Series Circuit Question

[mivey]

Here is another one of my bad drawings.

View attachment 7754

That is the most appropriate drawing and is similar to what is used in most texts (there is usually a small circle with a sine wave inside).

Internal resistance shown as symmetric just for the purpose of illustration.

That's OK but symmetry is not really important like that since the resistance is representative anyway.

To get more accurate for a POCO service through a transformer, we would use something like a Steinmetz model. In that case, part of the transformer internal impedance would be one the input side and part on the output side. The source impedance would be shown beyond the input side of the transformer.

A power supply or generator would have a detailed schematic and may look completely different.

However, most all of them can be simplt represented using the model you have shown in your post.

 

[K8MHZ]

That is the most appropriate drawing and is similar to what is used in most texts (there is usually a small circle with a sine wave inside).

That's OK but symmetry is not really important like that since the resistance is representative anyway.

To get more accurate for a POCO service through a transformer, we would use something like a Steinmetz model. In that case, part of the transformer internal impedance would be one the input side and part on the output side. The source impedance would be shown beyond the input side of the transformer.

A power supply or generator would have a detailed schematic and may look completely different.

However, most all of them can be simplt represented using the model you have shown in your post.

Thanks.

I teach radio stuff on occasion and I found that my best method, in spite of my lack of artistic skills, is to make simplified sketches and build from there.

I strongly disagree with adding things to the mix that do not need to be there. In this case, the lesson was about current reference, not internal resistance. All the inclusion of internal resistance did was to confuse most people. So, I decided to split that off and try to explain it separately so others would see that it's not difficult to understand, but really is superfluous to the original lesson.

 

[cadpoint]

George, did you explain the picture in class?

You use another or correct it, even?

I beleive this has gone way over-head of the average Joe and reflects more of how
an engineer looks at things verses what's trying to be implied! Whether correct of incorrect.

I enjoy the pictures, and the level of thoughts...

 

[George Stolz]

George, did you explain the picture in class?

I displayed the picture, explained that there was a discussion going on here regarding it's accuracy, and then noted that their book did not observe the resistance of the power supply, so they were asked to ignore it for now.

In all honesty, I have been watching this thread intently, and can't visualize where exactly this .8v lives.

 

[K8MHZ]

I displayed the picture, explained that there was a discussion going on here regarding it's accuracy, and then noted that their book did not observe the resistance of the power supply, so they were asked to ignore it for now.

In all honesty, I have been watching this thread intently, and can't visualize where exactly this .8v lives.

It doesn't 'live' anywhere.

It's just a representation of the voltage drop that occurs at the power supply when loaded. A larger load would have a larger value than .8 volts. For instance, if the voltage dropped from 120 to 118 volts, then the value would be 2 volts.

It's not a constant as far as the power supply goes. It will change as the load increases.

I think what you are looking for is a place where you could put the probes of a volt meter and read the .8 volts, like the graphic shows. Honestly, I don't think there is a place you can actually do that. Instead, it's a calculated value and representing it as a directly measured value is wrong.

 

[iceworm]

... In all honesty, I have been watching this thread intently, and can't visualize where exactly this .8v lives.

Maybe this will help. See what you think.

ice

 

[George Stolz]

Your macro is essentially what I described here, and got thoroughly scolded by Mivey.

My official position is that the loss is real, but is essentially seen as a loss due to transformer inefficiency. Since the secondary is constructed of wire that has resistance, that .8V is lost getting to the very center of the secondary. Essentially, the ends of the conductor extend internally into the transformer to the center of the winding.

Would y'all buy that?

I can picture what you drew, but it has been called woefully inadequate. :huh:

 

[mivey]

Your macro is essentially what I described here, and got thoroughly scolded by Mivey.

Thank you sir, may I have another!

If it helps you to look at it that way, it is the impedance you go through to get back to the EMF source. The "center of the secondary" is not that place.

 

[George Stolz]

The secondary of the transformer is not the EMF source of the derived system? :huh:

 

[iceworm]

Your macro is essentially what I described here, and got thoroughly scolded by Mivey.

First: You should consider being scolded by Mivey as a badge of honor. He gets just as screwed up as the rest of us.

Your macro is essentially what I described here, ...

