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Resistor order

  1. May 26, 2014 #1
    Hi, i've been struggling to find the answer to this questions, which are probably simple ones.

    Does it matter what order a resistor and LED ( or any component ) are in. For example:

    Vcc -----resistor------Led------gnd


    Vcc ------led-------resistor------gnd

    Are they the same? If so why?

    I don't quite get how electrons know what is coming next after hitting the first resistor for example in a voltage divider.

    Second ones:

    Let's assume that an ( a led or an led? EEE LEEE DEE ??? ) led has a forward voltage drop of 3v, and a voltage supply at 9v. No matter what resistor do i choose, the forward voltage drop on the led should be the same, but the current should vary right? which in practice isn't close at all...
    Should i expect in this case: 9v VCC -------- 100kΩ ------- LED (3v) ----- GND to have 3v on the led, 6v on the resistor and very very tiny current such that the led would not possible glow?

    But if we push 20ma of current through the led with less then 3v. Will the led get bright?

    Thanks for your time in advance, hope there are not stupid questions.
  2. jcsd
  3. May 26, 2014 #2


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    Last edited by a moderator: May 26, 2014
  4. May 26, 2014 #3


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    Order of elements in a serial path is irrelevant.

    Electrons of course don't "know" what coming up and in any case it does not matter since the actual speed of electron drift in a current is negligible (some like a meter a second, maybe slower). Current flow is like a chain. You pull the chain a little and all the links move by the same amount. Same thing with electrons in a current, with the "chain" moving really slowly.
  5. May 26, 2014 #4


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    Because the current flowing in a series circuit will be the same, regardless of where it is measured

    Note also ... tho the resistor does produce a voltage drop, its primary purpose is to limit current to the LED

    Ohhh and your example of a 9V supply, a 100k resistor and a LED

    The LED isn't likely to light up, or if it does it will be very faint .... for a 20mA, 3V LED and a 9V supply you will need somewhere ~ 4k7 to 10k for an avg brightness

    work through the formula and examples in the link given to you by UltrafastPED

  6. May 27, 2014 #5


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    How do things 'know'?
    That's the question that comes into everybody's head at some early stage in their electrical studies. The fact is that it takes time for circuits to settle down. An electromagnetic pulse travels around and along a DC circuit when it's first turned on. All the elementary questions deal with what happens after a steady state has been reached. Yes, the resistors nearer the battery may well have more instantaneous current flowing, at switch on, but that's of no concern to 'the student' or even the regular DC circuit designer.

    Consider the time when you played at damming a stream, as a child. It could take many minutes for the flow to settle down over all the weirs and through all the holes. Same thing with DC electrics. All circuits have bits of inductance and capacitance in them (not necessarily in the form of soldered in components - just in the layout).
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