# What is meant by "common ground" for a DC power source?

#### Loki_Liesmith

Joined Oct 16, 2017
5
Please excuse the freshman question, but I am confused about the concept of "common ground". I am used to building circuits that are powered by a battery or a two-lead power supply, with a + and a - terminal. The most basic such circuit is in panel A of the attached picture. Conventional current goes from the + terminal through the resistor, through the LED and into the - terminal. I've also seen the same circuit represented schematically as in panel B: conventional current goes from a positive voltage source, through the resistor, through the LED and into "ground". So, I always thought that the negative terminal of a power source and the ground were one and the same.

But now, I have a project where I want to operate a mass flow controller. The instructions for the mass flow controller say: "the power supply used must be bipolar,... +/-15 VDC. These voltages must be referenced to a common ground. One of the "common" pins of the instrument must be connected to the common terminal of the power supply. " This confuses me, as it implies a third terminal on the DC supply, in addition to the + and the -. Would it look like the scheme in figure C - a wire that is connected to both the + and the - terminals of the supply and the other end touches something metal that touches the earth? Even now, looking at my own hastily drawn panel C, I realize that what I drew is a short circuit across the battery. But I can't picture what it is supposed to look like. Can someone please redraw the schematic in a way that shows how "common ground" works and explain to me why it's needed? Thank you.

#### MrChips

Joined Oct 2, 2009
23,505
GND or COMMON refers to any node in your circuit that you wish to be used as a 0V reference point.

Generally speaking, when we say GND, COMMON GND, or EARTH GND, we mean 0V reference point that is at the same potential as mother earth.

A +/- 15VDC power supply is actually two power supplies, each supplying 15VDC.
The +15VDC supply has its negative terminal connected to COMMON (GND).
The -15VDC supply has its positive terminal connected to COMMON (GND) .

Here is a schematic (simply replace the 4.5V with 15V).

This dual power supply configuration is required so that the output of the amplifier can swing to both positive and negative voltages. This is very common when working with signals that are meant to swing in both directions.

You can do this with two batteries or with two power supplies as shown in the schematic above.

If you are using a laboratory bench supply that has three terminals labeled + GND -, this is not a dual power supply unless it says so on the front panel. Make sure the power supply explicitly states it is a dual power supply.

This not a dual power supply:

This is a dual power supply:

This is also a dual power supply:

#### Loki_Liesmith

Joined Oct 16, 2017
5
Thank you. It makes a lot more sense to me now.
A +/- 15VDC power supply is actually two power supplies, each supplying 15VDC.
The +15VDC supply has its negative terminal connected to COMMON (GND).
The -15VDC supply has its positive terminal connected to COMMON (GND) .

Here is a schematic (simply replace the 4.5V with 15V).

This dual power supply configuration is required so that the output of the amplifier can swing to both positive and negative voltages. This is very common when working with signals that are meant to swing in both directions.

Joined Jul 18, 2013
23,084
Common Ground.
Max.

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#### MisterBill2

Joined Jan 23, 2018
8,711
I predict either a lot more questions with a mass flow-meter installation or else a serious disaster, because they are both expensive and demanding in how they are powered and otherwise treated, and often times quite non-forgiving of any actual abuse. So I suggest that Loki_Liesmith, the Thread Starter, obtain as much application circuit information from the mass flow-meter maker as they can provide.