My questions thread (LED Q's)

Discussion in 'The Projects Forum' started by Alex., Dec 16, 2011.

  1. Alex.

    Thread Starter New Member

    Dec 16, 2011
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    Hello.

    So my friend and I are planning to make a super awesome beer pong table.
    We want to put LED lights in it for effect, and I'm the one who has to do the thinking.

    So I ordered 600 LEDs off eBay, (yellow, blue, and white), and now I need to figure out what size resistors to order.

    I only took 1 electronics class a year ago so I'm still n00b.

    First question: it says "Forward voltage: 1.8~2.6V"

    What does that mean? Do I design for 2.6V or 1.8? or average it out?

    And also, am I doing this correctly?

    It has a 20mA rating, and lets say we use 2.6V, and we run 3 in series.

    we have 7.8V / 0.020A = 390 Ω resistor required?

    Thanks in advance :eek:
     
  2. Wendy

    Moderator

    Mar 24, 2008
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  3. Audioguru

    New Member

    Dec 20, 2007
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    If your LEDs are actually 2.6V each and you have three in series needing 7.8V then a 390 ohm resistor will limit the current to 20mA when it has (390 ohms x 20mA)= 7.8V across it. Then the power supply must be double 7.8V which is 15.6V.
    If you use a 220 ohm resistor then a 12V power supply will light three 2.6V LEDs in series at (12 - 7.8V)/220 ohms= 19.1mA.

    If the LEDs are actually 1.8V then their current will be much more than 20mA and they will soon burn out.

    Since the LEDs have a very wide range of voltage then maybe you should measure and sort them.
     
  4. Adjuster

    Well-Known Member

    Dec 26, 2010
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    The white and blue LEDs will almost certainly have Vf values of more than 3V, something like 3.5V may be nearer the mark.

    You are likely to need different resistor values according to the LED colours.
     
  5. fireofenergy

    New Member

    Dec 17, 2011
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  6. Audioguru

    New Member

    Dec 20, 2007
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    But then you wrongly talked about using a voltage regulator.

    LEDs with the same part number are not all the same. Their forward voltage is a range of voltage and the graph on the datasheet shows only typical ones that you cannot buy. You buy low ones, maybe typical ones and high ones.

    If the forward voltage is lower than typical then a voltage regulator will burn it.
    If the forward voltage is higher than typical then the LED will be dim or not light up.
     
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