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 Post subject: Dr. Kirchhoff and the art of reading schematics
PostPosted: Apr Tue 07, 2020 6:44 pm 
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<<this is what we do when we can't go out>>

Gustav Kirchhoff left us with some useful concepts.....

Electric circuits can be described in terms of "nodes" and "branches". The easiest example is a battery and a lamp. The nodes are + and - on the battery, and the branches are simply the battery and the lamp.
Herr Kirchhoff gave us 2 simple rules:
--the sum of the currents in and out of a node is zero
--the voltage anywhere on a node is the same

Here is a absurdly-oversimplified audio amp, drawn in a node and branch format**:
Attachment:
nodes.jpg
nodes.jpg [ 37.89 KiB | Viewed 1049 times ]

There are 9 nodes, which can be named as follows:
1. input
2. V1 grid
3. ground (AKA "B-")
4. V1 anode
5. B+
6. V2 grid
7. V2 anode
8. speaker drive
9. speaker return

To relate this to a real-world device, the first--and possibly most important-- rule is that the nodes are not specific physical locations. "Ground", for example, is all over the place in a typical device---with some combination of structure (chassis) and wires. Further, any wire connecting a component to a node is functionally part of the node. IOW, any voltage or current measurement on said wire will still follow Kirchoff's laws.

Note that one thing needs to be changed to fit the tubes into Kirchhoff's laws: The tube symbol gets replaced by an equivalent circuit, and one or more RULES which relate the current in the cathode to anode branch to the node at the grid.

How about the missing branch from the battery + to B+? We (obviously?) need to add this for the amp to do anything useful. But--without it--do Dr. K's laws still apply??----YES: the sum of the currents at the nodes are still zero---because there ARE no currents. (similar answer for voltage....)

So, what does this have to do with reading schematics??? We can define the voltages and currents in the nodes and branches solely based on the schematic. But, to solve any common problem, we need to be able to find these features in the actual hardware. To do this, start anywhere........Let's start with B-. Look in the chassis and find any ONE of the items that are shown connected to this node.
There are a total of five:
--signal return
--V1 grid resistor
--V1 cathode
--V2 cathode
--battery return--AKA B-
Once you find the physical location of any one of these, you have found node 3. The important thing to recognize is that it will not look anything like it is drawn on the schematic. To verify that something is correctly wired, you have to inspect the actual wires, brackets, etc. to see that they electrically connect as the schematic shows. In some cases, you might have to use an ohmmeter to verify the connections.

To summarize:
A node is a functional intersection of 2 or more circuit branches----it is NOT a physical entity or location.
Conversely, you cannot define a node by describing some physical feature. metal structures, unused pins, terminals, etc. are not nodes---but any one of them might be PART of a node.


To be continued.... <<EDIT: EEEEK--I misspelled his name----fixed>>


**The alert reader will recognize that--as drawn--this amp will not have very good performance.....;)

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 Post subject: Re: Dr. Kirchhoff and the art of reading schematics
PostPosted: Apr Wed 08, 2020 5:29 am 
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it was always something that came very naturally to me, even as a little boy.


steve

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 Post subject: Re: Dr. Kirchhoff and the art of reading schematics
PostPosted: Apr Wed 08, 2020 6:53 am 
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One thing I found with the radio I've been working on is that sometimes these nodes can be unused pins on tubes or IF transformers, especially if the component you're looking at is terminating to ground. So you're trying to follow the schematic, and see a resistor going to one of the contacts for a tube socket, and you think "the schematic doesn't show it going there!" but really it's just using that point as a ground node for a few things. It would be nice if everything was neatly laid out in the chassis all in order with 90 degree bends in the wires, but that's not really practical in the real world. Instead, you have to solve the puzzle. It's frustrating and fun somehow.


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 Post subject: Re: Dr. Kirchhoff and the art of reading schematics
PostPosted: Apr Wed 08, 2020 7:37 am 
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Good point, but remember the admonition that a physical point in a chassis is often just PART of a node. In the purest definition, a node is not a physical point. The best example is often the B+ "bus". You might find it at several unused tube pins, but the "node" is typically all over the place

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 Post subject: Re: Dr. Kirchhoff and the art of reading schematics
PostPosted: Apr Wed 08, 2020 1:06 pm 
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I hate to be the party pooper but:

Kirchoff's law is simply an example of the conservation of energy or charge, for a steady state system, applied to moving electrical charges.

If you have a situation where charges were accumulating at some particular point (node) in a circuit, in other words the charges that were arriving over time, were exceeding the the ones leaving the circuit node, it appears to violate Kirchoff's law.

However, this is exactly what you have, if there is a capacitor on the node absorbing the charge, in the process of charging.

