# Schematic analysis for voltage drops

Discussion in 'General Electronics Chat' started by uriahsky, Jun 3, 2013.

1. ### uriahsky Thread Starter Active Member

Apr 27, 2009
43
2
I have been searching and searching for examples or books or videos on analysing schematics of any kind for all of the nodal voltage drops. I would like too see someone work their way through things like power supplies, TVs, motherboards, LCD monitors or almost any schematic and show the math and thought process used in the analysis.

We spend so much time learning serial and parallel resistors, caps and inductor formulas along with transistor and digital circuits but I never see anyone putting it all together and working through a complete schematic. Is there anything or anyone out there that goes through example after example of someone doing this?
Thanks,
Russ

2. ### WBahn Moderator

Mar 31, 2012
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First, you shouldn't be learning formulas, you should be learning concepts and techniques. If all you are learning is formulas, then you will only be able to analyze circuits that are an exact match to the circuits the formulas applied to.

Very seldom is it necessary to completely analyze an entire schematic for a substantial circuit. Instead, circuits are designed to be modular so that you can focus on a particular subcircuit of interest and, for the most part, ignore the details of what is driving it or what it is driving. When we can't ignore these details entirely, we like to abstract the driving and driven circuits into fairly simple (just simple enough to capture the important behavior needed for the analysis).

It's the same in many fields. How many people worry about the transmission gear ratio on a car when they are working on the brakes or the air conditioner. As much as possible, you work on the smallest subset of the whole that you can.

3. ### t06afre AAC Fanatic!

May 11, 2009
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Here I will also add that circuit simulation programs have taken over much of the work in this field. Circuit simulation are often included in many ECAD systems. Somewhat simplified you just draw a schematic and leave all the boring work to simulation software. That after crunching the data. Present the result in nice curves

4. ### uriahsky Thread Starter Active Member

Apr 27, 2009
43
2
Thanks for the response. Electronics is a life long learning experience and one thing that I think would help me learn is to see someone explain how they find voltages at any node on a schematic. I am doubting that it can be done. I have never seen anyone do this and I have close to a hundred books on the subject of electronics and this is never demonstrated to an advanced degree. Perhaps the leap from simple circuit to complex schematics is not that great but just to help close the learning gap I would like to see it done and explained.

The problem with ECAD software is having to draw in the whole schematic and also they never explain the thought process at how the voltage was arrived at.

I am not looking for exact answers to the micro volt just a close enough estimates to know that there is a problem, and if the voltage is higher or lower what could that point to.

Maybe it can't be done? Why else have I never seen this type of thing?
Thanks again,
Russ

5. ### MrChips Moderator

Oct 2, 2009
18,135
5,695
You don't get it.

Yes, it can be done.
Yes, it can be done at the microvolt level.
Yes, people do it for a living.

But it is not done the way and reasons you envision it.

Think of a whole house being built. You can hammer a nail into a piece of wood. You can analyze the stress and strain put on that single nail. You can spec the right size of nail to choose. The builder understands the purpose and specs required for that specific nail in that specific application. You don't do a structural analysis of the whole building to figure out what size nail to use.

Similarly, the function of a single resistor in a circuit of hundreds of components should be well defined and clearly understood. You do not have to analyze a single node from the perspective of the entire circuit. One can understand and spec the single resistor for the portion of the circuit in which it is employed.

So you do need to know how series and parallel circuits work, how to apply KCL and KVL and how diodes and transistors behave. That is where the analysis is done.

6. ### uriahsky Thread Starter Active Member

Apr 27, 2009
43
2
I am looking at this from the point of view of troubleshooting and repair not from an engineering perspective. I don't need or want exact voltages just rough estimates for repair. Just enough to know there is a problem if the voltages don't match from schematic to the pcb.

The reason I think I never see this kind of thing is that to gather enough knowledge to do this kind of thing requires math and experience of an engineer and if someone knows that much they don't spend their time with service repairs, they design and build things. Usually repair people don't need that much knowledge to do service work and so they never need to spend the time to learn the math required to figure the nodal voltages, but I still want to learn this.

Most electronic repair courses go into all of the usual theory on circuits along with KVL, KCL etc but I have never seen someone take a schematic and apply KVL and KCL and work through the schematic. What good is KVL and KCL to repair techs if it isn't used in actual repair work?

I am willing to pay someone to help me with this and I tried to to do this a few times but the people couldn't do it. I am sure some people can do this but I just wish I could find some actual examples of it being done along some explanations of the process.
Russ

7. ### MrChips Moderator

Oct 2, 2009
18,135
5,695
Let me see if I can narrow this down for you.

