Analyzing large circuits

[boogyman19946]

Hello folks!

Off the bat, this isn't really a homework question per se, but I decided it will probably fit best in here.

Anyway, I was reading through the tutorials on the forum and I was wondering, how can I analyze large circuits? A lot of circuits schematics I come across I can just barely make out what they do (I probably get more info from the title than from the circuit), but there are so many shenanigans going on where I don't really have an idea how to justify the connections the schematic presents or why certain components are where they are.

I figure that for vast majority of circuits I'm missing a few vital pieces of info, like for example I notice a lot of audio related circuits are usually peppered with capacitors in the strangest places (in one schematic I saw, three of them were connected in parallel, although one of them was placed between two pins on an IC). I'm guessing they're used for filtering or other kind of stuff for circuits that change with time.

I guess the bottom line question is... how can I get around to understand all this stuff? I have electronics at school, but I feel like I'm not getting the whole story. I've been told that capacitors store energy but never told how much energy is stored in a microfarad or how to calculate how big a cap I need to make it charge for a specific time before it cuts off current. It's more like, we're told what these components do but not really how to apply them.

I don't know if I expressed my question clear enough XD I'm just wondering how can I get around to a point where I'm in a reasonable position to design a non-trivial circuit myself? For now I'll keep on with the tutorials and I'll keep messing around with my breadboards and stuff and see what I get.

 

[davelectronic]

Hi there Boogyman. I know what you mean it bewildering in some circuits. I see it as blocks, as in different stages all play a part in the final circuit, to analyse down to the finest detail is hard i think, but start small in one specific part of the circuit, see what its doing before moving to the next stage. To do it as a whole would be to hard, well for me it would, i try to break it down, depending on voltage current applications you will learn why components value and orientations are chosen.
Dave.D

 

[duke37]

You can get round to understanding this stuff by working up gradually from simple circuits to slightly more complicated ones and to try to repair those that go wrong. Logic and method is the way to go.

You say that you have seen a circuit with three capacitors in parallel although one was connected between two pins of a IC. If one was connected between the pins, then all three were or they were not in parallel. Here is a case of getting the jargon right.

Energy.
Consider the case of a hammer, to accelerate the hammer, you need a force, F=M*dV/dT (if you know calculus) where V here is velocity and M=Mass.
The kintetic energy in the hammer is M*V*V/2. If the hammer is slowed rapidly, say by hitting a nail (preferably without a thumb attached) the force will be quite high, in my case, it will be high enough to bend the nail!
With an inductance, the voltage across the inductance V=L*dI/dT and the energy in the inductance is E=L*I*I/2. If the current is reduced rapidly, the voltage generated can be very high which is why a diode is placed across a relay coil to protect a driving transistor.
With a capacitor, the current flowing into the capacitor I=C*dV/dT and the energy in the capacitor is E=C*V*V/2, thus one can work out the voltage change for a specific current into or out of the capacitor.
A tuned circuit consists of a capacitor and inductor in parallel with the energy swapping backwards and forwards between them. Equating the two energy equations, you can get a relationship between current in the inductance and voltage across the capacitor.

Hope this helps, Duke

 

[boogyman19946]

Hey guys, thanks for the replies!

@davelectronic, You mean in general you look at a circuit in terms of groups of different functions? Like, breaking down a counting circuit into a portion that generates the clock and a portion that uses the signal from the clock to count on each cycle? That's not so bad an idea, I'll have to try it some time.

@duke37, I guess that's the case with everything really :] You must understand algebra before you do calculus, but that's just a normal progression.

Also, you are perfectly correct about those three capacitors! I actually haven't noticed that because when I was writing out the schematic (I was transferring a physical circuit onto paper rather than design it myself XD), the capacitor ended up looking like it's connected between two pins of an IC. All three capacitors, in reality, were connected the same way! XD I completely missed that.

I'm not sure I understand the portion about the hammer, inductance, and capacitance. It's not the math part, I'm just not sure what you're trying to illustrate with it.

 

[davelectronic]

Yes that breaking down the components function best works for me, plus some maths, but i dont go as far into analysis as Duke simply because my maths is not that good, but i can break down the bigger circuits, mainly in my reclaiming of stuff from electrical items destined for scrap or the tip etc, its unfortunate my maths hold me back a bit, some time frustratingly annoying, but i still enjoy what i can do.
Dave.D

 

[duke37]

You asked about the energy in a capacitor and the choice of capacitor size so I thought I would give three similar equations for energy, the hammer analogy is very good to understand the voltage when switching off an inductance and the necessity of using a flywheel diode or capacitor to limit the voltage.

The equation I=C*dV/dT is used to determine the size of a reservoir capacitor, re-arranging C=I*dT/dV, so if the current is 1A and 1V is the permitted ripple and the capacitor is charged with a full wave rectifier from 50Hz mains, then dT = 1/100.
C = 1 * 1/100 / 1 = 1/100 F = 10mF = 10000uF

More than one capacitor may be connected in parallel since large capacitors tend to have too much internal inductance to make them work well at high frequencies.

