Author Topic: Need somebody to talk to about AC concept  (Read 7290 times)

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Offline tester43Topic starter

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Need somebody to talk to about AC concept
« on: June 26, 2018, 09:01:48 am »
Dear All,

Do not hate me for stupid questions please :)

I doubted myself - after building multiple projects using micro-controllers I give up and went back to read about basics (definition of voltage etc).

I have a lot of basic questions for AC current.
I am looking for somebody to have a conversation (patience needed).

To ask the first question: what's the concept of GND in AC.
In DC it's extremely easy - low potential + high potential = difference.
What's the meaning of GND symbol in AC circuits?
 

Offline agehall

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Re: Need somebody to talk to about AC concept
« Reply #1 on: June 26, 2018, 09:31:21 am »
GND is always just a reference point. You can pick it arbitrary. Doesn't matter if we talk about AC or DC. This took me a while to realise but the way I grasped the concept was to look at a battery powered circuit, placing the ground reference in various places to see how calculations were done and realising that the result was always the same.

Then it is just a matter of applying the same concept to all other circuits.
 

Offline tester43Topic starter

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Re: Need somebody to talk to about AC concept
« Reply #2 on: June 26, 2018, 09:46:46 am »
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.
In the older times with first transistors GND was at the peak voltage - to make it more easy to calculate transistor point.

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

Offline agehall

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Re: Need somebody to talk to about AC concept
« Reply #3 on: June 26, 2018, 09:52:38 am »
Same principle. Pick one and say that is your ground reference. The other one is then relative to that reference.

I'm sure someone with much more AC experience will jump in and explain all about neutral and how that may (or may not) relate to ground in various countries, but in principle, ground is just a reference point you attach somewhere and everything else is then considered to be relative to that point.

To simplify, think about the battery powered circuit again. Now imagine that you (instantly) flip the battery around 50 times/second - all of a sudden you have AC.
 

Offline tester43Topic starter

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Re: Need somebody to talk to about AC concept
« Reply #4 on: June 26, 2018, 10:20:33 am »
i was waiting for this.
So ... Neutral wire. But: when you are using the plug with 2 pins you can freely flip the sides so I can't assume which wire is neutral.

Hmm in a way you could use line as GND. In this way voltage would be the difference with neutral....
 

Offline BravoV

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Re: Need somebody to talk to about AC concept
« Reply #5 on: June 26, 2018, 10:22:29 am »
What's the meaning of GND symbol in AC circuits?

The GND (Ground) concept is quite confusing for beginner to understand AC current.

My suggestion is, try imagine the GND (Ground) actually is not ground as we're standing at, but its more like a water surface, say like sea level.

Imagine we have a special balloon, that in order to lift it from water level, to certain height, you need a X effort, same again when you try to submerge the balloon under the water down to same depth as the previous height, still you need identical X effort too, as this illustration the balloon tries to resist when submerged.

Now, imagine again, there is a process that lift the balloon up to certain N height, and then move downward and submerge it under water at the same N depth, and pull it up again and move upward, and repeat.

The word "difference" as at your DC understanding now being expanded, that its now not static as DC as it stays constant, but its a "dynamic" difference.

Yes, to measure the difference as in volt at AC, assuming the movement is constant and with constant height and depth, you need to measure at least one cycle e.g: start from water surface, upward  + downward movement back to water surface, theoretically.

And yes, its getting more complex than measuring DC like just lifting the balloon and stay static at certain height.

To measure a DC is like taking a "still" photograph, but to measure an AC, still image is not valid, but must shoot in a video form, hope you get this.

My 2 cents.

Offline agehall

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Re: Need somebody to talk to about AC concept
« Reply #6 on: June 26, 2018, 10:32:39 am »
i was waiting for this.
So ... Neutral wire. But: when you are using the plug with 2 pins you can freely flip the sides so I can't assume which wire is neutral.
Correct. And in my opinion, you should never assume that one wire is neutral. Go back to the simple battery circuit as I suggested and think about what happens when you switch the battery around.

Which way the current flows only matters in "more complicated" circuits and they will deal with this using rectifiers to ensure that current flows the right way. If you consider a simple circuit, such as a lamp and a battery connected to each other, it doesn't matter which way the current flows.

I should also point out that I'm speaking more from a theoretical point of view here. Don't take my advice and go poking in a 240V system - if you want to learn, get a function generator that outputs 1-5V of sinewave and play with that...

Hmm in a way you could use line as GND. In this way voltage would be the difference with neutral....
GND is a reference point. PE (Protective Earth) is slightly different but I'm not the right person to go into details about that.
 

