Why is there no GND on my DC Powersupply?

nomurphy

Joined Aug 8, 2005
567
Lets consider a simple circuit: A battery connected to a bulb in a flashlight. The voltage across the battery is known, but the voltage between any terminal of the battery and ground is undefined. Therefore the whole circuit is floating with respect to ground. Most technical people would say that the circuit is isolated from ground.
No, not necessarily. The term "ground" is a matter of definition, and is usually the common point for all power within a circuit. However, there can be many "grounds" in a circuit, for instance: digital ground, analog ground, video ground, and let's not leave out chassis ground. Many times circuits are created without any thought or regard to earth ground.

I could easily draw your battery & light bulb circuit with a circuit ground. I would simply use a ground symbol on one side of the battery (probably negative, but in this case it wouldn't matter), run a wire connection between the battery and the bulb, and then use a ground symbol on the other side of the bulb. This circuit now has a common point that will be known as GROUND. Although it has nothing to do with earth ground, it is merely syntaxical and you really shouldn't get hung up on it. However, in this case of grounding the (-) side of the battery, you would now say that the bulb is running on +V relative to ground. Although this would not matter with a filament light bulb, it would matter greatly with an LED which has a specific polarity.

A power supply with a floating output will generate either a + or - voltage depending upon which polarity output is connected to the circuit ground. The LED would only light if proper polarities are observed during design & construction. One of the easiest ways of determining polarity, is to establish a circuit ground.

In most modern designs, circuit ground is one or more layers of solid copper within a multi-layer board. This "solid plane" ground technique creates a much lower impedance and contributes greatly to alleviating ground bounce and EMI issues, along with improving signal integrity.
 

chesart1

Joined Jan 23, 2006
269
No, not necessarily. The term "ground" is a matter of definition, and is usually the common point for all power within a circuit. However, there can be many "grounds" in a circuit, for instance: digital ground, analog ground, video ground, and let's not leave out chassis ground. Many times circuits are created without any thought or regard to earth ground.

I could easily draw your battery & light bulb circuit with a circuit ground. I would simply use a ground symbol on one side of the battery (probably negative, but in this case it wouldn't matter), run a wire connection between the battery and the bulb, and then use a ground symbol on the other side of the bulb. This circuit now has a common point that will be known as GROUND. Although it has nothing to do with earth ground, it is merely syntaxical and you really shouldn't get hung up on it. However, in this case of grounding the (-) side of the battery, you would now say that the bulb is running on +V relative to ground. Although this would not matter with a filament light bulb, it would matter greatly with an LED which has a specific polarity.

A power supply with a floating output will generate either a + or - voltage depending upon which polarity output is connected to the circuit ground. The LED would only light if proper polarities are observed during design & construction. One of the easiest ways of determining polarity, is to establish a circuit ground.

In most modern designs, circuit ground is one or more layers of solid copper within a multi-layer board. This "solid plane" ground technique creates a much lower impedance and contributes greatly to alleviating ground bounce and EMI issues, along with improving signal integrity.
If a line designated as ground is not connected to earth ground, then
that ground cannot be defined as zero volts. That ground has an undefined potential with respect to earth ground. A grounded chassis is connected to the ground prong of the power plug. When the equipment is plugged into an outlet, that chassis ground is connected to earth ground via the outlet wiring.

With regard to the ground terminal on a power supply, that ground terminal is connected to the minus terminal of the power supply if you want ground as a reference. If that ground terminal is not earth ground, than what is the purpose of having a ground terminal and a minus terminal? In the power supplies I have used, the ground terminal of the power supply was connected to the ground prong of the power plug.

At any rate, we are creating a very confusing scenario for anyone trying to understand the difference between common and ground.

When we say ground is at zero volts, we are implicitly defining that ground as earth ground.
 

jpitz31

Joined Oct 24, 2007
37
I have two power supplies on my bench. A single power supply with a (+), (-) and (Gnd) terminals. I took my volt meter and measured Red lead to Red Terminal, Black Lead to Gnd Terminal. The result was no voltage. Then took Red lead to (-) and Black lead to (Gnd). The result was also no voltage.

If I understand the last few posts then this indicates that my single power supply is isolated from ground.

I then took my dual power supply and measured the following:

Red lead to Red Terminal (+), Black lead to Black Terminal (Gnd) and measured 1.2 volts (+) voltage.

Red Lead to Yellow Terminal (-), Black lead to Black Terminal (Gnd) and measured 1.2 volts (-) voltage.

Red lead to Red Terminal (+) , Black lead to Yellow Terminal (-) and measured 2.4 volts (+) voltage.

This would then indicate that my dual power supply shares a circuit and earth ground when the (Gnd) terminal is used and is isolated when the (Gnd) terminal is not used.

Let see If I understand??

When I use the single power supply (Red) (Black) (Gnd) terminals I get the following conditions:

No earth ground, isolated Red and Black terminals, I get (+) or (-) voltage depending on which polarity is used as the common or circuit ground.

But If I earth ground the circuit I can only get a (+) voltage with reference to the ground.

I cannot get a (-) voltage in reference to ground when earth grounding the circuit from the single power supply.

On my dual supply I can get the following:

Using Red Terminal (+) and Black Terminal (Gnd) I can get isolated (+) voltage with a circuit ground.

Using Red Terminal (+) and Black Terminal (Gnd) (and earth grounding the circuit) I can get (+) voltage in reference to ground.

Using the Yellow Terminal (-) and Black Terminal (Gnd) I can get isolated (-) voltage with a circuit ground.

