Electronics > Beginners

Need somebody to talk to about AC concept

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tester43:

--- Quote from: dmills on June 26, 2018, 07:24:35 pm ---My real question about that design is why all the opto isolators,... The Arduino crowd do seem to love their superfluous opto isolators, seems to be a cargo cult thing in that scene.

--- End quote ---

Blasphemy!  :-DD


--- Quote from: dmills on June 26, 2018, 07:24:35 pm ---...
Connecting one end of the secondary to ground is also a good thing from a safety ...

--- End quote ---

Are you saying that because secondary coil is isolated it will not conduct to earth, because loop is between coil ends, because its isolated ( i repeated, i know)?  And it will once the isolation would get damaged so mains would appear in full glory on secondary... hmm

rstofer:
Take a battery circuit like you find in a textbook.  The battery is drawn vertical with + at the top.  Now add a resistor, drawn vertically along side the battery.  Connect the top battery connection to the top resistor connection and similar with the bottom.

You have a DC circuit.  Now add some other components in series or parallel until your eyes cross.  How in the world to analyze this mess?  Kirchhoff's Laws which I won't cover.  But to use those laws, we need a reference point (node) from which all measurements are made.  It can be ANYWHERE in the circuit but it usually the node connected to the battery (-) terminal.  For some reason, we call that GND or Vss or some other term.  All it means is that when we probe for voltage, the black lead is on GND and the red lead on the node we want to measure.  There is absolutely no intentional reference to Earth Ground.  There could be but it isn't necessary for this discussion.

Cool, we have a DC circuit.  Now, pull out the battery and connect the secondary of some transformer.  Not one thing has changed!  We still put the black probe on GND (wherever we decided it should be) and we measure AC with the red probe.  If it is a proper transformer with isolation between primary and secondary, we can connect our circuit GND to Earth Ground if we wish.  Or not...  There are reasons to do this, there are reasons not to do this.  Typically, we don't.  But that Earth connection has absolutely nothing to do with the circuit or how we take measurements.

Then there are center-tapped transformers and we USUALLY take the center tap as circuit ground.  But we don't have to!  Imagine a 12.6-0-12.6 transformer.  I could ground one end and wind up with 0V-12.6V-25.2V if that's what I want.  As long as the secondary is isolated from the primary, I can do anything I want with the secondary.  Just like stacking two batteries...


Earth ground is an entirely different subject.  According to the National Electric Code Article 250 (somewhere) it says something like: all exposed non-current carrying metallic surfaces shall be grounded.  Each word is important.

All: well that means every single piece
Exposed: possible to touch
Non-current carrying: normally doesn't carry current.  Exposed busbar is acceptable for certain applications
Metallic surfaces: something that conducts.  Not plastic...  This is how double-insulated works...
Shall be grounded: pretty obvious.

So, you have that Rigol scope and the whole thing is plastic EXCEPT the exposed non-current carrying BNC connectors which must be grounded.  And they are!  And people want to float their scopes to defeat this safety requirement.

What about cord connected drill motors with 2 conductor cords?  The housing is plastic, no problems there.  But what about that metallic chuck?  It turns out that enough of the gear box is plastic that there is no conductive path from the electrical components to the chuck.

I'm not going to get into switching mode power supplies.  Apparently there are some measurement considerations about which I know exactly nothing.  I buy power supplies, I don't build them.

rstofer:

--- Quote from: tester43 on June 26, 2018, 07:45:57 pm ---Are you saying that because secondary coil is isolated it will not conduct to earth, because loop is between coil ends, because its isolated ( i repeated, i know)?  And it will once the isolation would get damaged so mains would appear in full glory on secondary... hmm

--- End quote ---

That is a consideration.  In the 'electrical' world, we ground one side of control transformers that might be stepping 480V down to 120V (VERY common, we do it every day!).  If we didn't and the transformer developed an internal fault between the primary and secondary, our control voltage could get to 600V (the windings adding, for example).  Usually, that would be considered a bad thing!

There is good reason to carry Earth ground from the wall outlet to exposed metallic surfaces as well as tying it to circuit ground.  I use wall warts, there is no Earth ground available.  I'll worry about that some other time.

tester43:
Ok guys! Congrats to everybody in this thread - I think i got something when combined with asking myself a question "why do I feel ok with bridge rectifier and not with AC". See pt 3 below.

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)

OMG moment 1: Secondary Coil is isolated from mains - it means that PE is not a part of circuit. PE is part of mains - so as long as I do not close current circuit (create loop) it's just a piece of wire (situation changes rapidly  :-DD if I would damage isolation). So this circuit example i showed - it was just a PE safely attached to AC wire.

OMG moment 2: I think I understood one of the reasons to do the symetrical transformer - Voltage at center tap would be zero - I need to read more on that (do not understand yet why it would be zero at this physical point between coils and not zero on other end).

OMG moment 3: IMHO, example with "imagine battery with pulsing +" is incorrent. It would be good if we would explain Bridge Rectifier (Graetz) - but it helped me by thinking "I choose to measure everything on this wire versus the moment when electrons do not move - meaning when there is zero.

Still.... it needs to sink in but much better now.

-----
AAA i'm on the wave - I just understood oscilloscope - it is referenced to zero point (unless there is DC bias).

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Next topic for later - why would I want or not want to connect my secondary transformer AC zero with filtered DC gnd. Will search for this later.

tester43:
Symmetrical power supply - I was wrong with center tap being 0  :-DD
But combined with graetz it gives me a correct symmetrical + 0 - power supply.

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