# Purpose of the Power Supply Ground terminal?

I have a power supply with 3 terminals: "-", Ground and "+".

Now I know that Ground is mains ground, but what does it mean? I thought that negative "-" terminal and Ground should be very near to each other. So I took voltmeter out and to my suprise the difference between the "-" terminal and Ground was zero, but the difference between "+" terminal and Ground was zero as well. I mean power supply was ON and output was ON, but the difference between both terminals and Ground was zero. How's that possible when "+" and "-" was as set on the voltage dial?

Deeply misterious :drooling_face:

If a supply has those 3 output options available, then the one marked MINUS will be a voltage that is negative with relation to GND potential. It is, rightfully so, a potential below ground. In digital logic, only in special cases do you see a negative voltage rail be needed, however, use of dual polarity power sources is quite common in analog circuits, especially audio amplifiers for speakers and with operational amplifiers.

DROBNJAK: Now I know that Ground is mains ground...

If you're quoting from the power supply's manual, OK, else I would not make that assumption.

DROBNJAK:
I have a power supply with 3 terminals: “-”, Ground and “+”.

Now I know that Ground is mains ground, but what does it mean? I thought that negative “-” terminal and Ground should be very near to each other. So I took voltmeter out and to my suprise the difference between the “-” terminal and Ground was zero, but the difference between “+” terminal and Ground was zero as well. I mean power supply was ON and output was ON, but the difference between both terminals and Ground was zero. How’s that possible when “+” and “-” was as set on the voltage dial?

Deeply misterious

if there is no voltage between “GND” and “+” and no voltage between “GND” and “-” , and if there is a voltage between “+” and “-” , then I think that “GND” is just the metal box, not connected to anything else

OK, so my Ground is just "fake" and it is there for good looks. Is this normal with other power supplys, or should I get a new one?

If it’s fake, it’s not normal.

Thanks, at least I know. What is normal? Should "-" terminal have a negative voltage in reference to Ground terminal?

yes

There are no "rules". It depends on the design and the application.

Thanks, at least I know. What is normal? Should "-" terminal have a negative voltage in reference to Ground terminal?

Not necessarily. Since you measured no voltage between ground and either output terminal. I would guess you are correct about it being a mains ground, isolated from the +/- outputs. In that case you can leave it open, connect to the negative terminal, or connect it to the positive terminal to make a negative-voltage supply. Or, you could use two of these supplies to make a dual (positive and negative) supply.

If you leave it open, the negative terminal could be used as an isolated low-voltage ground for your Arduino circuit.

I've got some similar supplies. The "ground" is the chassis, isolated form everything else. I've connected it to the green wire (ground) on the AC power cord, so now it's a mains ground. The negative terminal connected to "ground" on the boards we are testing (completely isolated from earth-ground or mains ground).

Without measuring...

The + and - may be floating, and you are meant to choose if this is + with - attached to Common/Gnd, or - with + attached to Common/Gnd.

Lots of opinions so far, and any could be correct. IME (which is just one more possible perspective), the DC - and + terminals are your power supply. The Ground terminal is a safety ground, connected to the ground lug of the mains plug. It likely exists in the power supply to ensure that if the PSU develops a fault and AC mains is present where it shouldn't be, any conductive path to the chassis will cause a short and a blown fuse / breaker.

Giving you a convenient access point is useful if you'd like to do something similar with your load device. I.e., a metal chassis that has no particular potential WRT to the circuit within, but a safe return path for stray voltages, noise, or faults. Normally, the Ground will be galvanically isolated from your circuit. Connecting the - terminal to ground defeats this for probably no good reason -- and could do more harm than good.

For example, in telecom where lots of devices run on -48vDC, there's almost always a separate ground terminal. Connecting that to either PSU terminal could mean seriously destructive current loops through the chassis and equipment racks. It's there for saftey and noise immunity, and should never be connected to the PSU rails.