There are two types of troubleshooting/repair technicians.

Tech 'A' is told to go measure the voltage at node X. If the measured voltage does not fall within V volts +/- E volts, the circuit is bad. Throw out the board and get a new one.

Tech 'B' sees that the voltage at node X is out of specs. Tech 'B' understands how the circuit works, analyzes the situation and figures circuit C is at fault and replaces the faulty component. How is Tech 'B' able to recognize the scenario and pin-point the fault? Because Tech 'B' has the right knowledge, math and circuit design experience to be able to analyze the circuit correctly.

Which camp do you want to fall into?

This is the stereotypical view of the difference between a technician and an engineer. My day job is a technician. I think and function like an engineer.

8. ### uriahsky Thread Starter Active Member

Apr 27, 2009
43
2
I of course want to learn but I am looking for something I can never seem to find. Actual examples of someone figuring the voltages at all of the nodes on any schematics. Have you ever seen someone do this? I searched and searched but can not find any examples of someone going through a schematic and showing the math and thought process of the voltages. You would think this would be done because examples of various simple and advanced sub circuits are all over the place but someone going though a whole schematic I never see. That is what I am looking for. Have you seen this in a book or video or website somewhere?
Russ

9. ### MrChips Moderator

Oct 2, 2009
18,135
5,695
Ok, I give up. You are not listening.

One does not go about figuring the voltages at all the nodes on the schematic.
A complete schematic is composed of many simple circuits.
You take one simple circuit and analyze that one circuit.
This is done in all your text books. Learn basic circuit design and analysis. That is how it is done.

10. ### uriahsky Thread Starter Active Member

Apr 27, 2009
43
2
Your not listening to my question!

I realize that it a schematic is made up of many smaller circuits. This i know.

I can handle most simple circuits, but sometimes when i am looking at a large schematic i am not 100% sure that i am correctly following and figuring the voltages. I would like to see someone else go through a whole schematic so that i can learn and ask questions. But for some reason this has never been done on this planet earth, ever, once anywhere. I dare some to show me someone doing this for a complete complex schematic! I WILL PAY MONEY TO SEE THIS DONE!

That is what i am looking for.
Anyone?

11. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,692
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Show us some exemplary diagram. Maybee we can help.

12. ### WBahn Moderator

Mar 31, 2012
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What do YOU consider a complete, complex schematic?

Keep in mind that most circuits are not static. For instance, an AM radio receiver has oscillators and many of the nodes change continuously in response to the detected signal. So what are you looking for that would satisfy you that the circuit has been completely analyzed?

I've designed ICs that have had millions of transistors and could tell you the expected behavior of every node in the circuit, including the nodes in the protection circuits in the pads. I could even tell you the expected voltage drops along the major power distribution buses on the chip. Most of that work was done with pencil and paper and then verified by simulation. What can I say? I'm old school.

Last edited: Jun 3, 2013
13. ### studiot AAC Fanatic!

Nov 9, 2007
5,003
522

This is the first I have seen of this question or thread.

Rest assured such analytical methods do exist, along with the textbooks to go with them, the math has been around longer than the transistor.

You should consult a good book on cascadable two port networks.

For you purposes I would recommend

"Electronic Engineering Applications of Two Port Networks" by Gatland

However I should warn you that this approach makes servicing more, rather than less, difficult.
The days when every circuit diagram came with little circled voltage, as measured with an AVOmeter are long past.

The really important thing is to have some understanding of why a voltage drop is incorrect ie what change to the circuit has occured to cause it. For example what could cause the collector to rise to near the rail voltage?
Or what could cause the base - emitter drop to be zero. It doesn't really matter if it should be 0.6, 0.65 or 0.7 if it measures at 0.1, the transistor is not operational for some reason.
The outward effect of this may be that the transistor (and therefore the circuit) is not amplifying or even passing signal.
But if the BE drop is normal the fault lies elsewhere, perhaps an open coupling capacitor.
I do not need a fancy calculator to tell me this, just lots of experience and head scratching.

So yes, lets talk service techniques.

Last edited: Jun 3, 2013
14. ### JMac3108 Active Member

Aug 16, 2010
349
67
Listen to what Mr. Chips is telling you. The methodology you are describing is just not the right way to troubleshoot.

As a technician or engineer you need to learn how individual circuits work. The idea is that you have knowledge of basic circuits, and a good idea how the overall system works, and this allows you to break down the design into individual circuits to see if they are functioning correctly.

