Same ground, different voltages?

[TheShaner]

I am wiring up my buttons and some LED's inside of a cocktail I am working on.  The buttons are obviously wired to an iPac, the lights are getting power from a power plug coming off of the Power Supply of the computer.  Some of the lights (buttons) are running at 5v, and some of the lights (internal and under the cabinet) are running 12v.  Can I use the same ground across the board?  Or will I need to use the ground from the iPac for the buttons, the 5v ground for the 5v lights, and the 12v ground for the 12v lights? 

[SgtSlaughter]

I was wondering this myself, whether the 2 black wires were meant for 12v and 5v grounds, but everywhere I read said you can use them inter changeable.  The only downfall with running everything daisy changed is should a charge go through you have more change to fry multiple devices (at least thats what I understand).

I do think it probably is best practice to have your lights grounded seperately from your lights though.

[mgb]

If you meter the two black commons for continuity, do they ring out as being electrically the same?

[Nephasth]

Ground is ground is ground.

[matsadona]

Exactly, for a PC PSU all voltages share the same ground.

If you want the wiring more convenient you could add an extra resistor for the 5V LED's, and run everything at 12V.

[kahlid74]

Ground is ground is ground.

I'm not electrically super savvy but while this is true, the question becomes whether the components can handle a different voltage coming back through right?  If the IPAC is rated for 5V and you send 12 back through could there be components not expected this or are components on an electrical path only ever before the voltage and not on the ground path?

[DeLuSioNal29]

Think of a how a molex connector is on a power supply.  Both 12v and 5v share common ground, but have different voltages.

D

[Sarver Systems]

Think of a how a molex connector is on a power supply.  Both 12v and 5v share common ground, but have different voltages.

D

And 2 different ground wires.

But, back inside the power supply itself, they are connected together.

So, to answer your question....ground is ground is ground. Just as was stated before.

[Nephasth]

Ground is ground is ground.

I'm not electrically super savvy but while this is true, the question becomes whether the components can handle a different voltage coming back through right?  If the IPAC is rated for 5V and you send 12 back through could there be components not expected this or are components on an electrical path only ever before the voltage and not on the ground path?

There is no voltage on ground. Ground is ground. Ground is not hot.

[lilshawn]

all the black wires on a computer power supply are tied together. it's fine to attach them together.

in some cases where you have an external power supply brick, and you are having noise issues, you have to tie the negative voltage wire (-) to the ground of the computer power supply. make sure your output of your supply is isolated from line voltage before doing this.

you can even NOT use the black ground on the computer power supply. all the voltages are referenced together...for instance say you have some fans that make a ton of noise when running them at 12 volts, but won't start properly if you run them at 5 volts...you can attach the positive lead of the fan to the yellow 12 volt supply, and attach the negative lead to the red 5 volt line. your fan will run at 7 volts. (the difference of voltage between 5 volts and 12 volts.)

This is called ELECTRICAL POTENTIAL DIFFERENCE.

you can use this to your advantage by tapping all sorts of voltages in the power supply.

Positive on: + negative on =  ___volts DC.

12 volts + 3.3v = 8.7v
5 volts + 3.3v = 1.7v
12 volts + -12 volts = 24 v
3.3 volts + -5 volts = 8.3 v
12 volts + -5 volts = 17 v.

PLEASE NOTE, if you decide to use the negative voltages of your supply, the maximum load is that of the NEGATIVE rail. so even though your +12 puts out 45 amps, the - 5 rail only puts out 900mA.... this means the maximum load on your "17 volt supply" should be UNDER 900mA.

[matsadona]

The potential difference between two points can be illustrated like this:

[Louis Tully]

.

[lilshawn]

The potential difference between two points can be illustrated like this:

not ground...water 

But no, that's illustrating it perfect.

[matsadona]

The potential difference between two points can be illustrated like this:

not ground...water 

But no, that's illustrating it perfect.

