As you could see from the image below i want to understand what is the -V in a switching power supply unit and where i can use it?

enter image description here

  • 1
    \$\begingroup\$ Do you wish to know the Value of V, or what the -V is for? \$\endgroup\$
    – stanri
    Oct 9, 2013 at 9:37
  • \$\begingroup\$ What -V is for. \$\endgroup\$
    – user29966
    Oct 9, 2013 at 9:54

2 Answers 2


The V- is the more-negative terminal of the DC output.

Line (L) and Neutral (N) are the AC mains inputs. The ground () symbol is for protective earth.

The DC outputs are the +V and -V, where V- is the more negative connection.

The DC outputs may be floating, relative to the protective earth input. This would allow you to connect two of these power supplies in series, and ground the midpoint, which would produce a bipolar, protective-earth referenced power supply.

As to where you would use it, you would use it any time you use the power supply. You have to use it to use the power supply. It's where you connect for one terminal of the DC output.

  • \$\begingroup\$ If I understand correctly,in order to connect a device that needs 24V and 1.5A I connect the '+' to +V and the '-' to -V? \$\endgroup\$
    – user29966
    Oct 9, 2013 at 10:27
  • 2
    \$\begingroup\$ That is correct. \$\endgroup\$ Oct 9, 2013 at 10:28
  • \$\begingroup\$ Does this mean I can connect V+ to the ground symbol if I'm using this in series with another to generate -24V? Or would that give me smoke? \$\endgroup\$ Jan 26, 2014 at 19:16
  • \$\begingroup\$ If you want to generate -24V, you connect the positive on one supply to the negative of the other. This connection then becomes the node that you measure other voltages relative to. However, this can only be done if there is no internal connection in both power supplies to the protective earth, since that would produce a short through the earth connection. If you want to reference one of the nodes to the protective earth, you can do that, but you don't have to. \$\endgroup\$ Jan 27, 2014 at 11:01

+V signifies the output terminal that is the most positive and -V is the most negative terminal voltage. The marking "-V" is used because the output is isolated from incoming power circuits and calling it 0V or ground could be confusing.

So if you put your meter across +V and -V you'd measure 24V and this is adjustable by a few percent with VR1. This is your output.

In case you were thinking it don't think that -V or +V is anything to do with the protective earth connection (earth symbol within a circle) - this is used to ground the chassis and any other parts that are relied upon for safety of the users from electric shock on the secondary (24V) side.

  • \$\begingroup\$ I use -V for running diode in reverse bias mode. In question, guy asked what is the usage of -V terminal. I know one usage and I wrote in my answer. There are other usage of that terminal which I do not know and I did not mention in my answer. I wrote what I know. \$\endgroup\$ Oct 9, 2013 at 10:06
  • \$\begingroup\$ @User1551892 your usage of that terminal and the officially accepted normal usage of that terminal are two different things. Do you think the questioner will be wanting to reverse bias diodes or do you think there is a more common use for it like powering a circuit board or a motor or a small PC. \$\endgroup\$
    – Andy aka
    Oct 9, 2013 at 10:19

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