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.
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.