You could argue the law is not violated because if you consider the charges leading to the capacitor's plates, they still all "add up" (none are created or destroyed) but Kirchoff's law takes no account of the dynamics as that process occurs.

It is a lot like Ohm's Law that takes no account of dynamics or transient changes as inductors oppose a change in current and capacitors oppose a change in voltage. In DC systems, or stable current flow situations, the "reactive elements" disappear, and appear not to be there.

So what I am saying here is that Kirchoff's law is a very limited law, mainly for steady state DC systems and very limited utility for AC systems, in terms of shedding any light on them conceptually, or where there are transients and where charge storage and energy storage devices , such as inductors & capacitors are involved; ie Radio & TV circuits.

Kirchoff's law is great for simple DC circuits with resistors & multiple branches for sure, but our radios & TV's are much more sophisticated than that aren't they ? with energy storage elements such as L & C.

Perhaps more interesting laws appropriate to electronic apparatus are those of the Thevenin & Norton Theorem which help us understand source impedances, for example the parallel combination of an anode resistor with the tube's plate resistance determining the output impedance from an anode.


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 Post subject: Re: Dr. Kirchhoff and the art of reading schematics
PostPosted: Apr Wed 08, 2020 2:10 pm 
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Many, but not all aspects of an A.C. circuit can be treated like a DC circuit if considered in the context of a intanteous snapshot in time.

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 Post subject: Re: Dr. Kirchhoff and the art of reading schematics
PostPosted: Apr Wed 08, 2020 2:30 pm 
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Kirchhoff's laws apply to any and all currents and voltages---in any circuit. When we write loop equations, they can include all the complex impedances, and they are valid at any frequency.

But--there is a big difference: As the frequency goes up, the model of the circuit must become more complicated.

All this aside, the main reason for invoking Kirchhoff was to make the point about nodes---mainly that they are functional junctions between branches---and not physical locations.

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 Post subject: Re: Dr. Kirchhoff and the art of reading schematics
PostPosted: Apr Wed 08, 2020 5:48 pm 
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If you do not understand Kirchoff's Laws, Thevenin and Norton won't make much sense either. There is no contradiction in Kirchoff's Laws when reactive components are involved, but you have to realize that when reactances or time-varying elements are present, you cannot draw conclusions about steady state behavior from single instants in time. You either have to ensure that the instant you are looking at is indeed after a steady state has been arrived at, or put your calculus hat on and start integrating over the time that it takes to reach steady state. This may be forever in AC circuits, but there one can usually take one complete AC cycle as representative of what will happen for all other cycles as long as the source voltage remains constant.

Transient responses of networks is a very large and complex subject requiring a black belt in differential equations to fully comprehend. Fortunately not necessary for reading schematics or fixing radios!

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 Post subject: Re: Dr. Kirchhoff and the art of reading schematics
PostPosted: Apr Wed 08, 2020 7:09 pm 
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Chris108 wrote:

........ black belt in differential equations to fully comprehend.

I love it....I have maybe a "brown belt" in complex algebra for the frequency domain---when it comes to differential equations, I think I was IDed as untrainable very early in my education.

But---never mind all that. To understand schematics, one just needs to understand the basics of current flow and that a node is not a physical element in a chassis

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 Post subject: Re: Dr. Kirchhoff and the art of reading schematics
PostPosted: Apr Thu 09, 2020 8:28 pm 
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OK, then, on this schematic I have highlighted a "node".
Attachment:
AK9.jpg
AK9.jpg [ 226.47 KiB | Viewed 762 times ]


And next, the physical appearance on the chassis; some of the node is located on the top side.
Attachment:
DSC03384node.JPG
DSC03384node.JPG [ 658.96 KiB | Viewed 762 times ]


Correct?

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 Post subject: Re: Dr. Kirchhoff and the art of reading schematics
PostPosted: Apr Thu 09, 2020 9:35 pm 
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pixellany wrote:
I have maybe a "brown belt" in complex algebra for the frequency domain---when it comes to differential equations, I think I was IDed as untrainable very early in my education.

But that's the great thing about the Laplace Transform - it converts differentiation and integration into multiplication and division, so the (linear) differential equations become a snap to solve just using algebra.

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 Post subject: Re: Dr. Kirchhoff and the art of reading schematics
PostPosted: Apr Thu 09, 2020 10:04 pm 
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jrehkopf wrote:

Correct?

Can't tell for sure without seeing all the wires, but you are clearly "tuned in" to the concept.

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 Post subject: Re: Dr. Kirchhoff and the art of reading schematics
PostPosted: Apr Thu 09, 2020 10:13 pm 
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stevebyan wrote:
pixellany wrote:
I have maybe a "brown belt" in complex algebra for the frequency domain---when it comes to differential equations, I think I was IDed as untrainable very early in my education.

