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Resistor help

Discussion in 'General Discussion' started by No3ntry, 5 Feb 2006.

  1. No3ntry

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    Joined: 18 Oct 2002

    Posts: 1,297

    Location: Derbyshire

    I have an LED which is 3 volts

    what resistor will i need to allow it to run on a 12v system


    Thanks James
     
  2. Aod

    Sgarrista

    Joined: 7 Oct 2004

    Posts: 8,675

    Location: London

    what current does the LED run at?
     
    Last edited: 5 Feb 2006
  3. No3ntry

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    Joined: 18 Oct 2002

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    Location: Derbyshire

    30ma

    would it matter if i ran a little more current through it ?
    say for example 6 or 9 volts what effect is this likely to have long term ?

    i have tested it with both 6 and 9 volts and it workd fine but 12v blew one of these leds

    Thanks James
     
  4. lordedmond

    Wise Guy

    Joined: 14 Jun 2004

    Posts: 1,573

    Location: Tilchestune

    ok 3 volt led
    supply 12 vdc

    current though network = 30ma or 0.030 amp

    you need to drop 9 vdc

    thefore ohms law r = v/I

    r = 9 / 0.03

    = 300 ohm

    wattage of resistor p - v * I
    9 * 0.03 = 0.27 w

    therfor a half watt 300ohm would do the job

    increased current will blow the led quicker and give higher output
     
  5. No3ntry

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    Joined: 18 Oct 2002

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    Location: Derbyshire

    thanks for the reply

    I have tried the LED in place at 3V and it wont be bright enough but at 6 it is *just* bright enough and at 9V is spot on (using small 3v watch batteries)

    but wiring it directly to the cars 12v burnt it out almost straight away
    does the amount of increased current decrease the life of the LED on a linear scale ?

    eg 3v - life = 50,000 hours
    6v - 25,000 hours

    if current gives higher output what does the increased voltage do ?
     
  6. lordedmond

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    Joined: 14 Jun 2004

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    Location: Tilchestune

    OK

    power ( watts ) is v * I
    eg 1 watt = 1 volt * 1 amp

    therfore if the resistance remains the same increase the volts will increase the current as well

    if you put the volts up to 6 the network ( led + resistor ) will draw 60 ma


    BTW you can get 12 vdc leds high intensity ones as well and save you the trouble

    RS sell then as do the other electronic component shop they have a store opp. the old Players office in Notts ( now Nat West office )
     
  7. Aod

    Sgarrista

    Joined: 7 Oct 2004

    Posts: 8,675

    Location: London