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Voltage divider - Looking for a reference with respect to Vout resistnce

  1. Mar 24, 2012 #1

    Femme_physics

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    In voltage divider, how high must the resistance on Vout be so that voltage divider can be used according to convention? I'm looking for official references..

    If anyone can help I'd appreciate it.
     
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  3. Mar 24, 2012 #2

    sophiecentaur

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    This is a 'practical' question? You would need to do the sums for each case.
    The value of the 'load' resistor needs to be added in parallel with the resistor across which you are measuring the Vout. The 'bottom resistor' of the pair in the divider has been modified. The answer to "how high" that resistor must be depends totally upon the accuracy that you require.

    If the load is 100 times the resistance of the potential divider resistor then the error will be about 1% - because the effective resistance has been reduced by about 1%.
     
  4. Mar 25, 2012 #3

    Femme_physics

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    Do you have a resource for that?
     
  5. Mar 25, 2012 #4
    Why'd you need a resource for that? Is not the thing simple enough?

    voldiv.gif

    The relative error will be as follows:

    [itex]\Delta[/itex]V/V = [itex]\frac{|V_{out-load} - V_{out}|}{V_{out}}[/itex] = |[itex]\frac{R_{L}}{R_{L}+R_{1}||R_{2}}[/itex]-1|

    I guess now you can figure out that to reduce the error all you need to do is either make RL very large relative to R1||R2 or make R1||R2 very small relative to RL.
     
  6. Mar 25, 2012 #5

    Femme_physics

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    If it's an open circuit-- how do u know there's no load? Open means that you don't know what's attached to it.

    And if there's no load, doesn't it mean that the current won't bother to flow into R2 resistance? It'd much prefer to flow through the region where there is no load.

    As far as to why I need the resource...I just want to do proper research of the information I get on the forum :)
     
  7. Mar 25, 2012 #6

    sophiecentaur

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    Any textbook is sufficient resource. What you need is actual experience of doing some calculations to see what happens.
    I assume that you can work out 1. the equivalent resistor for two in parallel and I assume that the 2. potential divider equation is familiar.
    Just substitute some numbers in there and you will get a feel for it. There is no definite answer as to 'how much' error is acceptable.

    ??? Since when did current have any preferences? The expression 'No Load' means an Infinite Resistance so the voltage on the Potential Divider would not be affected.
     
  8. Mar 25, 2012 #7

    Femme_physics

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    I'll accept this :)

    In a shortcircuited scenario it has a preference to flow where there are no resistors.

    How can "no load" mean infinite resistance? Load by definition means burdens, or something which weighs down on or oppresses! Saying no load means no resistors, not infinite resistance, IMO!
     
  9. Mar 25, 2012 #8
    A load is useless if current does not flow through it and produce work. In infinite resistance no current flows through - thus "no load".
     
  10. Mar 25, 2012 #9

    I like Serena

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    I believe that as a "rule of thumb" 2% error is considered negligible in engineering best practices (sometimes 5%).
    However, I can't find such a rule of thumb with google (yet).
     
  11. Mar 25, 2012 #10

    jim hardy

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    That's wonderful ! I love it !! How logical !:approve:

    It's electrical jargon - no load is generally accepted to mean unloaded, unburdened, unattached and unencumbered, as opposed to undefined...
     
  12. Mar 25, 2012 #11

    sophiecentaur

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    This is just history; you just need to adjust your viewpoint to account for someone else's reasoning, I think. No power is delivered to an infinite resistance. The other extreme would, of course, be a Short Circuit, which would be drawn on a diagram as a line joining the two points.

    Other possible confusions are "Open Circuit" which means no way through, whereas " Road Open" means traffic can get through. When a switch is 'closed' current can flow but a Road Closed sign means traffic can't. Language is a funny thing. :smile:
     
  13. Mar 25, 2012 #12

    Femme_physics

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    Thanks, I accept :)

    :wink:

    Hmm... then according to your logic it appears that 'No load" means the load that the CURRENT applies on the Vout. If it's no load, then it means there must be infinite resistance. Does that sound right?
     
