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 Posted: Feb 21, 2012 - 01:23 PM |
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Joined: Apr 07, 2011
Posts: 38
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I have some really basic questions about grounding electronics and "grounds" in different power supplies. Any help to pointers would be helpful!
- Where do the "grounding pins" (some countries might have holes, but whatever) in a wall AC outlet go? Actual earth ground? Some ground that comes from power company?
- PC's PSU is usually plugged into wall with grounded cable - does that mean that any ground reference derived from PC (say, USB cable ground) are essentially connected to ground from "grounding pins"?
- What about the ground supplied by external, non-grounded AC-DC power supply? I've understood that AC mains voltage has "ground" and alternating wires, is the PSU's ground this same ground?
And here are some derived questions:
- If I make a signal generator that is powered by (ungrounded) 5V DC wall adapter, can I connect such device's ground to the ground of another, USB-powered circuit? If the 5V DC is "floating" I'd assume this is OK, but I'm not very certain about this...
- Would it be possible to connect the negative terminal of a 9V battery to USB ground, to get +9V from the positive terminal? (of course this should not get in direct contact with the +5V VCC from USB)
- In a similar line, could I use a 12V DC adapter with connected grounds to USB to provide an opamp with VCC above 5V?
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Posted: Feb 21, 2012 - 01:55 PM |
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Joined: Oct 20, 2005
Posts: 257
Location: Colorado
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Quote:
Where do the "grounding pins" (some countries might have holes, but whatever) in a wall AC outlet go? Actual earth ground? Some ground that comes from power company?
In USA, they are connected to earth ground. In a properly wired house. There are some improperly houses out there. And older homes do not have the third ground pin.
Quote:
PC's PSU is usually plugged into wall with grounded cable - does that mean that any ground reference derived from PC (say, USB cable ground) are essentially connected to ground from "grounding pins"?
Well, yes, I repeat, no. One way to tell if this is true is to ohm out the third ground pin to the case. Or to the suspect ground. I say yes because that should be the intent, but I did build a PC once where I did NOT connect the case ground correctly. Hence the no.
Quote:
What about the ground supplied by external, non-grounded AC-DC power supply? I've understood that AC mains voltage has "ground" and alternating wires, is the PSU's ground this same ground?
This would depend on the design of the power supply. Back in the day, this would normally be transformer coupled to the AC line. This provides some isolation from the AC and the ground would be a 'floating' ground which you could connect to many different potentials. In fact, when repairing TV's, an isolation transformer was a must. Or death could claim you. Which would not be good. (Depending on your friends total vote I guess.) This was in the 60's BTW. Non-isolated power supplies 'ground' may not be ground. I would be wary of wall warts for sensitive applications.
Quote:
If I make a signal generator that is powered by (ungrounded) 5V DC wall adapter, can I connect such device's ground to the ground of another, USB-powered circuit? If the 5V DC is "floating" I'd assume this is OK, but I'm not very certain about this...
If I were doing this one of, I would measure the difference between grounds in voltage or current to see if they were in fact the same potential. Or run the wall wart off of an isolation transformer.
Quote:
Would it be possible to connect the negative terminal of a 9V battery to USB ground, to get +9V from the positive terminal? (of course this should not get in direct contact with the +5V VCC from USB)
Aha! The true floating power supply! Yes. This would work. (Not sure what you want to do with it but it is fine to do this.)
Quote:
In a similar line, could I use a 12V DC adapter with connected grounds to USB to provide an opamp with VCC above 5V?
As long as the adapter is floating, sure. |
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Posted: Feb 21, 2012 - 02:02 PM |
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Joined: Oct 18, 2001
Posts: 354
Location: Eastern England.
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jokkebk wrote:
Where do the "grounding pins" (some countries might have holes, but whatever) in a wall AC outlet go? Actual earth ground? Some ground that comes from power company?
Have a read of http://en.wikipedia.org/wiki/Earthing_system.
Here in the UK two of those systems are in common use...
TN-C-S
TT
TN-C-S is commonly used in towns; TT is commonly used in remoter areas. |
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Posted: Feb 21, 2012 - 04:00 PM |
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Joined: Jul 19, 2011
Posts: 460
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jokkebk,
These are good questions about a topic which confuses many beginners trying to learn electronics and electricity.
Part of the confusion comes from a diverse list of synonomous signal names which EE types toss around quite cavellierly. These include:
Ground
Earth Ground
Earthy
Neutral
AC Neutral
Chassis Ground
DC Common
AC Common
Common
Power Supply Common
AC Return
DC Return
Power Ground
Power Return
Signal Ground
Shield Ground
Ask an EE to give you an explanation of these and he will generally give you HIS specific interpretation of all these terms. Ask another EE and you'll likely get a different explanation! The fact is that there is little universal agreement on the meaning of these terms, and virtually no restraint on their misuse in technical conversations and written articles. So, Good Luck! trying to figure out FROM THE WORDS ALONE the function these signals perform in the diverse applications & situations in which you will find them.
Nonetheless, let me give you one simple, general principle which I find helpful: A VOLTAGE SOURCE HAS TWO TERMINALS. Batteries, generators, power supplies, and an AC wall outlet are all examples of voltage sources we encounter constantly. Further, either terminal of the voltage source taken by itself is completely impotent and benign. The voltage source can only exert its strength when both terminals are utilized.
