Conventional flow notation

Discussion in 'General Electronics Chat' started by kaiosama, Dec 6, 2010.

  1. kaiosama

    Thread Starter New Member

    Dec 6, 2010
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    Hello,

    I am trying to learn about electricity and circuits. I am reading the first chapter of the DC ebook and it says that the flow does not really matter.

    Say i have a circuit with a LED and a resistor to lower the current before it enters in the LED, then the flow of electrons would be important because I want the electrons to go into the resistor before going into the LED because I don't want to destroy the LED.

    Can anyone please clarify this subject?

    Thank you
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    214
    I don't know which section of the ebook you are referring to but I suspect that the comment was most likely that it does not matter which way you assume the current flows as long as you are consistent in your choice throughout your analysis of a given circuit.

    hgmjr
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    It is a series circuit, so the same current exists in the resistor and the LED. Which circuit element gets the flow first is not significant - except that the polarity has to be correct to forward bias the LED.
     
  4. Wendy

    Moderator

    Mar 24, 2008
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  5. kaiosama

    Thread Starter New Member

    Dec 6, 2010
    29
    0
    Hello

    Say the electrons first go into the LED and second into the resistor. If the LED and the resistor are in series, then the current going through them is the same.

    On http://www.kpsec.freeuk.com/components/led.htm it is written that a LED must be connected in series with a resistor to limit the current that go through the LED, which implies that the role of the resistor in such a circuit is to limit the current that go through the LED.

    If the current going through the LED and the resistor is the same, then somehow the first electrons that travel when I close the circuit must be "guessing" that there is a resistor coming after the LED.

    I don't know much about electricity but this looks strange to me, can someone please explain?

    thank you
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    You might learn a lot reading in our Ebook.

    For your resistor and LED, there will be some voltage applied. LED's are essentially like all diodes in that there is a PN junction that must be forward biased in order for the device to conduct. For many LED's, that forward bias voltage is going to be close to 2 volts (the data sheet for the LED will state the exact voltage). But after becoming forward biased, the PN junction will conduct with no resistance. Current through the LED must be controlled. A fixed resistor is effective at this.

    Let us assume an applied voltage of, say, 10 volts. Some of that voltage is used in providing that forward bias, so we subtract the bias voltage from the applied, and find we have 8 volts left to drive current.

    Further, we wish to limit current to a level that will not damage the LED. Most ofter, that is 10 ma, or .01 amp. Using Ohm's law, we can find the correct value resistor. Since we know that E = IR (voltage equals current times resistance), we can rearange the equation to find the resistance - R = E/I. E is 8 volts, and the desired I is .01, so R is 800 ohms.

    Life is never so simple, of course. Standard resistor values never quite match what you need. For 5% tolerance resistors, one may obtain either 750 ohms or 820 ohms. The 820 value is probably a good choice. If you need something closer, there is a 1% tolerance value of 806 ohms, almost perfect.

    The last bit of concern is the size of the resistor. They come in different sizes because they dissipate power as they drop voltage. The formula is P = I2R (power equals the square of the current times the resistance). For 800 ohms, the dissipation works out to 80 mw, so even a 1/8 watt resistor will work.

    Frankly, ignore the electrons and just work the formulas. Electrons are supplied with energy to move by an applied voltage, and flow in a magnitude determined by the resistance in the circuit. They have no more free will than an air molecule.
     
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