Duke

 

[boogyman19946]

Meh, I really wouldn't sweat it all that much. From my experience, knowing higher level math is most definitely helpful, but in a lot of times it isn't exactly required. Take for example my physics class. For the entire first quarter and stretching into the second one, we were covering acceleration, velocity, and displacement. The relationships between them (at least the ones we learned about up to this point) can be described in calculus terms in less than 15 seconds, from which the student could use differentiation and integration to solve the problems give in class, and even extend further outside the limitations of the algebraic formulas. While everyone else struggled to memorize the formulas, I, being fairly proficient in differentiation and somewhat ok with integration at that point, used calculus to solve virtually every problem and more, and my answers weren't all that different from the ones given by the teacher, but the bottom line is, did I really need calculus to do it all? Not really, but it most certainly helped ;]

Yet then again, when it comes to applying math, I wouldn't recommend doing things by hand when the problems get really hairy. The more variables you have to keep track of and the more calculations you need to do, the more you should use a computer to solve the math for you, unless you want to try yourself in patience and accuracy. Going back to my physics class, after about a week of using calculus to solve all the problems, the problems kept using the same formulas over and over again so involuntarily, I eventually memorized them and didn't have to integrate/derive anymore because I already knew what was going to be the result. So in a vast amount of cases, the simpler you keep your math, the better

If you ever need math help, though, I recommend www.khanacademy.org . Khan covers a lot of topics in math and makes virtually everything understandable despite the level you're on.

 

[boogyman19946]

You asked about the energy in a capacitor and the choice of capacitor size so I thought I would give three similar equations for energy, the hammer analogy is very good to understand the voltage when switching off an inductance and the necessity of using a flywheel diode or capacitor to limit the voltage.

The equation I=C*dV/dT is used to determine the size of a reservoir capacitor, re-arranging C=I*dT/dV, so if the current is 1A and 1V is the permitted ripple and the capacitor is charged with a full wave rectifier from 50Hz mains, then dT = 1/100.
C = 1 * 1/100 / 1 = 1/100 F = 10mF = 10000uF

More than one capacitor may be connected in parallel since large capacitors tend to have too much internal inductance to make them work well at high frequencies.

Duke

Oh alright! So, to make sure I understand this correctly, if an inductor is being operated at a point where the current may vary rapidly (such as the mentioned relay switch), it's inherent property of generating reverse voltage (in order to resist change in current) can sometimes become too much for other components right? So using the equations, one can calculate how much energy, or otherwise voltage, will be generated by the inductor in order to determine what kind of a capacitor or diode to place to limit it. Am I right in that assumption?

 

[davelectronic]

I only use what i need in the maths, and thankfully its not the really hard stuff, i probably could go further than i am already, our local college offers free courses for higher level maths, as i have life long illness the course for me would be free, but at the moment i dont have the stamina to go the distance needed.

If the future is favorable for me then the option exists for me.
In a way if you've got it but dont use it, then its always there if you do need to call on your higher level of maths.
But if you need it but have not got it, like me your stuffed, lol.
My practical skill far out weighs my maths mental theory, i understand what it all means including psychics, but struggle to put it out on paper to show how i arrived at that figure as strange as it sounds, for me its like looking at a foreign language when it gets really complex.
I know its not, and understand all the terminology and its real time application and changes, electromagnetism is one example, i understand all about magnetic circuits as i like building psu's when my health will let me, synchrony of AC motors leading lagging current / voltage etc, it all makes sense DC analogy and application inductance capacitance resistance even laser theory and more, ive read loads of books, i can break down a modern jet engine and explain the physics there, but i cant read it nearly as well on or in a formula maths format, or write how i arrived at that description point.
Not to worry i am not going to lose sleep over it, i can use the maths needed to work out the majority of circuits, if the more complex stuff comes later, i will have to wait and see.
Dave.D

 

[duke37]

Basically yes but inductors do not generate energy, they store it.
The diode across the relay coil will pass current until the resistance in the coil will dissipate the energy and the current dies away.
If a capacitor is placed across the coil, on switch off, the energy in the coil will go into the capacitor and a voltage will be generated which can be much more than the power supply. This is the principle used by boost convertors and car ignition systems. In the ignition system the inductance (called a coil) is effectively in parallel with a capacitor (called a condenser) to generate about 300V. A secondary winding with 20 times the primary turns thus gives 6kV to provide the spark.

Duke

 

[boogyman19946]

I would give just about anything to have the ability to take, what my teacher calls, "a leap of faith" and assume things work. Unfortunately, my brain is wired in very rigid "must know why" format where I would be more fond of knowing why something works the way it does rather than how to readily apply it's function. The best way I can say it is that I have a hard time accepting abstractions.

 

[davelectronic]

That leap of faith for me is ( ive got to wire it and try it ) even if i mess up, and i have.
Playing with and modding an atx psu last week i got zapped with 240 volts mains AC , sloppy carelessness, my own stupidity, but i have no problem with practical stuff, fire up a microwave oven transformer for current unloading testing, fine not a problem, i know how things work, but fail the ability to show a lot of it on paper. ive had capacitors explode up on me, circuits go bang, oop's , oh well i move on learned that dont work, let alone explain it on paper in complex equation format.

