Need somebody to talk to about AC concept

[CatalinaWOW]

At the risk of damaging your growing understanding I want to point out a coupe of things.

1.  Even though the secondary circuit of the transformer is a single piece of wire, there is EMF along that wire.  Each turn of the transformer has induced current in changing magnetic field and thus voltage.  The turns are series connected so voltage increases as you go farther from your reference point.  This voltage across a conductor is the essence of the difference between DC and AC circuits.  The time varying fields and currents do things that don't happen in the static case.

2.  You might want to experiment with a resistor across the secondary of your transformer.  How does the AC voltage vary with resistance.  This is actually a dangerous experiment (not physically dangerous, but dangerous to a fragile understanding), because over a range of resistance values there is a simple relationship, but if you go outside of that range more complicated explanations of what is going on are required.

3.  One way to help your understanding of the artificial nature of ground is to take the secondary of your transformer.  Connect one lead to the negative terminal of a nine volt battery.  If you connect a DMM across that battery it will read nine volts, just as you expect.  If you use that DMM to measure the DC volts across the the secondary you will get zero volts.  But if you measure AC volts across the battery you will get nothing, and if you measure AC volts across the secondary you will get an appropriate value.  If you look at the voltage with an oscilloscope you will see the other end of the coil going up and down with respect to the negative lead of the battery.

Another dangerous experiment - hook the battery up across the secondary.  This one can be physically dangerous also so be sure to think about it before you do it, and take appropriate precautions in case your understanding is not sufficient to protect you.  It will take a fair understanding of both the transformer and the battery to predict what will happen in this case.

[tester43]

Very interesting. Thank you. so....

ad1. it's related to transformer ratio - each next cycle of wire over the core gives linear increase of voltage (without load)
ad2. I would expect to see voltage drop due to limits of transformer - Without load I have Peak of 17V - with normal load it would get lowered to typical 12.
ad3. I know what would be the multimeter result - need to read how MM are measuring the actual AC
ad4. this one I can't imagine for the moment - will need to do the actual test

[tester43]

let me elaborate pt 3:

Connect one lead to the negative terminal of a nine volt battery.  If you connect a DMM across that battery it will read nine volts, just as you expect.

Because it's potential difference between two points.

 

If you use that DMM to measure the DC volts across the the secondary you will get zero volts.

Because there is no DC bias in output of transformer.

 

But if you measure AC volts across the battery you will get nothing,

Because battery is not the part of AC circuit, it's not "in the loop".

and if you measure AC volts across the secondary you will get an appropriate value.

ok

If you look at the voltage with an oscilloscope you will see the other end of the coil going up and down with respect to the negative lead of the battery.

I think it's not relative to the battery - or did you mean one probe on battery + and second on transformer output?

[tester43]

For battery (DC) coil is just w wire.
At the first moment it will generate single spike of change in magnetic field.
Then it will consume as much current as possible from battery considering wire gauge, because resistance of this wire for DC is small or near zero.
Basically like a heater for the vacuum tube.

But.... did you mean to connect the battery with primary connected to mains?

[tpowell1830]

Spoiler alert....

Ground is not ground or negative side of DC.

In most cars, the negative side of the battery is connected to the chassis and is often referred to as 'ground'. In the 1950s, there were 2 cars that I knew of that had the positive side of the battery attached to the chassis 'ground'.   

[rstofer]

Spoiler alert....

Ground is not ground or negative side of DC.

In most cars, the negative side of the battery is connected to the chassis and is often referred to as 'ground'. In the 1950s, there were 2 cars that I knew of that had the positive side of the battery attached to the chassis 'ground'.   

And many cars had 6V electrical systems...

[tpowell1830]

Spoiler alert....

Ground is not ground or negative side of DC.

In most cars, the negative side of the battery is connected to the chassis and is often referred to as 'ground'. In the 1950s, there were 2 cars that I knew of that had the positive side of the battery attached to the chassis 'ground'.   

And many cars had 6V electrical systems...

... that were charged with generators (as opposed to alternators(alternators are AC, so segway back to the topic...))

[vk6zgo]

In DC it's plan and simple. Just pick a point and go on. For example japanese devices in the past had GND at the level of a few volts above zero - to make issues analysis easier.

Nope!---there is no "zero point".
For the purposes of measuring voltages, "GND" can be as good,a point to measure from as any.
Some older circuits used something called "back bias", which may be what you are thinking of.