But that's the great thing about the Laplace Transform - it converts differentiation and integration into multiplication and division, so the (linear) differential equations become a snap to solve just using algebra.

OP starts a thread
others derail the thread
because the derailment is more interesting than the original topic, OP decides to help perpetuate the derailment.....:)

I hated Calculus in HS, and the first 3 years in college. Then I took an extension course on doing circuit analysis with LaPlace transforms---I loved it. Now I tutor HS students in basic Calculus....some of us simply do not make sense....;)

ON-Topic:
I'll be happy if I can just help people to not ask---e.g.---"where does the brown wire go?"

another favorite of mine is how to draw the "DC Map"---meaning trace all the major DC current paths from source to return.
(And do it without using the evil word: "Ground" (shudder))

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 Post subject: Re: Dr. Kirchhoff and the art of reading schematics
PostPosted: Apr Sat 11, 2020 4:42 pm 
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Quote:
And do it without using the evil word: "Ground" (shudder))


I could add more words to that list:
"continuity"
"checks OK"
"tested good"

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 Post subject: Re: Dr. Kirchhoff and the art of reading schematics
PostPosted: Apr Sat 11, 2020 11:25 pm 
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jrehkopf wrote:
Quote:
And do it without using the evil word: "Ground" (shudder))


I could add more words to that list:
"continuity"
"checks OK"
"tested good"



Ok, I'll bite. What would be the proposed term to replace "continuity"?
Or is it just the misuse of the word? Need a little more information here.

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 Post subject: Re: Dr. Kirchhoff and the art of reading schematics
PostPosted: Apr Sun 12, 2020 1:15 am 
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As in “I have continuity from B+ to plate” instead of “resistance from B+ to plate is 3.5 ohm”. For example, a recent post told me there was continuity from the transformer to the detector plate, through the tickler coil. A more useful answer would have been “resistance from transformer to plate was xx ohms”.

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 Post subject: Re: Dr. Kirchhoff and the art of reading schematics
PostPosted: Apr Sun 12, 2020 1:34 am 
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jrehkopf wrote:
As in “I have continuity from B+ to plate” instead of “resistance from B+ to plate is 3.5 ohm”. For example, a recent post told me there was continuity from the transformer to the detector plate, through the tickler coil. A more useful answer would have been “resistance from transformer to plate was xx ohms”.


Agree. Some use the term continuity to describe just about anything that's not an open circuit.

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 Post subject: Re: Dr. Kirchhoff and the art of reading schematics
PostPosted: Apr Sun 12, 2020 12:30 pm 
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To establish "continuity", you need to "ohm it out".......

When I was learning this stuff, "continuity" was always an acceptable word---it meant simply that the little charges were getting where they were supposed to. It was understood that there might be some resistance in the path and that was taken into account when trying to verify things. Whoever was assigned to verify the "continuity" understood that they might need to look at the circuit diagram to see what range to use on their meter.
For the cases where the path had a low resistance, my trusty DMM has a setting where it beeps when there is "continuity". The beeper failed a few years ago---and i had to revert to thinking.......

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 Post subject: Re: Dr. Kirchhoff and the art of reading schematics
PostPosted: Apr Sun 12, 2020 1:23 pm 
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pixellany wrote:
To establish "continuity", you need to "ohm it out".......

When I was learning this stuff, "continuity" was always an acceptable word---it meant simply that the little charges were getting where they were supposed to. It was understood that there might be some resistance in the path and that was taken into account when trying to verify things. Whoever was assigned to verify the "continuity" understood that they might need to look at the circuit diagram to see what range to use on their meter.
For the cases where the path had a low resistance, my trusty DMM has a setting where it beeps when there is "continuity". The beeper failed a few years ago---and i had to revert to thinking.......


This has long been my understanding of acceptable usage of the word continuity. I think while working with other experienced technicians/professionals, the term included assumptions that there were other components that added some level of resistance in the node. That mutual understanding is what avoided misunderstandings.

Where miscommunication can and does happen in the ARF venue is when an inexperienced person does not fully understand the circuit theory.

This is when the communication sometimes needs to get very literal, much like Jeff describes and also sums up what being on the same page as the next guy is all about.

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 Post subject: Re: Dr. Kirchhoff and the art of reading schematics
PostPosted: Apr Sun 12, 2020 9:37 pm 
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For some this may be helpful. Basically it's the way the military taught the reading of schematics back in those old tube days. Just Google OD1725 D and look for Basic Schematic Interpretation. Actually there are even a few pages at the end with those new devices called transistors.

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