Newbie LED question

Discussion in 'General Electronics Chat' started by androiduk, Nov 17, 2012.

  1. androiduk

    Thread Starter New Member

    Sep 23, 2012
    2
    0
    Hi all,

    I am a RC truck enthuiast mainly Tamiya 1/14 scale , I wish to add a home built LED lighting systems to my trucks , all the LEDs will be wired in series to avoid issues should one go down.

    My problem is I am not sure what resistor to use , most of the LEDs are 3.2V forward voltage with 20ma forward current the supply voltage is 7.2V .

    I found an online calculator that says use a 220 Ω but a friend of mine says it should be nearer to 400Ω.

    I am now stumped.Please help.

    Andyb
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    When you say "series" I think you mean "parallel," or each LED gets it's own series resistor, and those pairs are put in parallel. That keeps one bad light from killing the rest.

    Here's how to get 20 mA into the LED: Given you have 7.2V battery and the LED takes 3.2V, you have 7.2-3.4=3.8V across the resistor. The resistor value is R= V/I, here 3.8V/20mA=190 ohms. 220 ohms is a close standard value and should work fine.

    That resistor will have V*I watts of power, or 3.8V*20mA=.076 watts, you want something at least twice this, so a 1/4 watts (.25) will work just fine too.

    If the LED needs a different voltage just run the numbers again for that LED's voltage.

    And good luck!
     
  3. androiduk

    Thread Starter New Member

    Sep 23, 2012
    2
    0
    Hi Ernie thanks for the reply , each LED will get its own resistor and run back to RC Mosfet latching switch seperate from any other LED then should the LED go down its easier to replace.

    If that makes sense lol.

    Andyb
     
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Your friend calculated 7.2V/20mA= 360 ohms without an LED.
     
  5. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
    463
    You could use a small MC34063 DC/DC converter, and step down to 5 volts, or even lower. Then you only need a very small resistor.

    The chip is not expensive or difficult to buy or difficult to use. It has been around for 20 years already.
     
  6. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    Or you could run two LEDs in series with one 43 Ω resistor, which would be quite efficient - on the order of 89%.

    And you could use 1/8 watt resistors.
     
  7. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
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    Yes but maybe the LEDs go out when the motor stall, and/or when the motor starts.

    It also depends on the battery. Some will maintain the voltage with a very flat discharge curve, but that is not neccessarily true for all batteries.
     
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