Ground

[wang]

Hi,

I have a power supply which provide DC voltage of 9V connected to my test fixture.

My power supply are plug into the wall socket. The earth plug is empty.

Only two plug are plugged into the wall socket. I would like to ask if my

test fixture ground point connected to the components is not referenced to

a system ground in this case. The ground in my test fixture is referenced to

the negative terminial of the positive terminial, right?

[LV_Pro]

The description you give appears correct. The ground pin of the AC outlet is actually more for safety, than as a ground reference. Usually it is a bad idea to use the ground return as the reference, it results in ground loops and other problems. Your measurement device (DMM, DAQ card, whatever) shouldn't be reference to the AC outlet ground, but rather connected, as you describe, across the signal to measure. If you have a sketch, or schematic of your setup it would help to confirm your connections. If it is a graphic, please use the jpeg format rather than a bmp file.

P.M.

Message Edited by LV_Pro on 08-31-2005 12:30 AM

[wang]

Hi,

You were saying my test fixture is a floating signal source, right?

If my power supply is reference to a system ground, does my test fixture in the block diagram ground would be what?

It would be just a negative terminal refer to the positive terminal or it would be referenced to the system ground?

I don't have my digram right not. But my diagram contain some capacitor connected to ground just like this. The symbol

is a ground symbol and it is like one vertical line with a few horizontal line.

You were talking about ground return, did you means a negative terminal point to the positive terminal for the common ground point,

not the system ground reference to the earth, right?

[tbob]

Without a schematic, it is hard to tell.  Forget the third prong in the A/C outlet.  It should not be used as a ground reference.  If you have a power supply plugged into the A/C outlet, and you are using the power supplie's positive and negative outputs, the negative is usually the ground reference.  I say usually because I have seen some positive ground systems.  System ground is whatever terminal (positive or negative) that all the circuit grounds are attached to.  If you look at your schematic or trace the negative terminal wire from the power supply, you will probably see that it is connected to some ground plane on the circuit board.  This ground plane on the circuit board will run to all points where a ground is needed.  If this is so, then the negative terminal of the power supply is the system ground and reference ground.  Some circuits have two grounds, and analog ground and a digital ground.  The digital ground might be connected to the negative terminal of the power supply, and a capacitor might connect the analog ground to the digital ground.  This is done for isolation.  To an analog signal, the capacitor acts as a short, so the ground reference is the same.  But DC wise, they are different.  Basically, if you put the negative lead of your DMM to the negative terminal of the power supply, and you read correct voltages on the circuit board with the positive lead of the DMM, then the negative terminal of the power supply is your reference ground.  It is not floating.

[Ray.R]

If you intend to plug more than one instument, make sure they have a common "ground".  Especially multiple power supplies.

The earth ground is to protect you from electrical shock hazard.  And may be used as an analog (or part of) reference for ground.  I would certainly make sure instruments are properly grounded as per the manufacturer's recommendation..... even if not recommended..  It is surprising how much potential difference (especially DC) can exist between a virtual ground and earth ground.

JLV

[tbob]

Good advice, Joe, when using multiple instruments.  Some DMMs (probably most DMMs) have a grounding strap option to connect the negative terminal to the metal case of the DMM, which is connected to earth ground.  For plastic cases, the connection is made to earth ground through internal wiring.  I've never used it, but I would say it is used to eliminate the potential difference between circuit ground and earth ground, if earth ground is used in any part of the system.

[Ray.R]

Good words tbob..

Also, I always install a larger ground strap or ground bar on a test rack, where all signals (gnd) are referenced to the same point.  Of course, I also make sure there is proper grounding of the instruments for the AC, which is for personal (and instrument) safety.

😄

[wang]

Hi

Thank you for your help!

Here is my diagram.

1) Is my test fixture a floating signal source in this case by looking at this diagram?

2) Is my power supply also a floating signal source in this case?

[xseadog]

Hi Wang
with regard to your block drawing: is the capacitor connected to the +ve terminal and earth of the test fixture ?

What size is it? Also important what voltage rating?

Are you are using it for ripple supression.?
Where do the current outpout connectors terminate?
Sorry more questions than answers but one should never assume anything!
chow
xseadog

[tbob]

More questions.  Floating is a relative term.  What do you want to use as a reference?  Earth ground, or the negative terminal?  Is the ground symbol connected to earth or to the negative terminal?  A lot of systems use the negative power supply terminal as a reference.  This is ground, and the circuit is not considered to be floating.  But if you were to put a negative terminal of a DMM to earth ground, you may read some potential difference at the power supply negative terminal with the positive lead of a DMM.  Does it matter?  Not unless this system is part of a total system with other power supplies that do not have all the negative terminals tied together.  I have a feeling you are spending a lot of time trying to get an answer to a question that has little or no reverence.  The circuit is floating with respect to earth ground, but if all your measurements are with reference to the power supply negative terminal, then the circuit is not floating because you have established a reference ground as the power supply negative terminal.  The key is this:  Do you have other power supplies where the negative terminals are not connected together?  Are you taking any measurements that are not referenced to the power supply negative terminal?  If the answer to both questions are no, then the circuit is not floating.  If any question is answered yes, then the circuit is floating.