Difference between COM and GND in power supply

Thread Starter

bullthistle

Joined Mar 2, 2019
4
I have built a signal amplification unit on a protoboard so that I can read the signal coming from each of 4 pressure transducers (see pic). The plan is to move everything to PCB boards as soon as possible. I am trying to understand how the COM port on my power supply functions, with the goal of eventually building my own separate 15V power supplies for each unit (to eliminate signal bleed between pressure transducers). Any insight is appreciated. Background: I initially had the system connected to a 2-port power supply (see pic), but the unit would not function correctly without grounding the OpAmps and pressure transducers. I also tried grounding everything to earth and that also did not work. Eventually, I switched to a 3-port power supply and grounded everything to the COM terminal, causing everything to work beautifully aside from a little signal bleed across the transducers. I'm still a little fuzzy on why the common terminal works for grounding my unit, but the ground terminal does not work. I have not yet been able to locate a clear schematic showing where the COM port attaches to inside the power supply. Thanks in advance for any comments or suggestions!
IMG_0502[1].JPG IMG_0501[1].JPG IMG_0500[1].JPG
 

dl324

Joined Mar 30, 2015
8,848
Welcome to AAC!

The COM on the Agilent supply should be connected to circuit GND. You're using the dual supply output and the '-' terminal is the negative voltage and "+" is the positive voltage. Some dual power supplies (yours is a triple) allow the dual supply to track or be independent.

The "-" on the QJE supply should be connected to circuit GND.
 

Thread Starter

bullthistle

Joined Mar 2, 2019
4
Thanks for the response! Yes, connecting circuit GND to COM is working for my system at this point, but I'm not sure 1) where the COM port connects to inside the power supply or 2) why it doesn't work for me to connect my circuit ground to the GND port.

Currently, my circuit is carrying a common ground for 4 different pressure transducers and some of the signal is bleeding between transducers as a result. (i.e. I can completely disconnect any one transducer and I will still get a small signal reading that mimics the general shape of the voltage waveform of the other transducers that are still connected, only with a much smaller amplitude.) I'm hoping to deal with this issue by constructing 4 separate power supplies, each with a +15V, -15V, and COM in order to power the transducers separately. However, in order to do this, I need to understand how the COM is different than an earth ground i.e. how I should correctly construct a common port.
 

WBahn

Joined Mar 31, 2012
24,670
Thanks for the response! Yes, connecting circuit GND to COM is working for my system at this point, but I'm not sure 1) where the COM port connects to inside the power supply or 2) why it doesn't work for me to connect my circuit ground to the GND port.

Currently, my circuit is carrying a common ground for 4 different pressure transducers and some of the signal is bleeding between transducers as a result. (i.e. I can completely disconnect any one transducer and I will still get a small signal reading that mimics the general shape of the voltage waveform of the other transducers that are still connected, only with a much smaller amplitude.) I'm hoping to deal with this issue by constructing 4 separate power supplies, each with a +15V, -15V, and COM in order to power the transducers separately. However, in order to do this, I need to understand how the COM is different than an earth ground i.e. how I should correctly construct a common port.
Imagine using batteries instead of a complicated commercial power supply.

You have two 15 V batteries. Each has a + and a - terminal. You connect the + terminal of one battery to the - terminal of the other battery and you call that point COM.

You also have a stake driven into the ground and you call that point GND.

You now want to power your circuit from ±15 V supplies. Which point are you going to connect the reference point of your circuit to? Would you expect it to work if you connected it to the other?
 

dl324

Joined Mar 30, 2015
8,848
1) where the COM port connects to inside the power supply
You can determine that by measuring the voltage from COM to the + output for the 6V supply. Or measuring from the 6V - to the + or - on the dual supply.

It occurs to me that the labeling might confuse a newbie. The - terminals on the 6V and +/- supplies would rarely be connected. The former represents the negative terminal (often connected to circuit GND), while the latter represents the negative supply of the dual supply.
2) why it doesn't work for me to connect my circuit ground to the GND port.
You seem to be confusing circuit GND with earth ground. The green terminal on the Agilent supply is earth ground. Earth ground and circuit GND aren't always connected.
 

MrChips

Joined Oct 2, 2009
19,269
COM is the reference point one chooses to which a difference in voltage is measured.

GND is the 0V reference point, usually galvanically connected to earth.
 

BobTPH

Joined Jun 5, 2013
2,016
The green GND terminal connects through the power cord to ground. It is not internally connected to any of the other terminals. If you want your device to have a common ground with with the electrical system of your house, you can connect to the green GND terminal to the COM terminal. It can also be connected to the -6V, giving you a +6V supply or even to the +6V, giving you a -6V supply.

Bob
 

MaxHeadRoom

Joined Jul 18, 2013
18,995
GND is the 0V reference point, usually galvanically connected to earth.
That is where the confusion comes in, most use GND to denote a circuit common regardless, this does not indicate in any way if it is earth grounded or not.
It would be much easier and less confusing to use COM as power circuit common and reserve GND to anything Earthed or Earth Potential. o_O
Another that perpetuates it, is the continuous use of the earth GND symbol for circuit common.
Max..
 

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Thread Starter

bullthistle

Joined Mar 2, 2019
4
Imagine using batteries instead of a complicated commercial power supply.

You have two 15 V batteries. Each has a + and a - terminal. You connect the + terminal of one battery to the - terminal of the other battery and you call that point COM.

You also have a stake driven into the ground and you call that point GND.

You now want to power your circuit from ±15 V supplies. Which point are you going to connect the reference point of your circuit to? Would you expect it to work if you connected it to the other?
Wow. That explanation makes it incredibly simply. Thanks!
 

Thread Starter

bullthistle

Joined Mar 2, 2019
4
That is where the confusion comes in, most use GND to denote a circuit common regardless, this does not indicate in any way if it is earth grounded or not.
It would be much easier and less confusing to use COM as power circuit common and reserve GND to anything Earthed or Earth Potential. o_O
Another that perpetuates it, is the continuous use of the earth GND symbol for circuit common.
Max..
True. I think a big source of confusion for me is that I am used to using the terms 'common' and 'ground' interchangeably.
 

WBahn

Joined Mar 31, 2012
24,670
True. I think a big source of confusion for me is that I am used to using the terms 'common' and 'ground' interchangeably.
Most people are, because in many of the initial applications we become familiar with, even just as lay people, they ARE interchangeable from a practical standpoint because they are physically connected together. Then, in many common situations where they aren't (think car electrical systems) they are not the same but we are only working with one and not both, so it doesn't matter which name we call it by because we don't need to distinguish between the two, so the norm is to talk about the negative side of the battery being "grounded", when it is really being "chassied". Then we start working with electronics and, especially at the hobbiest level and especially since most of us started with battery powered circuits, we continue to use the term "ground" to means the "common reference point of the circuit".

Even people that understand the distinction misuse the terms in the interest of facilitating communication by using the terms that most people are going to be familiar with unless the distinction is important to the present conversation. That fact that this is true in the textbooks that teach this stuff only embeds it that much more strongly. But even if you could somehow magically change all of the textbooks to be correct and even change the brains of everyone alive today to get them to use them correctly, I suspect that it wouldn't take but a few generations before the sloppiness would creep right back in because, for most people in most situations that they deal with, they ARE interchangeable in practice.
 

MaxHeadRoom

Joined Jul 18, 2013
18,995
The sad part is that publications such as 'The Art of Electronics' perpetuate the myth associated with the proper usage of the correct published symbols, they use the earth GND symbol throughout the book.
And do not even show a chart of electrical symbols, at least the copy I once owned.
Max.
 
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