Offline tester43Topic starter

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Re: Need somebody to talk to about AC concept
« Reply #7 on: June 26, 2018, 10:35:20 am »
this part about "baloon" and measuring the whole cycle was known - it's about the calculation of "what would be the dc equivalent of given AC wave in terms of work capability"(?)

i'm playing using small transformer.
AND YES: by intuition i did as you suggested: I put rectifier bridge to "produce" myself a gnd reference. So now I have just positive AC waves all the time for the price of diodes cutting voltage a bit.

but:
If you look at this picture of 2 tips: Hakko and JBC
At least for hakko it's clear - on the body of tip put PE wire to put it to gnd earth (and enable breaker at the same time if something goes through there);
BUT: JBC construction suggests something completely different - you can't just connect PE to the body that is powered with AC. At the same time you can't just put any wire from AC as GND..... <frustration meter rising>

http://dangerousprototypes.com/forum/download/file.php?id=12167&mode=view
 

Offline BravoV

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Re: Need somebody to talk to about AC concept
« Reply #8 on: June 26, 2018, 11:10:07 am »
this part about "baloon" and measuring the whole cycle was known - it's about the calculation of "what would be the dc equivalent of given AC wave in terms of work capability"(?)

AC is usually mentioned in RMS as in root mean square as in mathematical, so 220 Volt AC "RMS" is equal to 220 Volt DC.

Offline tester43Topic starter

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Re: Need somebody to talk to about AC concept
« Reply #9 on: June 26, 2018, 11:26:15 am »
thanks for reminder on RMS :)
 

Offline dmills

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Re: Need somebody to talk to about AC concept
« Reply #10 on: June 26, 2018, 12:08:00 pm »
The GND is a lie! Sometimes a useful lie, but a lie none the less.

All voltages are measured between two points, and all currents flow in loops, everything else stems from this.

"Ground" is a really crap word in electronics, especially around beginners because it has several subtly different meanings and in some cases more then one applies in a rather context dependent way.

Quite often we have some node in a circuit that it is convenient to use as a reference for measuring a pile of other voltages relative to, and quite often we will refer to this as ground, this is one of of the term.

Then we have the 'safety ground' use of the term, by which we really mean a conductor that does NOT normally carry current used to provide a path back to the supply neutral by some means that serves to blow fuses and open breakers in the event that a fault makes a metal case live with respect to ground. This is the second use of the term and can imply quite a different voltage to the 'ground' in the first use.

Them we have the 'ground' that is in some sense a connection to the mass of the planet, usually has little difference relative to the second case (by design) but some caution with that assumption is advised there are faults that can cause these last two to develop significant voltage between them.  This is the third meaning.

Regards, Dan.

 
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Offline agehall

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Re: Need somebody to talk to about AC concept
« Reply #11 on: June 26, 2018, 12:59:50 pm »
AND YES: by intuition i did as you suggested: I put rectifier bridge to "produce" myself a gnd reference. So now I have just positive AC waves all the time for the price of diodes cutting voltage a bit.

No no no, that wasn't at all what I was suggesting. That is how circuits that care about current flow deals with AC. You don't need a rectifier to "produce a gnd reference" - you PICK one, arbitrary.

Go back to basics - imagine a 1.5V battery and a lamp hooked up to it. The natural GND point for most people is right at the negative pole of the battery. However, that doesn't have to be the case. You can just as well say that the positive pole of the battery is the GND point. The result for all calculations will be the same (well, signs might change etc, but in essence everything is still the same).

You are stuck at the idea that GND is always at some global 0V reference point which simply isn't true. The sooner you get rid of that idea the sooner you will get the hang of this.
 

Offline tester43Topic starter

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Re: Need somebody to talk to about AC concept
« Reply #12 on: June 26, 2018, 02:07:11 pm »
I think my problem in this is that in AC in general there is no single "point" or potential that would not "float" - move constantly it's value.
Guys I'm doomed :(
 