My official position is that the loss is real, but is essentially seen as a loss due to transformer inefficiency. Since the secondary is constructed of wire that has resistance, that .8V is lost getting to the very center of the secondary. Essentially, the ends of the conductor extend internally into the transformer to the center of the winding. ...​

Well, I like the bolded part. But the part in red leaves me a bit cold. If you were to tell me the part in red means, "the losses are distributed along the each turn of the coil." - Then I would say, yeah, you got it right on.

... I can picture what you drew, but it has been called woefully inadequate. ...

Obviously the words of a Phillistine.

ice

 

[mivey]

I have been watching this thread intently, and can't visualize where exactly this .8v lives.

It doesn't 'live' anywhere.

It's just a representation of the voltage drop that occurs at the power supply when loaded.

That is the best way to look at it. It is just a linear circuit mathematical model using a single impedance to represent a complex combination of impedances that are upline of the terminals, like we do with a Thevenin equivalent circuit model.

There is no exact one place since it is a complex combination of source impedances. So if you want to measure it, measure the voltage across the open terminals, then short the terminals to measure the current and calculate Z = V/I.

FWIW, this complex impedance is what the Suretest tester attempts to measure but it does so using a delta voltage and delta current because shorting the terminals is a bad thing.

 

[mivey]

The secondary of the transformer is not the EMF source of the derived system? :huh:

Not the center of the secondary with two terminals finding their way to the middle.

I suppose one simplified way to look at it as the secondary is made up of a bunch of little EMF sources. Remember the secondary is made up of a long segment of wire that is exposed to an alternating magnetic field. It is all of these little pieces working together that gets us the total voltage we see at the terminal.

 

[George Stolz]

By Jove, I think I've got it!

Thanks guys.

 

[mivey]

First: You should consider being scolded by Mivey as a badge of honor. He gets just as screwed up as the rest of us.

Gets? I am perpetually screwed up.

Obviously the words of a Phillistine.

If one is going to the trouble to look closer, one should recognize that source impedance is not as simple as secondary turn resistance. However, as someone noted earlier, it is probably beyond the scope of what you should cover in an basic EC presentation.

 

[ActionDave]

That is the best way to look at it. It is just a linear circuit mathematical model using a single impedance to represent a complex combination of impedances that are upline of the terminals, like we do with a Thevenin equivalent circuit model.

There is no exact one place since it is a complex combination of source impedances. So if you want to measure it, measure the voltage across the open terminals, then short the terminals to measure the current and calculate Z = V/I.

FWIW, this complex impedance is what the Suretest tester attempts to measure but it does so using a delta voltage and delta current because shorting the terminals is a bad thing.

Bang! It has begun. Now we just need to get some discussion involving quantum theory. ...:happyyes:

Which way does the door swing to get me out of this room??? When I push on it it feels like it wants to swing toward me.:blink:

 

[ActionDave]

By Jove, I think I've got it!

Thanks guys.

For now you do. In time the certainty will slip away like warm jello being grasped by your toes.

 

[ronaldrc]

Time to turn on our BS firewall.
I see a mole Hill becoming a mountain.

Ronald :roll:

 

[mivey]

Bang! It has begun. Now we just need to get some discussion involving quantum theory. ...:happyyes:

No need for anything so complicated. It is just a series impedance like we have when we make volt drop calculations.

Time to turn on our BS firewall.
I see a mole Hill becoming a mountain.

Ronald :roll:

{In my best Yoda voice}"Always with you what can't be learned. You must unlearn what you have learned. Teach or teach not. Simplification one thing is, but no reason teach incorrectly is there."

Why does there sometimes seem to be an assumption that ECs can't be bothered with learning, and/or can't learn, something correctly?

 

[ronaldrc]

Like everyone else on here that understands what the example means,
I had to take a second look for a couple of minutes to register what the
.8 tenths of a volt was.


The example is correct but is deceiving, should say 119.2 volts with a .8 tenths volt drop.
It does give the indication to a inexperienced person that it would be okay to touch the
output without getting a shock.

I don't see any sense in making this more complicated than what it is.

Ronald

 

[mivey]

I don't see any sense in making this more complicated than what it is.

That's true. Unless source impedance was important (which it is not for the point being made in the graphic), I would have just shown a 120 volt source. If asked, then the depth of coverage would depend on the audience.

But if we are going to open the minutia box, whether it be the simplified version or the detailed version, we should state things correctly.