Lets say you see an op-amp circuit in the design. Yes, in this case you might have to analyze the circuit to determine if the output votlage is correct. You probably know how to do this already.

But, if for example, a solenoid is not turning on? You trace it back and see that the MOSFET that drives it is not turning on. You then find that the gate of the MOSFET is driven directly from a micro-controller and it is not in the right state to turn on the MOSFET. You just managed to troubleshoot the problem pretty far with only a few simple voltage measurements - no nodal anaysis required. And now you've discovered that the micro-controller is not driving correctly ... what do you do? Certainly not nodal analysis. I would check the power supply and the oscillator for the micro top see if they are present and look correct. If they are, then I would look to see if the micro is doing anything else its supposed to be doing in order to determine if it is completely dead or perhaps lost its program. Are there any inputs to the micro that are required to be there for the solenoid to turn on? At this point you have to have some knowledge of the system and how it is supposed to function. The point of this is that troubleshooting is not done by extensive nodal analysis, but by breaking down the functionality into small subcircuits that can be understood and tested.

15. ### kubeek Expert

Sep 20, 2005
5,444
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And since electrical engineers are lazy beasts they like to make the circuits in such way that the subcircuits influence each other only as little as possible, so that the who controls who question is easy to work out. This way the subcircuits are easy to design and test, and easy to verify they do what they think they should be doing.
Almost any device consists of blocks that are separate from each other and can be checked independently.

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16. ### WBahn Moderator

Mar 31, 2012
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I often point out to people that engineers became engineers because they are lazy and wanted to make cool gadgets that would do their work for them. They just didn't realize how much work being lazy ends up being.

17. ### screen1988 Member

Mar 7, 2013
310
4
I am totally lost when seeing the circuit like that.

It seems to me that when each small circuit are connected they affect each other.
Can you make an example to divide the circuit into subcircuit?
And with the circuit like that do you need to find which transistor is ON, OFF, active,...?
Do you need to find VB, VE, VC of each trans to know its operation region?

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18. ### WBahn Moderator

Mar 31, 2012
23,840
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The circuit could be better drawn to make things a bit cleaner and easier to see the stages.

There is also no output node indicated. Or input, for that matter. It's very poor form to draw a schematic and not even indicate what the inputs and outputs are or what the overall point of the schematic is.

To start breaking things up, one thing to keep in mind is that the base currents of most transistors are generally intended to be negligible compared to the other currents flowing through that node. Also, start recognizing things like differential pairs and the current sources associated with them.

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19. ### Brownout Well-Known Member

Jan 10, 2012
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This is easily analyzed. For simplicity, I am going to remove the 3v source and the 500K resisistor on the emitters of Q1,2. Now, consider the subcircuit made by Q1-Q5 and the associated resistors. Solving this subcircuit will allow you to solve for any node of mesh in the circuit. First, we need to know the current for Q3, the current source for the Q1, Q2 stage. Use the equation, IQ3=VEQ3/REQ3(15K ohms). Once we get this current, we can solve for the collector voltages fo Q1 and Q2. First, however, we need to solve for the base and emitter voltages of Q3. Obviously, the base Q3 is -15V + 30Kohms*I, where I is the current through the 30K resistor. That current is (VBQ4, 5 - .6v - 15V)/39.6kohms. If we only knew VBQ4,5, then the whole system can be solved, right?

So, Q4, 5 is 15V - I*30Kohm, where I in this case is the collector current of Q1 or Q2.

VBQ4,5 = 15V - I1*30Kohm
I1(Q1, Q2) = 1/2*IQ3
IQ3 = VEQ3/REQ3(15Kohms) from above
VEQ3 = [(VBQ4, 5 - .6v .15V)/39.6Kohms] - .6v from above

Note, I1 is IQ1,2; I2 is IQ4, 5

in the last eqn, the unknown is VBQ4, 5. One can solve for this using the other 3 equations. Substitute the 1st eqn for VB4,5 and the 2nd eqn for I1 and the 3rd eqn for VEQ3. In other words, you have 4 unknowns, VBQ4, 5; I1, IQ3 and VEQ3. And you have 4 equations, so you have everything you need to solve all 4 equations.

I would do the math for you, but I'm an engineer, and as someone correctly pointed out, I'm lazy.

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20. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,692
1,321
I think that it is to early for you to be able to analysis this type of a circuit.
You need to study some op amps internal circuits.
Because for me this circuit looks just like a very poorly design "op amp" in inverting configuration with gain equal to Av ≈ 20K/10K, the output at T7 emitter.

Last edited: Jun 4, 2013
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