On most home appliances, a metal chassis will have a connection back to the mains ground lug. However, all internal voltages will be floating with relation to mains ground (which is why you measure no voltage to either terminal -- the circuit is incomplete). This becomes necessary as you inter-connect appliances. For example, a TV and cable box. Each one's internal PSU floats with relation to the AC mains, so they're free to use single-ended positive supplies, dual-rail supplies with a center tap, or whatever they want. There's no inherent coupling between them except for the I/O points -- which could be AC or DC coupled, as appropriate. Still, since all of their chassiseses are at AC ground potential, you can set them on top of each other, or touch them at the same time, and all it does is equalize their safety ground potential. No harm done.

What does it mean that "voltage is floating"? And is that good or bad?

When I ordered a 2.1mm barrel jack, it had three pins. One of them wasn't connected to either tip or ring. I assumed it was there just to provide another contact point with the board, for mechanical strength, since power plugs are handled a lot. I couldn't imagine what its purpose was, if it was not electrically connected to either tip or ring.

Floating voltages have no common reference to another circuit. For instance, take two batteries. What's the voltage difference between the positive terminal of battery A, and the negative terminal of battery B -- assuming these two batteries are not connected together in any way.

If you try this with a volt meter, you should get 0v. There's no circuit between them, so there's no valid reference point to determine voltage.

It was once explained to me that it's like trying to measure how high the top rung on a ladder is. Well that depends, is the ladder lying flat on the ground? Propped against a wall on the second floor of a house? Floating in the atmosphere? This is what it's like when the circuits share no common reference point -- if two ladders are both propped against the wall, on the same level surface, then and only then can you compare their relative height.

In a power supply, the transformer has two windings. The primary is fed by the AC mains, and generates an electromagnetic field. This field generates a current in the secondary coil proportional to the ratio of turns in the coil. However, the two coils share no common reference point, so you can't compare the voltage of the secondary coil based on the neutral line of the primary. Its voltage is undefined.

However, the voltage of one end of the secondary is defined with regard to the other end of that same winding. Place a meter probe on one end, and the other probe on the other end and you will see voltage. You could also place one probe in the middle of that winding and have opposite voltages at each end (center tapped), or you can have multiple taps at different places to generate all kinds of voltages.

joshuabardwell: When I ordered a 2.1mm barrel jack, it had three pins. One of them wasn't connected to either tip or ring. I assumed it was there just to provide another contact point with the board, for mechanical strength, since power plugs are handled a lot. I couldn't imagine what its purpose was, if it was not electrically connected to either tip or ring.

Depends on the jack. Sometimes one pin is the center, and both other pins are the barrel. Other times, it's unconnected. Or it could be the outer metal casing (ground it to improve shielding). Or a switch, connected to one of the other terminals when a plug is inserted.

Barrel jacks like that are normally wired up as a switch, so when a plug is inserted, batteries are cut out of the circuit.

So, for instance, ground to pin 1 (the center terminal), internal battery to pin 2, and pin 3 goes to circuit.

If things are wired up so + is on the center, you are cutting power from the battery negative to ground. If wired so - is center, you are cutting power from the battery positive to Vcc.

What is the purpose of the Power Supply Ground terminal?

My answer would be: it is for safety. The ground connector is physically connected to earth's ground through mains. The earth has practically an infinite capacity, so whatever voltage is applied to it gets sucked down to earth ground. Normally the chassis of household appliances (microwave ovens, etc.) is connected to earth ground so that any stray voltages that are erroneously leaked to the chassis are neutralized.

PetriH: My answer would be: it is for safety. The ground connector is physically connected to earth's ground through mains.

Not always.

In fact I suspect his weird voltage readings are down to it not being connected.

pwillard:

DROBNJAK: Thanks, at least I know. What is normal? Should "-" terminal have a negative voltage in reference to Ground terminal?

yes

No. They're independent (or should be) so it can be higher, lower or even the same.

fungus:

PetriH: My answer would be: it is for safety. The ground connector is physically connected to earth's ground through mains.

Not always. In fact I suspect his weird voltage readings are down to it not being connected.

I think the problem is that the word "ground" is overloaded too much in electronics. My point is that for power supplies the usual purpose of a separate ground is for grounding the chassis to earth. It is true that true earth ground may not always be present, e.g. if there's a fault in the ground connection or the power supply is not plugged into grounded wall plug. But the purpose remains the same.