  14. Mar 25, 2012 #13

    NascentOxygen

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    Load can be expressed in terms of current, yes. Example, a load of 3 amps.

    So "no load" is a load of zero amps. And only if the load is infinite ohms can you have zero current flowing while you supply voltage to it.
     
  15. Mar 25, 2012 #14

    Femme_physics

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    Aha! :biggrin:

    Got it! o:)
     
  16. Mar 25, 2012 #15

    sophiecentaur

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    And as for "how much is an acceptable error?", the answer depends upon the application.
    If you wanted to make a reference voltage for measuring some other voltage to within 0.1% (not very common but some people are into that sort of thing) then your 'load' would need to be at least 1000 times the value of the bottom resistor. For use in a domestic room thermostat, you would only be interested in a percent or so - so your precision would reflect this - a higher precision would cost too much in time and effort.

    Strictly speaking, the word Load really refers to Power, rather than current. A 2kW load in the US involves twice the current of a 2KW load in the UK.
     
  17. Mar 26, 2012 #16

    psparky

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    If your truly interested in learning......click on the following link....starting on page 2. I attempt to explain the basics of electrical theory.....including your voltage divider. Give it a good read and your understanding will likely go up. Enjoy.

    https://www.physicsforums.com/showthread.php?t=588411&page=2

    Start with post #30.......V=IR
     
  18. Mar 26, 2012 #17

    sophiecentaur

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    In the same way that you wouldn't expect ot wave your arms about discussing the total on your restaurant bill, simple electronic circuits are far better discussed using Maths. Spreadsheets (even free ones) are so good for evaluating formulae and plotting graphs with varying values of volts, resistance etc. There really is never a good 'chatty' way of describing how all these simple circuits operate. If you don't bite the mathematical bullet with circuits then you may as well accept that you'll never really 'get it'.
     
  19. Mar 26, 2012 #18

    psparky

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    Yes....there is a good way to discuss with chat AND math. I have done that.

    Math is great....but even better with a REAL explanation. Teaching while pointing to a formula is lame......dry.....and boring. In my opinion.
     
  20. Mar 26, 2012 #19

    sophiecentaur

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    Unfortunately, we don't have 'real' equivalents in our heads for convolution, cross correlation, Fourier Transforms or, in fact, any of the operations that are vital when dealing with some of even the most elementary processes in Science. We do often 'wave our arms about' from a position of already being familiar with a certain level of the Maths in order to get to the next step, of course. In such a dialogue, however, it may be that only one of the participants is not actually way out of his depth. (Been there)

    Imo, it is actually bebasing Physics to think that it can be understood using just a few 'schoolboy' terms and concepts and no Maths. Many of the interchanges I read on these fora are the result of misconceptions which result from the reluctance or inability to appreciate just what the Maths is telling us.

    Your 'real' explanation, to my mind, is one that can suit you, personally, but is more of a self-affirming argument and it can be a final step in getting to the next level of (feel -good) understanding. So many people want to be able to leap in with a "So what about this particle nature" or a "The Fourier Series is just . . . . ." and they are very likely just fooling themselves. The proof of the pudding is when they can make the next step of a valid prediction, based on what they feel they know.
    I am only too well aware of my limits in this respect.
     
  21. Mar 26, 2012 #20

    psparky

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    This stuff is great....it's just that it only suits you genuis's. For the rest of us.....98% of us....this stuff is totally useless at our jobs in real life. For you 2%'ers....have a blast. You've earned it.


    I agree to some extent.

    I agree to a certain extend here as well....but me simply laying out the BASICS of EE.....is just me trying to lay a foundation for all the rest.

    You and I typically disagree on most things.....I'm ok with that. You are clearly much older than me and have way more experience. We both make valid points.....it's up to the reader to decide who's points' interest them more.
     
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