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Same ground, different voltages?
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lilshawn:

--- Quote from: matsadona on January 20, 2014, 02:39:09 am ---
--- Quote from: lilshawn on January 17, 2014, 10:15:42 am ---
--- Quote from: matsadona on January 17, 2014, 02:47:08 am ---The potential difference between two points can be illustrated like this:

--- End quote ---

not ground...water  :laugh2:

But no, that's illustrating it perfect.

--- End quote ---
Thanks. I was about to add a diver below the surface also in order to illustrate negative voltages...  :D

--- End quote ---

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:

--- Quote from: lilshawn on January 23, 2014, 07:41:21 pm ---"GROUND" or "Negative" as we so commonly refer to it is technically "zero volt reference"

--- End quote ---

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.  ;)
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