This is why birds can perch on 14 Kv power lines without harm - they are connected only one terminal of a voltage source. As many electricians know and practice, as long as you touch ONLY the neutral or ONLY the hot line of the mains, you will not get a shock. You can try this yourself in a safer manner using a DC power supply in the 40-50 volt range - just enough to give you a tickle. Hold either positive or negative lead and you will feel nothing, touch both and you get the "tickle". See?
In summary, a voltage source has two terminals AND you have to use both terminals to get the voltage source to do something.
NEXT, there is a distinction between the two terminals of the voltage source. By long-standing convention we call one the "negative" terminal and the other the "positive" terminal. This distinction makes sense for DC voltage sources, but it is frequently applied to AC voltage soruces as well, even though other terms such as "neutral and "hot" apply more appropriately.
The "negative' and "positive" naming convention is based on the direction current will flow thru a resistor attached to the voltage source's terminals. The resulting resistor current flows out of the positive terminal, thru the resistor, and into voltage source's negative terminal. Electrical measuring equipment (DVMs, voltmeters, ammeters, etc) are all marked according to this convention - they too have a "negative" terminal (usually of black color by long-standing convention) and a "positive" terminals (usually red by the same convention).
This "convention" keeps everybody "on the same page" when designing, constructing, repairing and measuring electrical and electronic equipment.
In AC systems the "negative" and "positive" terms are obfuscated by the fact that the polarity is constantly reversing by definition. Nonetheless, an AC voltage source still has two terminals and a convention needs to be adhered to in order to keep the designers, builders and maintainers in agreement.
For various reasons the negative terminal of real-world voltage sources are often purposely connected to the earth (via a metal rod driven into the soil, or a buried water pipe, etc). The main reason this is done is for safety, but there are other historical reasons as well. Why the negative terminal? I don't know. It's just been done that way forever as part of "the convention". As a consequence, and by long-standing convention, the negative terminal of a voltage source is commonly referred to as the "ground terminal" - whether it is actually metallically connected to the Earth or not. This is true for DC and AC voltage sources (even though in AC sources the choice of terminals is somewhat arbitary).
Thus, the terms "ground" and "negative" have become synonomous during the past nearly two centuries which have passed since the early telegraphers began the practice of connecting the negative termnal of their wet cell batteries to the Earth back in the 1820's.
The fact that current flows from positive terminal of a voltage source, thru a load, and back into the voltage source's negative terminal has given the negative terminal yet another name - the "Return Terminal".
If you have an electrical system or apparatus with multiple voltage sources, these voltage source must all be connected together in some manner in order for them to exert their voltages (remember, a VOLTAGE SOURCE HAS TWO TERMINALS). Voltage sources can be connected in various ways (series and parallel are the common terms used to describe the connections, though tese are a bit misleading). In most multiple voltage source toplogies one terminal of each source is connected to a "common" node. Typically this is the negative terminal of each voltage source which is connected to that common node. Thus, another term emerges for the negative terminal of a voltage source - "Common" and its variants "DC Common" and "AC Common".
So you see all of these diverse names (ground, earth, return, common, etc) refer in one way or another to the "negative" terminal of a voltage source, even though colloquially the terms are commonly misused, abused and slurred by my fellow EE-types. (I myself plead "guilty-as-charged".)
Looking at it in another way, one of the reasons beginners have such a tough time learning electronics is that they have a hard time overcoming the "speech impediments" of us EE types - of which the "ground-vs-negative" name confusion is just one of many. |
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Posted: Feb 21, 2012 - 07:11 PM |
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Joined: Jun 08, 2011
Posts: 300
Location: Maryland, USA
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Be aware that while negative ground in DC systems is a convention, it's not the only one! Some vehicles in particular bond the positive side of the electrical system to the vehicle chassis, though chassis negative is most common in cars and light trucks. When in doubt, double check, especially where vehicle batteries are concerned.
In premises AC wiring, the US National Electric Code uses the terms "ungrounded conductor" for the hot/line/phase, "groundED conductor" for the neutral, and "groundING conductor" for the equipment/safety ground. These terms reflect the specifics of the standard US electrical distribution where the neutral is bonded to the equipment ground (hence "grounded") at the service entrance (where the power lines enter the building). |
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Posted: Feb 22, 2012 - 12:01 AM |
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Joined: Dec 11, 2007
Posts: 6842
Location: Cleveland, OH
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For your question "c", regarding wall warts:
This Thread discusses wall warts and their isolation. Nard made a bunch of tests to see if they were isolated or not.
In the "Old Days" wall warts contained a classic transformer, with the primary side connected to 120 VAC, (or whatever the Mains voltage is in the country of interest), and the transformer's secondary was the output, perhaps being rectified, (but rarely "regulated".
Todays small wall warts are generally small switching power supplies, and the output often is not truely floating with respect to the primary.
JC |
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Posted: Feb 22, 2012 - 08:32 PM |
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Joined: Apr 07, 2011
Posts: 38
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Thanks for all the great replies, I'm constantly amazed by the effort people put into their answers.
I read the Wikipedia article on grounding and that was very useful. Also did some googling to understand things a bit more. I live in Finland and in a rather new house so grounding pins should actually be grounded. 
It seems rather logical that a transformed-based PSU would provide a "floating" voltage. And DocJC's link to the thread discussing switching wall warts was _very_ informative, I immediately made measurements of several DC supplies I had available - turns out all my Nokia phone chargers were very good, and all non-Nokia ones rather bad. |
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