Electronic – ground and what does it do

groundvoltage

I'm a bit confused about the concept of ground, and perhaps voltage as well, particularly when trying to analyze a circuit. When I learned about Ohm's law in grade school, I learned how to apply the law to calculate current, voltage, and resistance of simple circuits.

For instance, if we were given the following circuit:

enter image description here

We would be could be asked to calculate the current passing through the circuit. At the time, I'd simply compute (based on the rules given) 1.5V/1Ohms=1.5A.

Later on, however, I learned that the reason the voltage of the resistor would be 1.5V is because voltage is really the difference in potential between two points, and that the difference of the voltage across the battery would be the same as that of the resistor (correct me if I'm mistaken), or 1.5V. I got confused, however, after the introduction of the concept of ground.

The first time I tried to do the current calculation for a circuit similar to the previous circuit on a simulator, the program complained about not having a ground and "floating voltage sources". After a bit of searching, I learned that circuits need ground as a reference point or for safety reasons. It was mentioned in one explanation that one can pick any node for ground, although it's customary to design circuits so there is a "easy place" to pick ground.

Thus for this circuit

enter image description here

I picked ground at the bottom, but would it be okay to pick ground between the 7 ohm and 2 ohm resistor – or any other place? And what would be the difference when analyzing the circuit?

I've read that there are 3 typical ground symbols with different meanings – chassis ground, earth ground, and signal ground. A lot of circuits I've seen used in exercises either use earth ground or signal ground. What purpose is there in using earth ground? What is the signal ground connected to?

Another question: since the ground is at unknown potential, wouldn't there be current flowing to or from ground to the circuit? From what I've read we treat the ground as 0V, but wouldn't there be some sort of effect because of a difference in potential of the circuit and ground? Would the effect be different depending on what ground was used?

Finally: In nodal analysis, one customarily picks a ground at the negative terminal of the battery. However, when there are multiple voltage sources, some of them are "floating". What meaning does the voltage of a floating voltage source have?

Best Answer

The first time I tried to do the current calculation for a circuit similar to the previous circuit on a simulator, the program complained about not having a ground and "floating voltage sources".

Your simulator wants to be able to do its calculations and report out the voltages of each node relative to some reference, rather than have to report the difference between every possible pair of nodes. It needs you to tell it which node is the reference node.

Other than that, for a well-designed circuit, the "ground" has no significance in the simulation. If you design a circuit where there is no dc path between two nodes, though, the circuit will be unsolvable. Typical SPICE-like simulators resolve this by connecting extra resistors, typically 1 GOhm, between every node and ground, so it is conceivable that the choice of ground node could artificially affect the results of a simulation of a very high-impedance circuit.

I picked ground at the bottom, but would it be okay to pick ground between the 7 ohm and 2 ohm resistor - or any other place? And what would be the difference when analyzing the circuit?

You can pick any node as your reference ground. Often we think ahead and pick a node that will eliminate terms for the equations (by setting them equal to 0), or simplify the schematic (by allowing us to indicate connections through a ground symbol instead of by a bunch of lines connecting together).

I've read that there are 3 typical ground symbols with different meanings - chassis ground, earth ground, and signal ground. A lot of circuits I've seen used in exercises use earth ground or signal ground. What purpose is there in using earth ground? What is the signal ground connected to?

Earth ground is used to indicate a connection to something that is physically connected to the ground beneath our feet. A wire leading through the building down to a copper rod driven into the ground, in a typical case. This ground is used for safety purposes. We assume that someone who handles our equipment will be connected to something like earth ground by their feet. So earth ground is the safest circuit node for them to touch, because it won't drive currents through their body.

Chassis ground is just the potential of the case or enclosure of your circuit. For safety purposes it's often best for this to be connected to earth ground. But calling it "chassis" instead of "earth" means you haven't assumed that it is connected.

Signal ground is often distinguished from earth ground (and partially isolated from it) to minimize the possibility that currents flowing through the earth ground wires will disturb measurements of the important signals.

Another question: since the ground is at unknown potential, wouldn't there be current flowing to or from ground to the circuit?

Remember, a complete circuit is required for current to flow. You would need connections to earth ground in two places for current to flow in and out of your circuit from earth ground. Realistically, you'd also need some kind of voltage source (a battery, or an antenna, or something) in one of those connection paths to have any sustained flow back and forth between your circuit and the earth.

However, when there are multiple voltage sources, some of them are "floating". What meaning does the voltage of a floating voltage source have?

If I have voltage source with value V between nodes a and b, it means that the voltage difference between a and b will be V volts. A perfect voltage source will generate whatever current is required to make this happen. If one of the nodes happens to be ground, that gives you immediately the value at the other node in your reference system. If neither of those nodes happens to be "ground" then you will need some other connections to establish the value of the voltages at a and b relative to ground.