Using the Yellow Terminal (-) and Black Terminal (Gnd) (and earth grounding the circuit) I can get (-) voltage in reference to ground.

Using the Red Terminal (+) and Yellow Terminal (-) I can get either isolated double (+ or -) voltage if I do not earth ground the circuit and double (+ or -) voltage in reference to ground if using the ground terminal. (depending on which terminal is used as circuit ground)

This has been a great thread. I think I am learning a ton of info.

Thanks

Joe
 

bloguetronica

Joined Apr 27, 2007
1,424
I have two power supplies on my bench. A single power supply with a (+), (-) and (Gnd) terminals. I took my volt meter and measured Red lead to Red Terminal, Black Lead to Gnd Terminal. The result was no voltage. Then took Red lead to (-) and Black lead to (Gnd). The result was also no voltage.

If I understand the last few posts then this indicates that my single power supply is isolated from ground.

I then took my dual power supply and measured the following:

Red lead to Red Terminal (+), Black lead to Black Terminal (Gnd) and measured 1.2 volts (+) voltage.

Red Lead to Yellow Terminal (-), Black lead to Black Terminal (Gnd) and measured 1.2 volts (-) voltage.

Red lead to Red Terminal (+) , Black lead to Yellow Terminal (-) and measured 2.4 volts (+) voltage.

This would then indicate that my dual power supply shares a circuit and earth ground when the (Gnd) terminal is used and is isolated when the (Gnd) terminal is not used.

Let see If I understand??

When I use the single power supply (Red) (Black) (Gnd) terminals I get the following conditions:

No earth ground, isolated Red and Black terminals, I get (+) or (-) voltage depending on which polarity is used as the common or circuit ground.

But If I earth ground the circuit I can only get a (+) voltage with reference to the ground.

I cannot get a (-) voltage in reference to ground when earth grounding the circuit from the single power supply.

On my dual supply I can get the following:

Using Red Terminal (+) and Black Terminal (Gnd) I can get isolated (+) voltage with a circuit ground.

Using Red Terminal (+) and Black Terminal (Gnd) (and earth grounding the circuit) I can get (+) voltage in reference to ground.

Using the Yellow Terminal (-) and Black Terminal (Gnd) I can get isolated (-) voltage with a circuit ground.

Using the Yellow Terminal (-) and Black Terminal (Gnd) (and earth grounding the circuit) I can get (-) voltage in reference to ground.

Using the Red Terminal (+) and Yellow Terminal (-) I can get either isolated double (+ or -) voltage if I do not earth ground the circuit and double (+ or -) voltage in reference to ground if using the ground terminal. (depending on which terminal is used as circuit ground)

This has been a great thread. I think I am learning a ton of info.

Thanks

Joe
This would show that your PSU has a common ground shared by the red and yellow terminals, but tells nothing about isolation. If your PSU has an earthed plug, then measure the resistance between the ground ant the earth. If it is infinite, the PSU ground is isolated. If it is close to zero, it is not isolated.
 

nomurphy

Joined Aug 8, 2005
567
When we say ground is at zero volts, we are implicitly defining that ground as earth ground.
No, absolutely not. Again, ground is a matter of syntax, and where the designer decides is the common or ground point. By definition, ground is the reference or 0V point, it is NOT necessarily earth ground, which is why I used the term circuit ground.

If that ground terminal is not earth ground, than what is the purpose of having a ground terminal and a minus terminal?
Indeed, if it is as you say, then isn't the negative (black) post redundant?
No, because the PS potential exists between the (+) RED and (-) BLACK, the green or earth ground, is provided for safety purposes. It can be used to connect your circuit ground to earth ground, but it is not necessary. Again, circuit ground is 0V by definition -- it may very well be at a different potential than earth ground (not safe), but it's quite possible that earth ground has nothing to do with circuit ground (such as with your light bulb example).

One of the biggest lessons electronic/electrical engineers learn *eventually* is that ground, is not ground, is NOT ground. Impedance/resistance even exists within earth ground, so there is NO true point of 0V. Again, that is what makes ground planes within circuit boards desirable (BTW -- nothing to do with earth ground), the impedance is much lower than that of routed signal ground and performance is signficantly improved (there are numerous other reasons ground planes are preferred, but I won't go into that here).
 

chesart1

Joined Jan 23, 2006
269
No, absolutely not. Again, ground is a matter of syntax, and where the designer decides is the common or ground point. By definition, ground is the reference or 0V point, it is NOT necessarily earth ground, which is why I used the term circuit ground.

Indeed, if it is as you say, then isn't the negative (black) post redundant?
No, because the PS potential exists between the (+) RED and (-) BLACK, the green or earth ground, is provided for safety purposes. It can be used to connect your circuit ground to earth ground, but it is not necessary. Again, circuit ground is 0V by definition -- it may very well be at a different potential than earth ground (not safe), but it's quite possible that earth ground has nothing to do with circuit ground (such as with your light bulb example).

One of the biggest lessons electronic/electrical engineers learn *eventually* is that ground, is not ground, is NOT ground. Impedance/resistance even exists within earth ground, so there is NO true point of 0V. Again, that is what makes ground planes within circuit boards desirable (BTW -- nothing to do with earth ground), the impedance is much lower than that of routed signal ground and performance is signficantly improved (there are numerous other reasons ground planes are preferred, but I won't go into that here).
Thanks for the explanation.

I do not have much experience in circuit design. Although I have a BSEE, my job experience has been mostly assembly language programming [4040, 8080] and the PLM language.

Can you recommend any books that discuss grounding techniques and the problems associated with bad grounding techniques such as ground loops?
 
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