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?
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The V- is the more-negative terminal of the DC output.
L) and Neutral (
N) are the AC mains inputs. The ground (
⏚) symbol is for protective earth.
The DC outputs are the
-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.
+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.