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Wondering 'why'

  1. Apr 23, 2004 #1
    I've been reading, trying to learn electronics. Everything I've read so far seems to be either components or finished circuits. I want to read about the 'inbetween' process. The 'why' for each step of the design. Why use this battery. Why use that size wire. Why put a resistor here. Why use a capacitor there.

    What is the logic? What is the person thinking when designing a circuit?

    Please, can anyone recommend a web site or book that presents the 'inbetween' stuff? (Or other advice too.)


  2. jcsd
  3. Apr 23, 2004 #2


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    The in between stuff that you refer to is what an electrical engineer decides. Basic things like wire size are easy to understand with basic electrical training. 'Why' do you use a certain size light bulb in a room? 'Why' do you use a certain size engine on a lawn mower? And 'WHY' is there a smaller size air cleaner on a lawn mower compared to a car? All of the 'why' questions I asked I'm sure you are able to answer as they are fairly obvious. The questions you asked are all fairly obvious to an electrical engineer/circuit designer. Most decisions on component placement and selection are arrived at for very good reasons. I suggest if these things really interest you then you should take some classes on electronics. Any idiot can look at a circuit board and say 'yep, that's a capacitor'. But it takes a little more skill to look at it and say with certainty specifically why it is there or sometimes more importantly why it has failed instead of saying: 'Yep, it's bad' and replacing it.
  4. Apr 23, 2004 #3


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    What you're looking for is commonly called a bachelor's degree in electrical engineering.

    - Warren
  5. Apr 23, 2004 #4


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    Why ask why? :)

    There should be a logical reason for each step. Here's some basic questions:
    1. Do you understand the basic components for analog (resistor,capacitor,inductor) in terms of what they do and why they would be used?
    2. Do you understand ohm's law?
    3. Do you know about voltage/current around a circuit (Kirchoff) and series/parallel and division?

    Those aren't absolute requirements and a good circuit explanation may give you clues as to how those mechanisms work so you can learn them in process instead of ahead of time, but they are fundamental reason's as to why things are done the way they are done. Here's a background link:

    And here's an example of a schematic on that website that does a good job explaining why its setup the way it is and why each part is there:

    They have plenty up there, maybe worth looking around:

    Last edited by a moderator: Apr 20, 2017
  6. Apr 24, 2004 #5
    Thank you for your responses.

    Cliff, thank you for the links.

    F.Y.I. I don't have the option to attend school (restricted to bed mostly), so I only have the Internet and books to learn from. Any help would be appriciated. (Book titles+authors, or links to web sites.)



    P.S. To start with, I would like to build some sort of hand-crank electrical generator, and then maybe a simple motor, then maybe a digital rpm counter.
  7. Apr 24, 2004 #6


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    I found this site to have pretty good resources:


    You probable want to start with this course:

    http://ocw.mit.edu/OcwWeb/Electrical-Engineering-and-Computer-Science/6-002Circuits-and-ElectronicsFall2000/LectureNotes/index.htm [Broken]

    They have classnotes and audio to listen to. The book they use is: "Foundations of Analog and Digital Electronic Circuits" by Agarwal and Lang.

    After you master that course, you should move on to this one:

    http://ocw.mit.edu/OcwWeb/Electrical-Engineering-and-Computer-Science/6-012Microelectronic-Devices-and-CircuitsSpring2003/CourseHome/index.htm [Broken]

    There are plenty of courses to keep you busy.
    Last edited by a moderator: May 1, 2017
  8. Apr 24, 2004 #7
    The why is inventing something new, you created it to make electrical devices better or something we see on tv that you could make reality ie; a time machine. "Why use this battery?" What else are you going to use, you might need your device to be portable. "Why use that size wire?" Too small a wire for high amps is not a good combo and "why put a resistor?" Well every circuit needs a load or a resistor to restrict the amount of current going to your lamp so not to blow it up. Same with a cap, except a cap is used to filter AC eg; a DC power supply should have minimum ripples. When you use AC convert DC through a bridge it should have the correct cap to reduce the ripples and give you pure DC.

