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alpine2beach
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I feel as if the difference between grounding in a circuit and grounding in mains power is not very clear. So my questions are, what does a ground do in circuits and is it necessary?
mdjensen22 said:
Voltage is a differential quantity. To measure the voltage of a single point, a reference point must be selected to measure against. This common reference point is called "ground" and considered to have zero voltage. This signal ground may not actually be connected to a power ground. A system where the system ground is not actually connected to another circuit or to Earth (though there may still be AC coupling) is often referred to as a floating ground.
if I were to hook up the positive terminal of a battery to a ground (such as one used in AC mains), would there be a current?
What purpose does a circuit common/0V reference provide?
Studiot said:Suppose you have a microphone running off a 1.5 volt battery feeding a preamplifier, running off a 15 volt supply, driving a power amplifier running off a 60 volt supply.
You would designate (in this case) the negative terminal of each supply as the 'common' and call the common value zero.
This way you could measure voltage from common to any point in any of the parts and get a consistent set of readings.
This is using the common as a reference.
Sometimes we designate the high side of the supply as reference.
Studiot said:No I didn't mention ground and I said use the negative in this case.
An Earth is a body whose potential does not alter, regardless of the current flows into or out of it, within the design limits of the system.
As far as I am aware the term Earth is synonymous with ground - One is basically UK usage, the latter US.
Potential with respect to what?Studiot said:An Earth is a body whose potential does not alter, regardless of the current flows into or out of it, within the design limits of the system.
jim hardy said:In house wiring that bare wire is regarded as "Circuit Common" because it is power supply return.
alpine2beach said:I feel as if the difference between grounding in a circuit and grounding in mains power is not very clear. So my questions are, what does a ground do in circuits and is it necessary?
Evil Bunny said:Thanks Jim. Nice write up... One question:
Isn't the white neutral the power supply return?
In this case the transformer is the power supply and everything (120V anyway) is "returning" on the center-tapped neutral, correct?
EDIT: I just realized you're talking about the large bare wire between transformer and service entrance. Not to be confused with the bare (sometimes green) wire in the branch circuits of your house. My apologies...
I was going to be a wise guy and ask if the L1 and L2 leads of the transformer were 180 degrees out of phase with each other, but I decided against it ;-)
Not at all. It is a privilege to hear things from an experienced person like you.jim hardy said:Y'all must be tired of me by now, g'night...
well i just lost a half hour's typing due to that %#@^%&%(@U(*QRT!~~*** login gauntlet.
Potential with respect to what?
One of the connections has to go to ground.
Studiot said:In fact points K,J, L, A and B are all in the signal path and therefore subject to a varying potential with respect to other points in the circuit.
Well, G, C, D, E and F have same (constant / non-changing ) potential w.r.t themselves.Studiot said:Well G, C, D, E and F are all directly connected and in simple theory at the same potential so what if we called this zero reference and worked from there?
Studiot said:Reality, however, intervenes as soon as our circuit passes current because significant currents flow in the output circuit around TR1 and the loudspeaker, introducing a significant voltage difference between C and D which varies with time. A similar voltage appears between D and E due to TR2. Furthermore, power currents flowing in the rectifier/filter circuit introduce significant voltages between E and F.
When I say significant, they are not significant in relation to the output, but if any point between C and F are used as a reference for the input to A1 these voltages will be in series with the input and may be a substantial fraction (or even equal to) the signal voltage at this point.
Moving on to Fig 2 which shows two pieces of equipment, one mains powered, connected by signal leads.
If the return lead is not earthed the signal generator chassis may attain significant voltage (up to 100 volts) by electrostatic coupling with respect to the other equipment.
This effect is shown as the dashed capacitor .
The chassis of the second equipment is pulled up to this voltage by the return connection between the two pieces of equipment, labelled AB.
Now suppose we wish to make a circuit change to the circuit under test so we switch off its power supply and apply an earthed soldering iron.
If the chassis is not earthed, the charge flow to Earth from the induced voltage on the chassis can easily be enough to destroy semiconductor components.
Fig 3 shows the classic ground loop where two mains powered apparatus are earthed at their respective mains plugs which is also connected to the signal return between the equipment.
Shown dotted is the large area of a ground loop that picks up voltages (especially hum) from any magnetic fields that thread the loop.
This is all a good basis for discussion as to why we don’t want the Earth potential to vary and in relation to what this unwanted variation can arise. Part 2 will include safety Earth's.
jim hardy said:The car's power supply return (chassis) forms one plate of a capacitor, the Earth below it is the other plate, and you feel it when that capacitance discharges through you.
old jim
jim hardy said:If you are standing barefoot on moist ground and touch a point in a circuit that is earthed, you will not get a shock.
If instead you touch circuit common you will experience whatever voltage exists between circuit common and earth, which may be quite a shock..
If circuit common is earthed you will experience zero volts and will not get a shock.
You experience this in cold dry winter weather when exiting your automobile - that shock you get if your hand is stilll touching metal when your foot first hits the ground . The car's power supply return (chassis) forms one plate of a capacitor, the Earth below it is the other plate, and you feel it when that capacitance discharges through you.