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Need somebody to talk to about AC concept
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hamster_nz:
Also looking at the different power grid designs make it apparent why GND is used and how neutral differs from phase.

Mains power is distributed as three live phases, and neutral is the central connection on a three phase transformer, it is usually connected to a metal rods bashed into the earth at various points along the way.

Most of the power travels in the three phases, and only the imbalance between phases travels though the actual earth (i.e. rock and soil).

Because of this wirimg scheme you shouldn't be able to get a shock from a neutral wire, because it should be at something close to the potential of most things around you, but I wouldn't suggest you bet your life on it.
Zero999:

--- Quote from: tester43 on June 27, 2018, 11:13:56 am ---i did this project in the past in a very similar way to what you are showing, maybe some details were different like 7805 instead of lm317, maybe logic level at 3,3 not 5V etc. But I started to freak out with AC GND concept. Could not wrap my brain around the paradigm "there is nowhere in ac a point with CONSTANT potential that I could make my reference". In such case i did not know what is the meaning of GND symbol on the diagrams.

edit: and it's obvious that I used an optoisolator :) I bought them several years ago and they must be used ;D

--- End quote ---
No need to freak out, perhaps you should focus more on your understanding, rather than rebuilding the same project again, in a different manner?

Here's another analogy. 0V in a circuit is a reference point, from where everything else is measured. A similar concept is mean sea level on earth. Mountains have positive altitudes and craters have negative altitudes. Now imagine the waves on the sea, bouncing up and down, above any below mean sea level. If you float a buoy on a rough stretch of ocean, at mean sea level, you'll see its altitude continuously changes, oscillating above and below mean sea level i.e. going between positive and negative. The average (DC) altitude of the buoy is zero, yet the AC component is the height of the waves, which is analogous to the peak to peak voltage.

And save those opto-isolators for something else: there's no need to waste them on a project, which doesn't really need them
C:

Many have stated that ground is a reference point.

You are missing a fine detail    POINT !!!

You like to add a reference point to a DC battery.
You have added two wires to that battery.

Think of the two wires as being very long.
1000 feet or meters, 10,000 feet or meters.

Now in the physical world that reference point at battery is a long distance away.
The non-battery end of the two wires is not the same as at the battery end.
Your reference point does not exist at the non-battery end.
Simple to see when you remember that wire has resistance.

Now think of your two wires, The wires have a protective coating to prevent shorts, the insulation.

Insulation still has some resistance.
You can have a length of insulated wire pair that is so long that you can no longer read the battery voltage with a meter at the non-battery end.

The insulation resistance is the load on the battery.

When you connect a resistor across the end of that very long wire pair, your circuit is actually three resistors in series.
Two resistors here is the resistance of each wire.

Now think of making that pair of wires shorter.
That resistance of the wire just gets smaller, it never goes away.

Now think of your meter.
To measure the voltage drop of that long wire would take very long leads to the meter if that pair of wires was strait. The leads have resistance and would mess up the meter reading.
You could make a loop and get the two ends of the pair of wires a short distance apart. Then it would be easy to measure the voltage drop of the wires.

So keep in mind that a  a reference point is a POINT.
If you add any distance, you are not at that POINT.

====
Now think of something simple as why people want to add a ground to a circuit.
If the positive half of AC cycle does not match the Negative half of AC cycle then you have the equal of a PWM signal.
The circuit can and will create a DC offset. You can find this on capacitors in the circuit. You may not be able to measure this due to the resistance drain being greater the what is producing the offset.

Even a simple serial port connection will try to build this DC offset. A async serial connection that uses even parity is keeping the DC offset small by making data positive & data negative match over time.
The faster com links like HDMI & SATA have a bit just for DC balance.

Now think of what a capacitor is. Two conductors separated by an insulator.
The DC offset is created on the connecting wires.

=====

Go back to basics on AC.
One source for AC is an Alternator.
https://en.wikipedia.org/wiki/Alternator

Most of the worlds power comes from Alternators.

In simple terms an Alternator is a transformer with a rotary magnetic field.



C



tester43:
Hi again everybody :)
So it seems the analogy with waves on the water is good.
Average level of waves would be "selected" as reference point.

btw. as "point" I meant potential value - not physical "point" like a place on the copper.

So... imagine vast space of water with waves that are regular etc. You would like to measure the height of the waves below and above selected reference point/height.
In DC you would be standing on the large solid, not moving, made of concrete with iron bars inside ;) platform.
To make the discussion more visual: here you can find a gibraltar rock ;) https://en.wikipedia.org/wiki/Rock_of_Gibraltar#Sayings
Soooo... it's solid.

You would take your measuring device and could say "water level is 27 measurement units above me" or " from my platform I have water level 4 units below the platform".

But in AC you are sitting in the boat. Water is constantly kicking you up and down.There is no possibility to be aware is you are at the time at height 2, 6, -2 or other. That was my confusion. At least I know I can detect zero EMF (zero volts).
So to overcome this situation I used Bridge rectifier to have my solid reference point - to get of the boat, stand of the rock and measure how waves are coming and change its height with time.
C:
If you want to compare electricity to Water then

Voltage = pressure of the water.
Current = the flow rate of water
Resistance = size of pipe.
Wire = pipe
Capacitor = water tower where you store a mass of water
Inductance = mass of water flow resisting change in flow rate.

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