what is ground GND for?

Hello, I dug out my dad’s trunk, found a PSU, It has three outputs (black) negative, (red) positive, and a (green) in the middle marks GND, What is the GND for? Thanks.

PSU.JPG

Most likely: GND = 0V and it's a symmetric power supply. Quite rare.

PSU.JPG

The green terminal is connected to earth ground.
That power supply was made with the + and - terminals floating (not connected) from earth ground.
If you need to reference the supply to earth, you can put a 'link' between the green jack to the black.
One reason would be to reduce problems with ESD. (you could also connect your ESD wrist strap to the green terminal)
They have designed convenience into the supply for you to do this. :wink:

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larryd:
The green terminal is connected to earth ground.
That power supply was made with the + and - terminals floating (not connected) from earth ground.
If you need to reference the supply to earth, you can put a 'link' between the green jack to the black.
One reason would be to reduce problems with ESD. (you could also connect your ESD wrist strap to the green terminal)
They have designed convenience into the supply for you to do this.

Alternately you could put a link to the red jack and have a negative voltage supply. It's not uncommon for op amp circuits to require a split power supply which one can do with two such floating supplies.

Thanks for the replies.

I think I got the terminologies mixed up. I always thought GND == nagetive. I guess that is not the case.
So GND != nagetive and GND == neutural is this correct? Thanks again.

tony1812:
Thanks for the replies.

I think I got the terminologies mixed up. I always thought GND == nagetive. I guess that is not the case.
So GND != nagetive and GND == neutural is this correct? Thanks again.

No. that is not correct. GND is ground. Neutral is an AC power connection.

GND == neutural is this correct?

No, see;.

Mistaking neutral for earth is potentially fatal. neutral is supposed to be a low voltage, close to
earth potential, in a working mains distribution system. In fault conditions neutral can be live,
and even in non-fault conditions it can be pretty far from the local earth due to phase-imbalance.
From a safety perspective you treat neutral just like its live, and you can live a long and happy life!!

There is a difference between ground and mains protective earth (PE) which is there for safety and
in buildings with metal pipework this is usually all bonded to PE. Ground is a more general
term for the part of a circuit at a low, common potential, usually defined as 0V. Battery operated
circuits can have a ground, but have no connection to mains PE.

In AC analog circuitry “signal ground” is the reference voltage for AC signals, and doesn’t have to
be 0V at DC.

Much mains equipment which is earthed will use that mains earth as its circuit ground. (This wasn’t
true for ancient vacuum tube TVs though, they typically had live chassis inside the box and were
hazardous to work on).

MarkT:
Mistaking neutral for earth is potentially fatal.

  • 1 Million.

A brief discussion of supply relationships

MrMark:
Alternately you could put a link to the red jack and have a negative voltage supply. It's not uncommon for op amp circuits to require a split power supply which one can do with two such floating supplies.

You do NOT need any kind of a link to make a negative power supply. As long as the output is isolated, all you need to do is decide that the positive terminal is 0V.

A lot of people at school did this. They just connected the black and green terminals with a jumper wire. It'd be good to ask them why they did that. When they can't answer, you ask why they're doing something that they don't understand.

Guess you could say that mistaking neutral for earth will put you in the gnd.