see this symbole behind electronic devices.


A close-up image of the symbol:


What does it mean?

  • 5
    \$\begingroup\$ You always see it? I never see it and get really angry because it's so useful! If I need to replace a power supply, I need that information to choose a replacement. \$\endgroup\$
    – Cort Ammon
    May 25, 2019 at 3:18
  • \$\begingroup\$ To me, this is the only self-explanatory icon on that power box, except the forbidden disposal. All the other 22 things mean nothing to me. \$\endgroup\$ May 25, 2019 at 17:23

2 Answers 2


Negative on the outside of the barrel connector, positive on the inside of the barrel connector.

It is the standard method of showing the polarisation of the barrel connector. The positive pin is usually (as in this case) the inner contact, and negative is usually the outside of the barrel.

If you look at the image, it shows a surrounding area labelled "-", an inner point, which it labels with a "+". Hence, negative surround, positive inner contact.

  • \$\begingroup\$ Why do they draw it on the power supply? why not near the connector? \$\endgroup\$
    – Roh
    May 24, 2019 at 12:40
  • 9
    \$\begingroup\$ They need the space to fit the label. As the big power supply brick has all the rest of the information, it's where people will look anyway. \$\endgroup\$
    – Puffafish
    May 24, 2019 at 12:43
  • 5
    \$\begingroup\$ And do pay attention to that symbol! While almost everything uses negative on the outside, there are some evil devices that use positive on the outside and mixing plugs probably isn't good for the devices. \$\endgroup\$
    – Hearth
    May 24, 2019 at 15:47
  • 1
    \$\begingroup\$ @Hearth Hey, guitar pedals aren't "evil" they're just misunderstood! :) \$\endgroup\$
    – JYelton
    May 24, 2019 at 23:22
  • \$\begingroup\$ @Roh If I have a power supply fail, I can look at the one label and get 1) Voltage 2) Amperage 3) Polarity (the symbol you are asking about). Then I just need to know the size of the plug (there's only a few of them), and I'm able to go get a new power supply. \$\endgroup\$
    – Cort Ammon
    May 25, 2019 at 3:20

To answer your comment to Puttafish,

Why do they draw it on the power supply? Why not near the connector?

The connectors are purchased with no polarity assignment. It's up to the PSU designer what the polarity is or if it is AC. Thus it makes sense for the PSU builder to label the polarity on the PSU along with all the other technical information.

There's not a lot of room on the connectors either!

  • 2
    \$\begingroup\$ Very nice answer. How bad that we cannot mark two answers as accepted answer! Thank you \$\endgroup\$
    – Roh
    May 24, 2019 at 16:18

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