Offline whalphen

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Re: Need somebody to talk to about AC concept
« Reply #13 on: June 26, 2018, 02:40:34 pm »
In your circuit, ground is whatever node you want it to be!
The current is continuously changing.  But you can't say the voltage is changing unless you first indicate how you are measuring the voltage.  Voltage measurements represent a potential difference between two points.  So voltage measurements require two contact points.  You can call either of those contact points ground.  Or you don't need to call it anything at all.  But when you communicate or record the voltage measurement, you need to also communicate which contact points were used.  To simplify this, most people call one of the contact points ground and use that same contact point or node as a reference in all of their voltage measurements -- and label this as ground on the schematic.  It's then implied that, unless otherwise indicated, that ground node is the measurement reference point (i.e., one of the contact points) for all the voltage measurements.  It's also usually convenient to equate (attach) the ground reference point inside the circuit with conventional ground reference points used outside the circuit.  And those could be the cabinet, the mains ground, and even the Earth itself.
Think of an AC supply as a DC supply that is always changing.  You can measure voltage across the two supply lines just as you can measure voltage across a battery.  In either case you may call one of your contact points ground.  It's arbitrary and entirely up to you.  But, typically people call the negative terminal of the battery ground.  And, often, most people will call one of the contact points on an AC supply ground.  If the AC source is the mains, then it's typical to attach the mains ground to the circuit ground.
And, don't confuse the mains neutral with ground.  It's intended as a current carrying conductor and is also tied to the mains ground and the Earth ground at the source.  The neutral does not always have zero volts potential.  At distances away from the source, it can have a significant potential due to resistance when it's carrying a large current!
« Last Edit: June 26, 2018, 02:54:02 pm by whalphen »
 

Offline JourneymanWizard

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Re: Need somebody to talk to about AC concept
« Reply #14 on: June 26, 2018, 02:49:55 pm »
You'll get this.

Pick a wire on the output of your transformer.  Color or tape that one blue.  That is now your Return/Neutral/V-.  The other wire is Source/Hot/Live/V+.

The blades that plug into the wall on US outlets are different sizes due to safety in household wiring.  The smallest should be the LIVE/Hot connection; the larger should be NEUT/Return.  The third (round) connection on a 3-prong plug is Earth.

The blades are that way mostly for Edison (light bulb) sockets and the like: at your distribution panel in your house, the NEUT wire is supposed to be bonded with the Earth connection (which is connected to a grounding rod and/or your cold water pipes).  This is to help with safety and give a local safety reference.  The NEUT is supposed to be connected to the most touchable part of a light bulb socket, so if your fingers brush it during installation you don't get zapped (NEUT is connected to earth, you are connected to earth, the difference is small enough for safety).

Don't stress overmuch** until you are above 40V ac rms (the threshold for potential human dangerous) and if there is something referenced to earth ground (e.g. household wiring).


** which is to say normal amounts of care and attention need to be paid
 

Offline tester43Topic starter

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Re: Need somebody to talk to about AC concept
« Reply #15 on: June 26, 2018, 03:38:43 pm »
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)  :-[
 

Offline Brumby

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Re: Need somebody to talk to about AC concept
« Reply #16 on: June 26, 2018, 03:55:14 pm »
This is, perhaps, the most straightforward statement of an important fundamental - even if it does seem a bit dramatic:
The GND is a lie! Sometimes a useful lie, but a lie none the less.

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.

All voltages are measured between two points, and all currents flow in loops, everything else stems from this.

"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.)
 
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Offline Brumby

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Re: Need somebody to talk to about AC concept
« Reply #17 on: June 26, 2018, 04:04:58 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)  :-[

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:



 

Offline tester43Topic starter

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Re: Need somebody to talk to about AC concept
« Reply #18 on: June 26, 2018, 06:02:03 pm »
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?


« Last Edit: June 26, 2018, 06:13:21 pm by tester43 »
 

Offline dmills

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Re: Need somebody to talk to about AC concept
« Reply #19 on: June 26, 2018, 07:24:35 pm »
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.
 

Offline tester43Topic starter

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Re: Need somebody to talk to about AC concept
« Reply #20 on: June 26, 2018, 07:45:57 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.

Blasphemy!  :-DD

...
Connecting one end of the secondary to ground is also a good thing from a safety ...

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
« Last Edit: June 26, 2018, 07:48:29 pm by tester43 »
 

Offline rstofer

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Re: Need somebody to talk to about AC concept
« Reply #21 on: June 26, 2018, 08:15:16 pm »
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.
 

Offline rstofer

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Re: Need somebody to talk to about AC concept
« Reply #22 on: June 26, 2018, 08:26:58 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

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.

 

Offline tester43Topic starter

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Re: Need somebody to talk to about AC concept
« Reply #23 on: June 26, 2018, 08:59:01 pm »
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).

------------------
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.

« Last Edit: June 26, 2018, 09:02:50 pm by tester43 »
 

Offline tester43Topic starter

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Re: Need somebody to talk to about AC concept
« Reply #24 on: June 26, 2018, 09:37:49 pm »
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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