Grounding is one of the most difficult, and in some ways, one of the worst understood issues in electronics.
It's not often fully explained in a single text, rather people are left to put the picture together for themselves from a number of sources.
People look at the difficulties of understanding what's going on and they either try to ignore it, or they shut down completely. It's not difficult though, it makes use of something we use easily in some situations, the idea of relativity, but which we sometimes struggle with in the abstract.
A simple example of relativity is hot and cold.
Is boiling water hot?
Compared to what?
Compared to red hot iron, it's cold. Compared to water ice, it's hot. It's all relative.
Every circuit has a point which can be labelled ground. It's important to understand though that this can be more for the convenience of people thinking about the circuit rather than for any special physical attributes of the circuit at that point. Old cars used to have the positive battery terminal nominated as ground. New cars have the negative battery terminal nominated as ground. In neither case was there an explicit connection to ground-the-earth.
Since ultimately electronics includes radio it's difficult to deal with the idea of ground without dealing with some of the phenomena associated with radio. Since radio can pass through a vacuum, we may not necessarily be dealing with points that are connected together by conductors.
Ground, in radio terms, means the planet Earth, and ground in any circuit is sometimes called earth. The Earth is a big conductor, the biggest one local to us, and when we are in contact with it, which we usually are, we are at the same voltage as the Earth. We call this voltage 0 volts. We could call it 100 volts, but what would be the sense of that? It would make no difference in practice, but it would make calculations more difficult.
Similarly, in every circuit, there is a point where it makes sense to call it 0 volts, or ground. If we have a circuit with a single power rail, usually the negative terminal of the power supply is called 0 volts, because then all the voltages in the circuit are positive, and we prefer to do positive arithmetic than negative arithmetic.
Very often the point called ground in a circuit is physically connected to the Earth, through a conductor that makes a physical connection to the planet.
Very often, however, the point called ground in a circuit is NOT connected to the Earth (the planet) by any explicit wired connection. Does this mean that it is at 0 volts compared to the Earth? NO. It just means that it's the point treated as 0 volts for the purpose of thinking about the circuit and for doing calculations.
Does whether circuit ground is connected to Earth-the-planet make any difference to the way the circuit operates?
Sometimes YES, and sometimes NO.
The most obvious difference is: in general terms, if it's connected to Earth (and you're connected to Earth, which you usually are), you can't get a shock from it.
Audio circuits often have 2 wires, called signal and ground. This is just to let you know which output wire goes to which input wire on the next piece of equipment, if it makes any difference, which sometimes it doesn't.
Many circuits, even complex radio circuits, will operate perfectly satisfactorily without an explicit connection to ground. A car radio is an obvious example of a device that will work without a ground connection.
Now let's think about how you can get a shock from a device without an obvious ground connection.
It might be thought that no complete circuit could be formed without there being an explicit connection to ground, but this is not the case.
Almost all mains power supplies are connected to ground at some point. Almost all power is supplied to a local substation as 3-phase AC. 3-phase AC is a power distribution system which works with a reduced usage of wire (3 wires + 1 ground). It can do this because the currents in all the phases cancel. If the distribution demand is unbalanced then the unbalanced current may be returned via a light gauge wire, or through the Earth. At the substation the 3-phase is broken down into 3 pairs of (so-called) LIVE and NEUTRAL. Neutral, because they all join together and should be at 0 volts with respect to ground (when the distribution between phases is equal).
Even when a point in a circuit is isolated from the mains (as in the case of an isolating transformer), the voltage may be enough to cause sufficient leakage current to flow to be painful or dangerous.
Currents which flow through your shoes or through the dirt and water on your shoes are called leakage currents. These currents can be large enough to kill. They flow virtually everywhere, even on the surfaces of insulators.
Capacitors have a high resistance at DC, but are transparent to AC, to a degree depending on the frequency and the capacitance. Most objects on the Earth's surface have a capacitance to ground, through which AC current can flow.
Grounding schemes are important in electronics and particularly in audio, because currents flow in them.
Because the ground network is a network of conductors, which have a small, but not insignificant resistance, voltages appear between separated points in the grounding scheme. These voltages can cause unwanted noise or hum at the output. So we have to take care how the grounding is organised to keep the noise voltages away from points in the circuit where they could interfere with the signal and appear at the output. One straightforward scheme for doing this is called star grounding, where every single ground connection comes back to a single point (usually the negative output of a single rail PSU or the ground output of a dual-rail PSU) on an individual wire. A scheme often adopted in tube amplifiers is called ground follows signal.
I think that's enough for one post.
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