Electronics > Beginners

Need somebody to talk to about AC concept

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tester43:
This concept of "you decide" is obvious. Pre-requisite to understand mechanics of any tranzistor, diode, etc.

But I think I need to go back to basics with reading, in terms of induction.
What is confusing to me is this part: "pick any wire from transformer and it's your GND".
In fact it's a coil. Single, long, enamel coated wire which is sunk in pulsing magnetic field. Field is triggering movement of current carrier (let's call it electrons for simplification). In the whole length of this coil wire I have potential generated.
At the infinite short moment [t] do I have the same potential on both ends of coil or is it shifted somehow - I need to read more to understand it

Short version: in AC there is no output from transformer that if I would attach an oscilloscope to, that would not .... alternate - constantly move in time. That's why I can't understand how one can say:
- this end of coil is my reference - because it's constantly moving (in terms of potential)  :-[

Brumby:
This is, perhaps, the most straightforward statement of an important fundamental - even if it does seem a bit dramatic:

--- Quote from: dmills on June 26, 2018, 12:08:00 pm ---The GND is a lie! Sometimes a useful lie, but a lie none the less.

--- End quote ---

This is the key to understanding.

In short, you never have a Ground - unless you MAKE one.

In this respect, the two terminals of a battery and the two wires from the (isolated) secondary of a transformer are NO DIFFERENT.  Neither of them have a Ground.  It's only when you connect one terminal of the battery or one wire from the secondary of a transformer to a grounding point that you have a Ground.

The only difference between the transformer secondary and the battery is that one is AC and the other DC.  There is no mysterious Ground that magically exists for either of them.

The mains supply to your home does have a grounding point, but it is one put in place by the electricity supplier - very deliberately.


--- Quote from: dmills on June 26, 2018, 12:08:00 pm ---All voltages are measured between two points, and all currents flow in loops, everything else stems from this.

--- End quote ---

"All voltages are measured between two points"  This is the basic truth that you need to step back and understand in its entirety and simplicity.  You must also understand that this is only meaningful when the two points being used are part of the same circuit.  (This can get a bit fuzzy in some circumstances, but for basic understanding, this is the way to think.)

Brumby:

--- Quote from: tester43 on June 26, 2018, 03:38:43 pm ---Short version: in AC there is no output from transformer that if I would attach an oscilloscope to, that would not .... alternate - constantly move in time. That's why I can't understand how one can say:
- this end of coil is my reference - because it's constantly moving (in terms of potential)  :-[

--- End quote ---

ONLY with respect to the OTHER END of the coil.

If the secondary is isolated, then there is no real potential difference to ANY OTHER point you care to choose that is not part of that coil.  You may get environmental hash and capacitive coupling, but there will be no true potential difference.

In the matter of scopes, I suggest you watch this through - as the key subject is about "Grounding" and the use of scopes:



tester43:
Hi,

I understand everything from the video linked.

What I have learned: there is nothing like ground or reference point in AC - because of constant change of EMF (voltage) in the whole circuit. Before I start crying over the question: what do I see on the
oscilloscope screen when probing both ends of secondary transformer coil, let me try with something more simple:

This guy here did a simple triac controlled soldering station:
http://jacoburge.co.uk/diy-jbc/

Here is the schematic:
https://static1.squarespace.com/static/561b7180e4b05a82a1747f0b/t/59da3b7bf09ca452b52cdbe8/1507474303866/Capture.JPG?format=1500w

Going from left:
- Transformer gives 24Vac. (so in every cycle power comes in both "directions" with EMF / "force" of 24V), max current (Amps) is regulated through wire gauge, size of the core etc.
- 4n25 on top is used to detect the zero of AC.... so basically there is no GND. Using definition of Voltage there is no difference of potential on end of my coil. Every time the EMF is zero for short period of time I see the "interrupt signal" from 4n25 - that's perfectly ok.
- rectifier is there just to properly use 4n25
- moc3020 is a triac driver - to safely enable triac single cycle conducting
- three resistors on moc3020 left side set triac control voltage and protect moc3020 internals
- max6675 is an integrated thermocouple sensor reader giving digital temperature reading

Grand question is:
What is this ground symbol on left low, near _green_ wire symbol.
If it's PE, why his Transformer does not get a shortcircuit to actual ground?


dmills:
Remember the transformer secondary is sort of like an AC battery, there is no inherent ground connection to it. Remember further that current always flows in loops, no loop, no current.

You could connect ANY **SINGLE** point on the left of the opto isolators to ground and the circuit would work exactly the same, no loop, so no current flows! Now as it happens grounding one side of the iron (as well as the irons tip) is not unreasonable from a static protection perspective, but that is a slightly more advanced issue.

Connecting one end of the secondary to ground is also a good thing from a safety perspective in that it ensures that in the (very unlikely) event of a transformer isolation failure putting mains somewhere on the secondary a loop back to the power company will be completed, blowing the primary fuse.

My real question about that design is why all the opto isolators, they would make some sense if the whole thing was a non isolated mains design, but I am reasonably sure that they add little here. The Arduino crowd do seem to love their superfluous opto isolators, seems to be a cargo cult thing in that scene.

Regards, Dan.

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