Take the leap of faith in your own time, but believe in your own judgement.
Dave.D

 

[boogyman19946]

Basically yes but inductors do not generate energy, they store it.
The diode across the relay coil will pass current until the resistance in the coil will dissipate the energy and the current dies away.
If a capacitor is placed across the coil, on switch off, the energy in the coil will go into the capacitor and a voltage will be generated which can be much more than the power supply. This is the principle used by boost convertors and car ignition systems. In the ignition system the inductance (called a coil) is effectively in parallel with a capacitor (called a condenser) to generate about 300V. A secondary winding with 20 times the primary turns thus gives 6kV to provide the spark.

Duke

I think I have an fair idea of what's going on. A question for my own sanity though, doesn't the main current (the one not generated by the inductor) charge the capacitor? So if the inductor was to release the energy, wouldn't the capacitor be at it's limit? Or does the capacitor release the energy while the inductor is transferring its energy to the capacitor?

 

[(*steve*)]

I would give just about anything to have the ability to take, what my teacher calls, "a leap of faith" and assume things work.

DON'T

It's a bad thing.

Faith leads you to believe stuff that isn't true.

Learn *why* something operates as it does. It will serve you better in the long term.

 

[davelectronic]

What i meant is if you do the maths and the equations work on rock solid theory then take the leap of faith and believe in ones own ability to carry out a circuit construction and finally function and practical analysis.

My comments on things going bang is personal experiences, not that i am proud of my failures as i am not, i had an Uncle an electrical engineer, he tried or should i say drummed the safety aspect of electricity into me.

I would tell him of my latest venture, and get the lecture for my own good. So the leap of faiths meaning to me is believing in ones self, not over confidence or disregard for safety.

You or i can achieve anything if you put your mind to it, this i truly believe that .
Dave.D
PS, If it stays on paper to scarred to try your never know, but of course with the utmost safety in mind when you do go practical.

 

[(*steve*)]

Dave, what you mean by leap of faith is not "leap of faith".

You are saying, base your beliefs on experience and evidence -- exactly the opposite of faith. Perhaps your leap of faith (if you have one) is that your experience is good and your evidence valid.

Your understanding of electronics can come from theory or practice (but it's almost always a combination of both). People who are stronger in mathematics tend to be able to gain more understanding from equations. Some people need to "see it in action". Others need to take it apart, break it, fix it, and test it to destruction.

For example, consider the example of three capacitors connected in parallel in a circuit.

Based on a very basic understanding of capacitors I might think that I could replace them with a single capacitor having a capacitance equal to their sum.

But practical experience, and greater understanding that I've accumulated tells me otherwise. For example, I'm aware that real capacitors have resistive and inductive qualities that theoretical capacitors don't. Because of this, two capacitors in parallel may behave far differently than one. I also know that wires have resistance and inductance (and capacitance), so that it may actually matter where I place the capacitors. 10 0.1uF capacitors scattered around a circuit may be strategically placed.

Whilst you will never know everything, you need to have a mental working model of a component (that faith doesn't give you). When you learn a new thing (say about voltage ratings of resistors) you can add that to your mental model and incorporate it into your understanding. Without that understanding it is just another fact that can't actually be applied.

Faith = "I believe in the absence of (or even in the presence of contradictory) evidence".

You will search long and hard without much success for "faith-based" electronics courses, and there is a reason for this.

 

[davelectronic]

No i understand what you mean Steve. My faith is as your post starts first line or so in, faith in my own belief that i can do it from with in myself, not a higher power, i wont go down the religion road, but put it this way i am not atheist, so yes i have two faiths, the first my own self belief is what i was meaning in leap of faith, but yes i totally understand your explanations.
Dave.D

 

[(*steve*)]

I'm not talking about religion. I'm talking about electronics. (Just to make it abundantly clear that I am not arguing about people having faith in other things)

 

[(*steve*)]

To return more to what the original poster was talking about..

Think "building blocks".

Gain an understanding of a voltage divider (for example) and whenever you see that in a circuit you'll know what to expect.

Learn a number of other building blocks (say a common emitter transistor amplifier -- which contains 2 voltage dividers (one slightly hidden)) and you can apply your knowledge more widely.

Eventually you'll be able to look at a moderately complex circuit and see what it does. At the very least, you'll be able to quickly determine what sections are not immediately obvious and require further thought.

You will find that more complex devices often are described at some level with block diagrams. In these cases the blocks tend to be chunks of the device that you can understand in isolation -- in this case the designer is breaking apart the device into those easily understandable parts to assist you with understanding the whole.

The design process proceeds similarly. At some level you decide what circuit functions you require and you pull them out of your bag of tricks. People with more experience may have a larger bag of tricks and/or they may understand those tricks better.

 

[davelectronic]

Hi again Boogyman. just to clarify, my and i believe your tutors leap of faith is in the ability of your self beliefs, and is no relationship to any religious beliefs, although all individuals are entitled to any religious belief they choose, so not to sound odd, but i am christian protistant, but my religious views wont help me build circuits, my own experience and self belief will.
Dave.D