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Newbie electronic resistance question

Discussion in 'Electronic Design' started by ragtag99, Aug 2, 2006.

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  1. ragtag99

    ragtag99 Guest

    Thanks for taking the time to look at this post. I am new to the field
    but we all had to start at some point somewhere right.

    I'm trying to study on my own and I came across this web page
    http://library.thinkquest.org/10139/small/electro4.html and the very
    last question, just scroll all the way to the end and theres a
    'challenge question', I don't know where im going wrong. My answers
    aren't the same as the answers given. Could someone instruct me on how
    to solve that very very primative cirucit.

    Also could someone give me a practical example of why you need to know
    Ohms law, which i am studying on faith alone right now. Would it be
    something along the lines of "I have X component which runs on Y amps
    but I have 3Y amps running though, what resistance would i need" or
    something like that? Or am i way off.

    Thanks for any help.

    Jesse
     
  2. Jim Thompson

    Jim Thompson Guest

    The answer given for question 5 IS correct.
    Please post on newsgroup sci.electronics.basics

    ...Jim Thompson
     

  3. Jessie, newbie questions should be asked in
    <

    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  4. In multiple choice questions where you have no clue as to what they
    are on about, always pick the longest answer.


    Best regards,
    Spehro Pefhany
     
  5. leeps

    leeps Guest

    Im not going to bother with the link just yet, im going to help you
    with why you should learn ohms law. Actually you shouldnt learn ohms
    law you should understand electricity and in turn come to understand
    ohms law, it should just be 'common sense'.
    If you want some examples with why you would need it.
    You want to save money by using the smallest resistor possible.
    What do you need .25 watt .5 watt or 1 watt or 5 watt
    well if you know the voltage and the resistance you can figure out
    current and with the current and the voltage you can thus figure out
    wattage watts=volts*amps or watts =resistance*voltage squared
    or you have an analog to digital converter that will only take 5 volts
    but your signal goes up to 12 volts. What do you do, have no fear, for
    ohms law is here!
    use a voltage divider and knock that voltage down
    divide by 4 now your giving your a/d 3 volts not 12
    see it comes in handy
    what you should be doing is studying whats a volt, whats an amp,
    whats inductance, whats capacitance whats resistance and in doing so
    ohms law will become "like yeah DUH i didnt need them to tell me that"
     
  6. C.
     
  7. You have to essentially work from the inside, outward.
    Solve the easiest bits first, and build your way up to the final
    circuit. The previous questions are designed to give you this skill.

    There are obviously three resistors in parallel (2,3,5ohm), so solve
    them first, that gives 0.9677ohms

    Next solve the two resistors in series, 10ohm + 12ohm = 22ohm

    Now you'll notice that the 3 resistors you solved first are in parallel
    with the two series ones you solved second, so solve 0.9677ohms in
    parallel with 22ohms, that's 0.9269ohms.

    This total value of 0.9269ohms is then clearly in series with the 3
    ohms resistor on the right. So 3 + 0.9269 = 3.9269ohms

    Hope that helps.

    Dave :)
     
  8. ragtag99

    ragtag99 Guest

    yes that helps immensily. I was way out there using the resister on the
    right as a series with the middle resister in the center and all other
    sorts of lunacy. Now i see where i was wrong in so many situations
    thanks a lot.

    for now on ill take everyones advice and post in the .basic newsgroup
    (this was the first one that google came up with :/ ) Ill be posting
    here soon enough though ;)

    thanks again

    Jesse
     
  9. Great!
    Now it's time to move onto the famous resistor cube problem:
    http://www.radioelectronicschool.net/files/downloads/resistor_cube_problem.pdf

    *evil grin*

    Dave :)
     
  10. You should post to sci.electroncis.basics
    which is specially for helping new people.
    What answers do you get? That may help to detect where you are going
    wrong.

    The three low-value resistors in parallel are 2 ohms, 3 ohms and 5 ohms.
    Maybe you misread the values.

    The way to calculate the total resistance in your head is not to use
    directly the formula 1/R = 1/R1 + 1/R2 + 1/R3 but an equivalent:

    2 x 3 x 5 = 30. 2 ohms = 30/15, so 1/(2 ohms) = 15/30. Similarly,
    1/(3ohms) = 10/30 and 1/(5 ohms) = 6/30.

    Then 15/30 + 10/30 + 6/30 = 31/30. This is 1/R, so R = 31/30 ohms.

    Then the series branch: 10 ohms + 12 ohms = 22 ohms.

    22 ohms in parallel with 31/30 ohms: Work in the same way as for the
    three resistors: 22 x 30 = 660. 30/660 + (31 x 22)/660 = 712/660, so the
    resistance is 660/712 = 0.927 ohms (use calculator)

    Add the 3 ohms in series to get the total resistance = 3.927 ohms.
    I can understand that view. The trouble is that you need to learn more
    about practical circuits before you could understand why and how Ohm's
    Law is used. Some examples on the web page, especially fig 3, rarely
    come up in real life.
    You are way off. Sorry.
     
  11. The mind boggles. DON'T, whatever you do, EVER get a teaching or
    training job.
     
  12. leeps

    leeps Guest

    Actually i do a pretty good job when the person is in front of me. But
    yeah i agree after i read what i wrote i know i didnt do much help. I
    hope at the very least he'll take my advice and look up the voltage,
    amperage , etc instead of just learning ohms law cause thats what your
    supposed to do
     
  13. English may not be your first language but you write it as if you know
    it well. So why not go the further step of punctuating properly and
    writing 'I' instead of 'i'?

    You give the impression of semi-literacy.
     
  14. PeteS

    PeteS Guest

    Amperage?

    Is that the amount of '&' per line?

    or did you mean current?

    ;)

    PeteS
     
  15. In message <>,
    No, that's 'ampersandwich'
    He's just paying ohmage to technician-speak.
     
  16. leeps

    leeps Guest

    I suppose I deserve it. The thing is i grew up in the states and spent
    the last two years in france and i hate to say it im forgetting some of
    my english. im actually starting to put the adjective after the noun
    sometimes when i speak english ill say things like food hot or girl
    blond. Its actually starting to scare me.
    but hey give a guy a break im trying to help
     
  17. Lord Garth

    Lord Garth Guest

    One of my pet peeves as well!
     


  18. I didn't think you were supposed to pet peeves?


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  19. Sjouke Burry

    Sjouke Burry Guest

    Watch out, they might bite,especially when they
    have been estivating.
     
  20. ehsjr

    ehsjr Guest

    You got the answer to your other question.

    A practical example is putting a resistor in series
    with an LED, so that you won't burn the LED out.
    Say you have a typical LED, which has a maximum current
    rating of 30 mA, and you want to light it from a 9 volt
    battery. The LED uses 1.8 volts, so you need to drop
    7.2 volts in the resistor. E=IR, so 7.2 = .03 R, therefore
    R = 7.2/.03 or 240 ohms.

    Ed




    Would it be
     
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