Should resistor place before LED?

Discussion in 'General Electronics Chat' started by cht, May 3, 2015.

  1. cht

    Thread Starter New Member

    May 3, 2015
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    In conventional flow chart, we draw a circuit with LED like this:
    [​IMG]
    We put resistor here to limit the current and to protect the LED. But in reality, the current actually flows from the negative side of the battery, so in fact the electrons flow through the LED first and then the resistor. My question is why this still works? To me, it makes more sense that the electrons should pass the resistor first, so the current will be limited. In this chart, however, the electrons flow through the LED first. Why the LED won't burn up? Thanks.
     
  2. MikeML

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    Current in a series circuit is the same in all components; the battery, the resistor, the LED, even the wires between the components, so it makes no difference.

    btw, carefully read the blue part of my signature line...
     
    Last edited: May 3, 2015
  3. ScottWang

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    Although they are the same, but I always like to in series with a resistor at the first position as the current came from +Vcc power source or the battery, it will remind me to add a resistor and calculate the limiting current.
     
  4. #12

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    The force, the flow, the current, the electrons, whichever model you are thinking of, they all move at the same time and the same rate in this kind of circuit. It's a closed loop. Where, "first" is located is only a matter of your frame of mind.
     
  5. cht

    Thread Starter New Member

    May 3, 2015
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    Thanks for all of your reply. I understood now. I just think of one more concern, the voltage drop. I guess it would be better to place LED nearer to the negative pole than the resistor to avoid the voltage drop. Correct me if I am wrong. Thanks.
     
  6. #12

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    Wrong again. The voltage drops that can be measured across resistors, diodes, and batteries all happen at the same time and the same rate in every part of the circuit, in every direction. Like a litter of kittens, they all get treated equally, regardless of the order in which they appear.
     
  7. #12

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    "What happens first" is a concern for more complicated circuits with series and parallel paths, and time constants formed by inductors and capacitors. I go through the so called, "start up sequence" for these circuits. This circuit, simple series flow, with no delays involved, it's all instantaneous. There is no, "first".
     
  8. wayneh

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    Yup, it's like twirling a hula-hoop - it all revolves at once. No part of it gets ahead of another.
     
  9. cht

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    May 3, 2015
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    Guess I got it. Thanks.
     
  10. BillB3857

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    Feb 28, 2009
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    Another way to look at it would be, if the resistor is seen first by electron flow, it will limit how much gets into the LED. If the LED is first, the resistor will limit how much can get out of the LED. Either way, it's the same going through the LED.
     
  11. MikeML

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    I hate the damn water analogy, but if electrons were water molecules, wouldn't the flow be the same everywhere? Water molecules are not created or destroyed.
     
  12. wayneh

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    It is indeed like water in a hose - water flows out the end as it is replaced at the source.

    But circuits are one place where the water analogy falls apart. Water typically flows in one direction - through a pipe or down a hill - and does not recirculate inside a closed system. A lot of electronics can be viewed this way but it ignores the return ground current, and ignoring that can be a problem.
     
  13. MikeML

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    That's why I hate the water analogy...
     
  14. BillB3857

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    I'm sure glad the cooling system on my car is a closed system. Otherwise, I'd need a VEEEERRRRRY long hose?
     
  15. #12

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    The water analogy works if you call the battery a pump...
    Problem is, people in general know there is a time lag from when you turn the faucet on and when water arrives at the end of the garden hose.
    I like the hula-hoop idea. No time lag there. :p
     
  16. sheldons

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    As if this is a black art......it really makes no difference to the led .....either connect the resistor to the anode side of the led with the cathode grounded or the resistor can be connected to the cathode side of the led with the anode directly connected to supply positive.....either way the led lights with the forward current determined by your series resistor....let there be light and there will be......
     
  17. wayneh

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    [responding to #12 in #16]

    Yes, and you can even draw an analogy between inductance and the momentum/inertia of water flowing in a closed system. But the analogy gets more and more cumbersome as soon as you get away from Ohm's law. The water analogy is great for helping to explain the 3 terms. After that, not so much.
     
  18. kyu9971

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    Mar 28, 2015
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    Interesting analogy with water.
    The water can ' be compared has many things ... in the busy city the traffic is like a water in an close circuit , where it find way through get through.
     
    Last edited: May 3, 2015
  19. bertus

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