In the older times with first transistors GND was at the peak voltage - to make it more easy to calculate transistor point.

Nope again, early transistors were mostly PNP,  which required a negative supply to the collector, with the emitter to the positive side of the supply.
As in most circuit configurations, the collector is connected to its supply via the load, & the emitter normally returned to "common", it made sense to have the collector connected to the negative side, & the emitter from the positive side of the supply.
It also made the schematic layout similar to that used in the tube circuits people were familiar with.

BUT with AC: you have 2 wires coming from transformer - what now

OK, let's forget AC for a second.
Grab a 9 volt battery & connect two nice long wires to it.
I will call the one connected to the positive terminal the "positive wire" & that from the negative terminal,the "negative wire".

Hang a DMM across the two wire ends on the volts range with the red probe lead on the positive wire & the black one to the negative wire.

You should see "+9 volts" near as dammit.


Swap the DMM leads over----- you should now see  "-9 volts".

Find two metal objects in your house which are likely to be connected to "Earth" (as in the real "dirt").
In most older homes, they will probably be water taps ( faucets to those who talk American).

Leaving the other battery wire disconnected, measure between each side of the battery & one of the taps.
You should see "zero volts"both ways around.

Now connect the negative wire to the tap.

If you now measure between that,  or the, other tap   & the positive wire, you will see "+9 volts".
Now remove the negative wire from the tap, replacing it with the positive wire.
You should now see "-9 volts" between the tap & the negative wire.

What have we determined?

(1)There is no measurable voltage present between earth (ground) & either battery terminal if neither is connected to earth.
(2) If you connect one side of the battery to earth, you can see a voltage from the other terminal to the earthed device.
(3)Not so immediately obvious, but it shows you can use the earth/ground as a current carrying path.

At this point, you will say "Ahh! You cheated!--- the taps will be connected together anyway!
If you are that hard to convince, make one of the "earthed" objects a large appliance, like a stove, fridge, or whatever.
Here, the path is via the taps to the "dirt", through the dirt to the earth stake, & through the "protective earth" system to the large appliance.

Or, if you are really hardheaded, go out in the backyard, drive two earth stakes  into the ground, & redo the test.

[agehall]

OMG moment 0: Whole AC circuit is single wire with the same EMF along its whole distance ends (when I read my words now seems obvious, because of Kirchoff) - THERE IS NO SINGLE CONSTANT REFERENCE POINT OF ZERO POTENTIAL (or any other CONSTANT value)

Well, that isn't exactly true - when you define a ground point, that will always be a constant reference point. This has nothing to do with AC or DC - it's just the way it is.

Also, make sure you don't confuse mains earth/PE with GND. Different concepts and different purposes even though GND may be at the same potential as the PE.

[tester43]

hmmm... this will have to wait till evening Digging might rise questions

[Zero999]

This is about your soldering iron controller project, isn't it?

Attached is a way to control the AC to the soldering iron, without an opto-coupler. It will have lower switching losses, than running the soldering iron off rectified DC and switching it with a MOSFET. The TRIAC needs to be the type which will fire, when the gate current is positive and anode is negative but most small TRIACs full-fill this criteria. If not, the TRIAC can be AC coupled to the MCU circuit and fired on with negative pulses, when the anode is negative, but cross that bridge, if you get to it.

Note that the neutral of the soldering iron, is different than the 0V reference of the 5V supply: it's on the phase node of the transformer, but this doesn't matter: all voltages are relative. The TRIAC is switching the phase conductor of the soldering iron, yet the 0V for the DC supply is the other side of the transformer's winding. What's ground/earth in one context, is completely different in another. You'll see this quite often with circuits run off a capacitive voltage dropper, running from the mains.

No part of the circuit, other than the soldering iron's metal case is connected to PE, the Protective Earth conductor, which is there to stop the soldering iron's tip from floating at high voltages, due to capacitive coupling through the transformer and the element insulation. The PE conductor carries no current and should never do so, other than a tiny leakage, through any RFI suppression components, but that's another matter.

I/O:1 is the zero crossing input. R4 will limit the current to a safe level and the MCU's internal ESD clamping diodes, will limit the voltage to a safe level. The MCU will see a square wave, at the mains frequency. To trigger the TRIAC, I/O:2 should go high, for a few ms, after I/O:1 has gone high, then high again for a few ms, after I/O:2 has gone low, as that will be when the mains has crossed over the zero point and gone negative. If this doesn't make sense, I can post a timing diagram.

