Why use resistors on LEDs?

Discussion in 'Electronics & Electrics' started by alvarez, Sep 14, 2006.

  1. alvarez

    alvarez Member

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    Iv been adding Leds to most gadgets i own im now pretty handy with a soldering iron, but my wireing is always messy,

    Everywhere Ive read says to use one resistor to each positive terminal on each individual LED would it not be neater to put say 2 LEDs to one resistor in series?

    Is this acceptable why do we use resistors for each LED and would I still need a resistor if the power source was rated the same as the LED?

    Thanx for clearing this up I think it will work but Ive only done basic electrcity in Year 10 so Id feel better if you guys cleared me on this,
     
  2. Stygian Shane

    Stygian Shane Member

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    A led is a diode. A diode is essentially a one-way wire, but in a LED's case; it emits light at current passes through. Don't use a resistor, and it's effectively a short circuit, which is quite obviously NOT good for whatever's running the led.
    (Someone please correct me if I'm wrong!)
     
  3. dazzawul

    dazzawul Member

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    An LED has a certain amount of resistance, when you apply a voltage across it, according to how much resistance it has, it will draw that much current, (you could run a 12 volt device off of a car batt. if you wanted, as long as the voltage is right)

    LEDS are meant to run off of 3.4 volts or something like that, when they are fed more than that, they draw more current than they should, so they heat up (which affects their resistance, lowering it, making it pull more current, which makes it heat up more, thermal runaway, and why LEDs burn out :p) thats why you wire a resistor in, to lower the voltage its drawing and keep it from dying..

    Buuuuut, I got another question, how the hell do 12v LEDs work? :p
    (like in cars, is it a few diodes in series packaged together, or just a normal LED with a resistor built in )

    Cheers lads.
     
  4. dakiller

    dakiller (Oscillating & Impeding)

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    Wikipedia sum it up pretty well
    To sum up, Variations in the forward voltage bias of LED's will mean that you wont get an ideal current sharing between them
     
  5. dakiller

    dakiller (Oscillating & Impeding)

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    Just about all of what you wrote is wrong, LED's don't have resistance, they have diode like non-linear I/V characteristics
    They have a built in resistor
     
  6. ener

    ener Member

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    i though it was to protect the leds from any flucuations in the elctricity passing thru them
    (if thats what the wiki said then im sorry but i hate reading quotes i read half of it)
     
  7. GooSE

    GooSE New Member

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    Read what dakiller has posted. Basically it's just to stop over-current. If you leave the resistor out, the LED will draw enough current to destroy itself every time.

    To answer the question in the original post regarding using multiple LEDs with a single resistor... It will work as long as the resistor is able to dissipate enough power. To calculate this:

    P = V * I, where P is power (Watts), V is voltage drop across the resistor, I is current in Amps.

    Common resistors are either 1/4W or 1/2W.
     
  8. dephilile

    dephilile Member

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    Be very careful using one resistor on multiple LED’s. since LED’s are CURRENT devices, NOT voltage devices slight variations in how they were manufactured gives them slightly different characteristics. What can potentially happen is all the current will flow through one LED. It will burn out. Then the current path will move onto the next LED and burn that out until there are none left. Also note that characteristics of LED's change with temperature.

    If you wish to power multiple LED’s with one resistor it’s better to put the LED’s in series and have one resistor at the end of the LED string.

    EDIT - if you say for example have a microcontroller and want to attach a number of LED's to different pins you have to have multiple resistors. one for each LED. NEVER RELY ON THE CURRENT LIMITING OF THE MICROCONTROLLER.
     
    Last edited: Sep 14, 2006
  9. Elder

    Elder Member

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    http://www.overclockers.com.au/wiki/LED

    This was started by Goth, unfinished section started by me.

    My section shows the configurations you can use. The blank sections were going to be filled with a basic description of practical limitations (supply voltage limiting number of leds in series, supply current limiting number of leds in parallel, failure cases, etc/whatever)

    Someone else can jump in and write in my section if you want :p I have no motivation to complete it because I'm setting up & managing my works wiki now.
     
    Last edited: Sep 14, 2006
  10. Butcher9_9

    Butcher9_9 Member

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    i got some leds from pc case gear
    there was 3 leds in series and no resistor (they where prewired)
    will they die ?
     
  11. dazzawul

    dazzawul Member

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    Crap I should pay more attention in physics :Paranoid:
    (though I blame my metalwork\electronics teacher for not teaching us the right thing in the first place :lol: )

    But if theyve made it prewired it should be fine..

    Cheers lads.
     
  12. dephilile

    dephilile Member

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    i'm guessing they are running off 5 volts? hard to say without having access to a datasheet for them. if you run more power than they are designed for through them it will shorten their life.
     
  13. Butcher9_9

    Butcher9_9 Member

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    i would have said 4 volts but i know shit about electric stuff

    3 x 4 = 12
     
  14. azzachaz

    azzachaz Member

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  15. TRG.dOinK

    TRG.dOinK Member

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    http://forums.overclockers.com.au/showthread.php?t=433137&page=3&pp=15

    I've explained on page 3....

    You need a resistor to...
    A. Limit current so the led does not blow up/destroy itself.
    B. Have a voltage drop across the resistor.

    Leds do not run off 12 volts. Each colour is different. Red is around 1.8V to 2V. Blue was around 3.5V from memory, etc etc etc.

    Say you have a 12 volt car battery. You connect the red led straight across it. The red led does not like 12 volts across it and will draw too much current. It only wants 2 volts. Connecting a resistor in series with the led, allows the led to put a voltage drop across the resistor. Therefore you want the resistor to have 10v across it, and 2v across the led. Now you need to find out the maximum current the led can draw, usually 20mA. Using V=IR.. R = V/I.. R = 10v/.020A
     
    Last edited: Sep 18, 2006
  16. dephilile

    dephilile Member

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    when i said they are running off 5 volts i meant all three were running off 5 volts. 5 volts/3 LED's=1.6volts per LED
     
  17. brodsta

    brodsta Member

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    Looks like the <math> tags don't work on OCAU's wiki. I just spent a bit looking at the Ohm's Law entry on wikipedia to copy the <math> tag formatting, to no avail.
     
  18. Goth

    Goth Grumpy Member

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    FYI, the math formatting code is just basic a LaTeX implementation, but our software doesn't seem to support it.
     
  19. soul

    soul Member

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    you would still need at least one resistor to limit the current.
     
  20. Goth

    Goth Grumpy Member

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    If the combined forward voltage of the LED's is greater than the supply voltage, the LEDs operate in the Shockley region, so there is no risk of thermal runaway, so no resistor is needed.
     

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