Series Circuits(LEDs)

Discussion in 'General Electronics Chat' started by vindicate, Jul 24, 2009.

  1. vindicate

    Thread Starter Active Member

    Jul 9, 2009
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    Here is what I don't get. You have a 9V PSU. 3 Red LEDS with a Voltage Drop of 2V and a Forward Current of 20mA.

    To figure the resistance needed 9-6 = 3/20mA. 150ohm resistor.

    Ok so that is 20mA for the whole circuit. How is it that they all share the same current? It seems like it should be 3/60mA. So each gets 20mA. Doesn't current get used up to make the LED's light? Like if you have 20mA in the whole circuit and you use that to power 3 LEDs it seems to me that each would get 6.66mA. Can anyone explain this?
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    Each does get 20 ma. It is a series circuit. One of the definitions of series circuit is that each element in the circuit has the same current. There is no path for current to follow except through each circuit element in turn. If you draw the circuit, you will see that this is so.

    Current is not "used" to illuminate a LED. Once the internal junction becomes forward biased enough to conduct, the internal structure causes electrons to traverse an energy gap. The excess energy they have after going across is shed in the form of photons. The energy gap (not technically correct, but an illustrative term) determines the amount of excess energy, and therefore the color of light emitted.

    You can slightly change the color by increasing current through the LED, but it is most common to control the output intensity by limiting current.

    As far a the circuit goes, current through the 150 ohm resistor makes the voltage dropped across it equal to 3 volts. Each LED drops 2 volts. The total voltage drops add up to 9, which agrees with the total voltage out of the power supply. The energy used is the power in each device, given by E times I. That is 60 mw in the resistor, and 40 mw per LED - 180 mw for the total circuit.
     
  3. vindicate

    Thread Starter Active Member

    Jul 9, 2009
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    Is this true for everyhing? What about 2 speakers in series, or 2 Fans in series? 2 IC's that need power to function. None of these actually "consume" current? Is there anything that does "consume" current in a series circuit?
     
    Last edited: Jul 24, 2009
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    How would define "consuming current"?

    Current is the flow of electrons in a conductor. None of the electrons are used up, or disappear out of the circuit.

    In your examples, not to be overly fussy, it is more conventional to have the items in a parallel arrangement. Especially with regards to the fans and IC's.
     
  5. vindicate

    Thread Starter Active Member

    Jul 9, 2009
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    So neither current or voltage is ever consumed? Or is voltage drop what would be described as "consuming" voltage?
     
  6. beenthere

    Retired Moderator

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  7. vindicate

    Thread Starter Active Member

    Jul 9, 2009
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    Yeah I did read that actually. :)

    As for how do I define consumed. I mean like gone, not able to be used by anything else. Like burning gas in a car, used to make power and then it's gone.
     
  8. millwood

    Guest

    think of the current as workers. they start the day fresh out of the power source. full of energy. they go on and turn on the 1st LED, that got them tired a little bit.

    from there, they go on and turnn on the 2nd LED, tired more.

    by the time they turned on the last LED, they were totally exhausted and couldn't work anymore. so they went back home to get recharged for the next day.

    end of story.
     
  9. vindicate

    Thread Starter Active Member

    Jul 9, 2009
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    When you talk of them "getting tired" and "working" are you referring to voltage and voltage drop?
     
  10. rjenkins

    AAC Fanatic!

    Nov 6, 2005
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    Power (in Watts) is Volts x Amps. As each component in your circuit has a voltage drop across it, it is 'consuming' power.

    One analogy would be a gate controlling water flow down a river (the resistor) then waterwheels in the river (the leds).

    The current is exactly the same all the way through, but the power depends on how much flow and what height of water there is available to drop at each wheel. More height (pressure, voltage) means more power available.

    You can keep using power from the current until you get to sea level (= zero volts).

    Hope this makes some sense..
     
  11. ELECTRONERD

    Senior Member

    May 26, 2009
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    Ha, ha, ha...I have to say that was a great story Millwood! :D
     
  12. Wendy

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    Mar 24, 2008
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  13. going_in_deep

    Member

    Jul 25, 2009
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    this is the principle
    IN SERIES CURENT WILL BE IN SAME QUANTITY , SO IF 20 MA IS THE CURRENT CUMMING OUT FROM THE 9V BATTERY THE ALL THE 20MA WILL BE SUPPLIED TO THE 3 LEDS AND THE RESISTER WHICH R IN THE SERIES CONNECTION.
     
  14. indianhits

    Active Member

    Jul 25, 2009
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    so can i ask why use resistor to CONSUME(oppose) current since as above mentioned current remains the same at every point so even if resistor resist the flow.Doesn't current be same...so please help me out.
     
  15. beenthere

    Retired Moderator

    Apr 20, 2004
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    Current is not consumed nor opposed. The resistance determines the total current so the LED's don't burn up. The formula to use is I (current) = E (voltage )/R (resistance).
     
  16. Wendy

    Moderator

    Mar 24, 2008
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    Resistance isn't futile, it's necessary!
     
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