    I don't think theres really any logic behind the design, its had to be logically when time and money is involved. I prefer to install circuit breakers instead of using fuses but if I had a sponser who makes fuses. Well what choice do I have.

    Please, can anyone recommend a web site or book that presents the 'inbetween' stuff? (Or other advice too.)



    I recommend you go to Radio Shack. They have some do-it-yourself projects which explain some details. But if going to college in not an option as you mentioned the internet has a lot of great stuff.
  9. Jul 8, 2004 #8
    A book on network synthesis teaches to make circuits by requirements, meaning that the you have to make a circuit with this and the other features and you select then circuit. That'll give you the idea of why someone designed the circuit as you see it. But I think a book on NS would be overkill compared to dduardo's suggestion.
  10. Jul 8, 2004 #9
    that's a nice site... :) you sure can gain a lot from there.
    Last edited by a moderator: May 1, 2017
  11. Jul 14, 2004 #10
    Radioshack has a pretty good book called "Basic Electronics." It's a good book that actually breaks down electronics to a sub-atomic level. It's a good book to get your feet wet in electronics.
  12. Jul 14, 2004 #11
    There are a couple of aspects to your question, the first is to understand the detail of what a particular component is and what it does, it helps to do this both through reading and by practice on relatively simple 'circuits' -- if you try something too complex you can get bogged down and frustrated. I agree that Radio Shack can provide some interesting projects but to learn the why you need to read up as well.
    Actual professional why's are based on many years of 'bitter' experience. Ray
  13. Jul 16, 2004 #12
    Here's the first circuit I built: (It's not my design. I found it on the internet.)

    Code (Text):

                            | 7805|
                            |     |
                            |     |
                             | | |
                             | | |
    +7 to 30 Volts DC --+----+ | +----+---------+---- output +5 Volts
                        |      |      |         |
                        |+     |      |+        @ (LED shows power on.)
                     C1 =      |   C2 =         |
                        |      |      |         > R1
                        |      |      |         <
                  (-) --+------+------+---------+---- ground (-)

    C1 = C2 = electrolytic capacitors (anywhere between 100 and 1,000 microfarads)
    R1 = 330ohm (anywhere between 200 and 500 ohms)

    C1 takes out any ripple coming from the transformer so that the 7805
    is receiving a smooth input voltage.
    C2 acts as a load balancer to ensure consistent output from the 7805.
    Instead of a transformer I connected the circuit to my home-made DC generator (powered by an AC motor). It worked fine until the generator rpm increased and the output current exceeded some level, then the LED went dim. Decreasing the rpm brought it back to normal again. Taking a guess, I placed a resistor on the (-) wire between the generator and the circuit, and the LED stays bright at all rpm's now.

    I've learned a little bit, but I still don't understand the "why's". I know a capacitor stores power (amd releases it?) and a resistor reduces the current allowed through. But why did the designer decide in the first place to put the capacitors where they are. How is it that the capacitors 'smooth' out the current? Why is the resistor below the LED instead of above it?
  14. Jul 16, 2004 #13

    The circuit you show is a great example of something instructive and yet simple to build. But and I warn you it has a great many complexities also, which should be digested in bits.
    TRy looking through the application notes available from the regulator chip manufacturer for this or a similar chip ( National Semi is usually a good source)
    Here you may get the circuit for the regulator which is usually quite daunting
    but it will also point to some application circuits ( AC rectifiers example) and some indication on the values ( and why) of the components. They may lead you through a ripple reduction exercise (C1) , and the problem of circuit stability ( C2) .
    The LED/ resistor order is of no importance -- but there are conventions in circuit diagrams such as +ve at top , and circuit flow left to right , which engineers use without thinkng about them very much.
    In Asia circuits may look very strange due to such conventions.
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