problem with resistor pls help

Discussion in 'The Projects Forum' started by oolesh, May 21, 2012.

  1. oolesh

    Thread Starter New Member

    Apr 21, 2012
    hi everyone i have a question i have bought 200 white led on ebay and i got 470Ω 1/4 resistor for free the problem is when i connect it on a 12v battery the resistor get hot is it normal??it's a 5mm led 18000 mcd forward voltage is 3.2v,forward current is 20ma :confused:!!!sorry for my bad english!!!
  2. strantor

    AAC Fanatic!

    Oct 3, 2010
    there's 8.8V across the resistor, with 25.5mA flowing through it. This means it is dissipating .224W, or almost max rating. I imagine it would be pretty warm. I believe this is normal

    EDIT: you could reduce the dissipation in the resistor by putting multiple LEDs in series. 3 max. With 3 LEDs in series, there would be 2.4V across the resistor and it would dissipate .054W. Much less.
    oolesh likes this.
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    While you can use a 1/4 watt resistor to dissipate 1/4 watt for an experiment this is not a reliable way to select a resistor. It is best to run resistors at most half their rating, so I would put a 1/2 watt resistor there.

    That way it will keep working for a long long time.