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LED + Resistor question!!

Discussion in 'General Hardware' started by Docaroo, 7 Jul 2006.

  1. Docaroo

    Mobster

    Joined: 27 Jun 2005

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    Location: Aberdeen, Scotland

    Hi guys,

    I'm using the LED calculator on Metku mods site to figure out what resistor to use for some LEDs and I need a quick check...

    I have 12 LEDs wired in PARALLEL as they are going to be running from 3 AA or AAA batteries (1.5v batt times 3 = 4.5v total power supply).

    The LEDs have a forward voltage of 3.2 - 3.6v so I guess suppling 3.5 v to them would do me.

    Now here's the question, do I use a resistor to take the current down to 3.5v as if there were only 1 LED? This would require a 26ohm resistor.

    OR do I believe what the LED calculator says and use a 2.2 ohm resister - the figure changes depending on how many LEDs are in the parallel connection.

    This doesn't make sense to me as I thought the voltage would be shared across all the LEDs meaning you would need the 26 ohm?

    Also, if the calculator is correct, can I get away with having no resistor since it's such a tiny value (2.2 ohm)?

    thanks in advance guys!
     
  2. bitslice

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    Joined: 18 May 2006

    Posts: 9,041

    Vsource= 4.5
    Vfwd = 3.5
    led forward current = 20mA

    load resistor = 56ohm 1/4 watt (req 12 off)


    (this is way too early for me...)
    current doesn't get shared over all leds, one would suck it all,
    each led needs a resistor
     
    Last edited: 7 Jul 2006
  3. Docaroo

    Mobster

    Joined: 27 Jun 2005

    Posts: 2,863

    Location: Aberdeen, Scotland

    but in parallel wiring this is different!

    I'm putting 50mA through the LEDs so when I use the metku calculator I get this:

    Vsource = 4.5v
    Vfwd = 3.5v
    current = 50mA
    No. of LEDs = 12

    resistor = 1.8 Ohm

    However, when I use a different number of LED's:

    Vsource = 4.5v
    Vfwd = 3.5v
    current = 50mA
    No. of LEDs = 1


    resistor = 22 Ohm

    another calculator said use 26 Ohm! This is the part confusing me...

    Do I need one 22 ohm at the start of the circuit, one 1.8 ohm at the start of the circuit or one 1.8 ohm before ever LED in PARALLEL wiring?? :S :S :S

    eek!
     
  4. bitslice

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    Joined: 18 May 2006

    Posts: 9,041

    Vsource= 4.5
    Vfwd = 3.5
    led forward current = 50mA

    load resistor = 22ohm 1/4 watt (req 12 off)


    it should look like this:

    +Ve ---///----|>|---- -Ve
    +Ve ---///----|>|---- -Ve
    +Ve ---///----|>|---- -Ve
    +Ve ---///----|>|---- -Ve
    ....
    .... etc
    ....
    +Ve ---///----|>|---- -Ve



    post link to your calc, it looks wrong



    If you had one LED, it would still use the same value resistor

    V=I x R
    R= V/I

    4.5-3.5/0.05 = ~ 22ohm



    .
     
    Last edited: 7 Jul 2006
  5. Docaroo

    Mobster

    Joined: 27 Jun 2005

    Posts: 2,863

    Location: Aberdeen, Scotland

  6. Docaroo

    Mobster

    Joined: 27 Jun 2005

    Posts: 2,863

    Location: Aberdeen, Scotland

    thanks for the explanation though, thinking about it - it makes sense to need a resistor before ever LED, as you need to take the voltage down to 3.5 every time you have an LED there...

    Is there a way to take the overall voltage down so you only need 1 "master" resistor??

    thanks again :)
     
  7. bitslice

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    Joined: 18 May 2006

    Posts: 9,041

    ah, I see, (stick + wrong end)

    sorry, the one resistor thing is not normally a circuit that's used.
    It will work but the LED's may have uneven brightness.


    If the LEDs require slightly different voltages only the lowest voltage LED will light and it may be destroyed by the larger current flowing through it


    btw,
    the forward current of 50mA seems a tad high, normally they are about 20-30mA.?
    ...not a problem, just something to double check :)

    hth
    .

    .
     
    Last edited: 7 Jul 2006
  8. BenST

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    Location: Southampton

    Yep, it's not good practice to share one resistor with multiple LED's as you can never guarentee that all LED's will use the same amount of current, resulting in brighter/duller LED's.

    In theory if you wanted one 'master' resistor you would simply add up the current of all LED's, take the voltage that needs to be dropped across the resistor and divide, i.e 12x30mA = 0.36A , 4.5 - 3.5 = 1v , 1/0.36A = 2.7ohms. Then you would need to work out the power dissipated across the resistor i.e V2 / R , 1/2.7 = 0.37W. You would need a 2.7ohm 1/2w or greater resistor.
     
  9. Docaroo

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    Joined: 27 Jun 2005

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    Location: Aberdeen, Scotland

    so the calculator is essentially correct?

    all the LEDs are the same make/model/type/fwrd voltage (as only LEDs of the same voltage can be used in paralllel as you said)...


    So if I used a "Master" resistor of around 3 Ohms I'd be ok? :)
     
  10. bitslice

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    Joined: 18 May 2006

    Posts: 9,041

    yep.

    about 1 watt 1R8

    (you can put 4 x 1/4W resistors in parallel if you quadruple their resistance)

    4 x 6R8
     
    Last edited: 7 Jul 2006