The only downside to this circuit is, a half wave rectifier, results in DC flux in the transformer, which can cause saturation and overheating, if this is too high, however it won't be a problem if the transformer is overrated and the DC current taken by the MCU circuit is low, compared to the overall rating of the transformer. Suppose the iron uses 60W maximum: it's unlikely to be a problem the MCU circuit uses under 100mA and the transformer is rated to 75VA. R5 helps to alleviate this to some degree, by limiting the high current surges, taken from the transformer, every time the smoothing capacitor charges. It should be rated to >1W, as it will dissipate a reasonable amount of power.

[tester43]

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

[pwlps]

I'm sorry if my comment will add confusion to your understanding, but actually the statement that any point of a circuit is as good as another to choose the ground is a bit misleading: there is also a "true" ground in the sense of earth ground.  This is important if there are stray electromagnetic fields generated by your circuit because the earth ground acts as an infinite conducting plane i.e. a conducting plane with infinite capacitance.  Consider a half-dipole antenna feeded by a coaxial line. You can connect the ground of the line (outer shield) to the earth ground but if you connect the antenna to the ground it will not radiate (then the shield of the coaxial line will radiate instead of the antenna but the radiation impedance and pattern will be different).  In RF circuits where connecting wires can radiate the choice of ground is more important

[tester43]

it's all good. Actually that's the first time in my life that I am tempted to buy a better oscilloscope instead of new graphic card for my pc

[CatalinaWOW]

This thread reminds me of the old story about blind men describing an elephant.  There is nothing I would point my finger to as wrong, but certainly many different ways of coming at the subject. 

Good for all of us to remember that there are a lot of ways to skin a cat, and that all of our understanding of the universe is a working approximation.  Some just have more or less detail, or emphasize on aspect over another.  Electricians will have one focus, RF guys another, the guys operating in the solid state/quantum world will add their own details, the hobby guys do just fine with some simple models.   Horses for courses.

[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]

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

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.

[hamster_nz]

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.

If you make a measurement in an AC circuit you might be the equivalent of sitting in a kayak, moving up and down on the waves. One end will go up slightly with respect to the other as the waves pass. 

However, think of GND as being a very large barge or super-tanker. It is so large that waves are unable to make it move significantly. It is still 'floating', but can be used as a stable reference for measuring the height of waves while still being out in open ocean.

... musings...

To me it seams DC/AC thing is a matter of scale. Everything AC acts like DC at a very small timescale or when operating at low frequencies, and everything DC starts acting like AC at very large time scales or when operating at high frequencies.

The tricky bit is the bit where things are "sort of DC, and sort of AC", and doing a simple DC or AC analysis doesn't tell the whole story. Typically that is also where all the interesting, useful things happen (like filters, and power supply ripple/noise, and PWM modulation, and high speed digital, and transmission lines, and ...)

[NorthGuy]

Imagine, you have an output of the transformer, which, you know, produces AC. You decide one of the wires is your ground. You can connected it to earth. Then you draw the waveform of voltage on the other wire. This will be a sine wave centered on zero.

Now add a rectifier. It'll have two terminals - "+" and "-" with DC between them. Now, looking at the diodes in the rectifier and at the waveform on the AC wire, draw the waveforms on the "+" and "-" terminals relative to your ground. This may take you a while to do.

Look at the resulting "+" and "-" waveforms. What do you think?

[BravoV]

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.

Inductance = water wheel resisting change in water flow rate, or changing (speed up or slow down) the water flow if the wheel has been rotated that has it's own rotating inertia

There, a bit improvements on inductance analogy  ... 

[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.

Inductance = water wheel resisting change in water flow rate, or changing (speed up or slow down) the water flow if the wheel has been rotated that has it's own rotating inertia

There, a bit improvements on inductance analogy  ... 

Actually I think this change is bad.
Your water wheel example of use of mass but does not match up to electrical inductance.

Inductance is created by the magnetic field. More or less current changes the magnetic field.

Using my deflations above with current = flow rate of water, A change of flow rate is a change in mass per unit of time. This mass of water is the equal of inductance. Where mass is resistant change in speed, inductance is resistant to current change.

This fits in with the other deflations.

A wire has some inductance, A coil of wire has more inductance.
A pipe contains x amount of water(mass), A coil of pipe adds the centrifugal force  to the mass of water causing an increase in resistance to change.

[BravoV]

How about the wheel it self has a mass which is significantly greater than the mass of the flowing water (the parasitic inductance) ?