Thanks. I was about to add a diver below the surface also in order to illustrate negative voltages... 

[cw]

you might use both grounds to make a ground loop. In the event of a break in the ground wire you will still be up and running.  I wire all my cps like that..

[lilshawn]

The potential difference between two points can be illustrated like this:

not ground...water 

But no, that's illustrating it perfect.

Thanks. I was about to add a diver below the surface also in order to illustrate negative voltages... 

and you wouldn't be for from the truth.

think of the voltage you get as if you strung a string from person to person...the length is the voltage. if you had a diver at -90 feet (volts) and a person on a diving board at 108 feet (volts) the string would be 198 feet long (volts)... such as the high voltage display in a pinball achieves high voltages using limited power feeds.

[Mysterioii]

"ground is ground is ground" is somewhat misleading.  For a single power supply with multiple outputs then yeah it's a safe assumption.  Across different systems and supplies, or anywhere where there might be an isolation transformer or some such, you CAN'T assume they're the same.  Things may, or may not, be connected to earth ground, but aside from that, ground is just a reference point.  It is NOT the equivalent of "absolute zero" for example.  It's just a reference value. 

If you're using multiple power supplies, tie the grounds together unless they're powering completely separate systems.


http://en.wikipedia.org/wiki/Floating_ground

[lilshawn]

"GROUND" or "Negative" as we so commonly refer to it is technically "zero volt reference"

[SavannahLion]

"GROUND" or "Negative" as we so commonly refer to it is technically "zero volt reference"

Hmmmm..... I think that M is trying to say is that it's not uncommon for circuits to use the term "ground" as the common reference point and that reference is sometimes not at 0v. Eg we might connect the "ground" lead of a meter to 100v line and the "positive" to the 101v line to measure the difference.

In addition I would also argue using the nomenclature "negative" to technically mean 0v is patently false and dangerously misleading. What happens when you start dealing in circuits that require a negative voltage and 0v? Tie the negative to the ground of a system with +Xv/0v scheme and you could have potentially disastrous consequences.  Or to put it into perspective. What happens if you decide to tie the -v of your old RAM to the true 0v of the case? Doesn't work.

Yeah, I know that in a lot of circuits, ground and negative do end up being the same. But the terminology really shouldn't be exchanged so freely, especially if negative really does mean negative and is readily distinct from 0v.

[lilshawn]

we try to not talk about referring to things as "0v" because it can lead people to believe it's safe to touch.

often japanese cabinet schematics refer to the neutral wire as "0v" while technically it can be considered so, if ANY of your plug, outlet, house wiring is wired improperly, you chance electrocuting yourself on that line.

a power supply could be labeled as 12 volts in respect to another wire or part of the circuit, but as you mentioned, that negative reference could be at 100 volts. and the one you measure be at 112 volts with respect to earth ground.

you have to be cautious when measuring voltages especially when you have an "earth ground" and a "chassis ground" and possibly multiple "zero volt references" in the power supply While not common, it's always a possibility.... you must ALWAYS prepare for that possibility, no matter how remote it might be.

whew. 

[Nephasth]

A live wire is perfectly safe to touch... As long as you are isolated from ground.

[PL1]

A live wire is perfectly safe to touch... As long as you are isolated from ground.

Mythbusters rules apply here:
Don't try this at home, kids.  We're what you call 'experts'.     


Scott

[lilshawn]

if it weren't for the people who do stuff at home, we wouldn't have what you call "dumbasses".

[MTPPC]

All your dc ground circuits should be tied together. Then tied to the ground planes in the cabinet. Then tied to the green wire on your power cord and plugged into the ground hole in your 110vac socket. This way, when you touch your game with static from walking across the carpet, all your ICs are protected. You'll also notice that your computer case is grounded through the ground post in your AC plug. Most electrical weirdness is caused by poor grounding.

[the_gamer]

The potential difference between two points can be illustrated like this:

It's more like how many people/electrons are on the level but I am not that good at explaining