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Electronics Overly bright power LED's.. Resistor size?

Discussion in 'Modding' started by scoob8000, 13 Dec 2006.

  1. scoob8000

    scoob8000 Wheres my plasma cutter?

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    I want to dim the power and HDD LED's in my new Coolermaster case. They are blinding.

    I believe they run off of 5 volts from the mobo.

    Anyone have a tried and true resistor size that would make a good fit?

    100-150 ohm would get me about half the voltage assuming 20-25ma

    Any other dimming suggestions? Guess a resistor would be the easiest..
     
  2. cpemma

    cpemma Ecky thump

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    Think bigger. Try 1k, maybe 4.7k if that doesn't calm them down.
     
  3. bender386

    bender386 What's a Dremel?

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    or if you lazy like me some bluetack works great
     
  4. scoob8000

    scoob8000 Wheres my plasma cutter?

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    I guess this bugger is going to be some trial and error.. Guess I could try not to be lazy and try a few different ones before soldering them in.. :)
     
  5. scoob8000

    scoob8000 Wheres my plasma cutter?

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    Glad I grabbed a handful of sizes. 4.7k was the highest I got. Ended up needing to put two of them in series to get the dang LED's dim enough.

    These are nice bright lights, I hate to not steal them for something else and put some dimmer LED's in there.. :)
     
  6. Stuey

    Stuey You will be defenestrated!

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    I would have though that 4.7k is too high. Cpemma, can you please explain the high number values?

    Figuring that .02 A, I'd think a 4.7k resistor would drop close to 100 volts.

    As technical as you can get, I can handle it, so please lay it on thick!
     
  7. Captain Slug

    Captain Slug Infinite Patience

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    The power and HDD led header supply voltage is only 2 to 2.2Volts.
     
  8. cpemma

    cpemma Ecky thump

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    No, that's the forward voltage of a typical case LED you're measuring. There's a current limit resistor dropping a few more volts at about 10mA current.
    Go back to the start, a 5V supply, 2V LED, typical motherboard resistor 220 ohms

    Voltage left for the resistor to drop = 5 - 2 = 3V

    Current through the resistor will be 3/220 amps = 13.6mA

    Now add a 4.7k resistor. Same 3V to be dropped.

    Current through the resistors will be 3/4920 amps = 0.61mA

    Series circuit, so the same current goes through the LED in each case.

    Basic Ohm's Law V = IR or R = V/I or I = V/R

    In this case, voltage and resistance are the known quantities, current is the unknown to be calculated.
     
    Last edited: 16 Dec 2006
  9. Stuey

    Stuey You will be defenestrated!

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    Got it, thanks!

    Ok, so with the LED, there is now 0.61 mA as opposed to 20.0 mA, right? My problem was that I was assuming that the current was held constant at 20 mA and that the voltage needed to be dropped further my an additional resistor. I was considering the LED as a lightbulb. :duh:

    Thanks for the clarification!
     
  10. scoob8000

    scoob8000 Wheres my plasma cutter?

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    cpemma = guru :)


    It still kinda behooves me too.

    If I'm understanding this right, there's really a resistor on the board before the LED headers. Reading the header pins on the mobo show 5 volts on a multimeter because there's not much current flowing really. Add about 20ma, and it drops to 2ish volts.

    Right? :)
     
  11. cpemma

    cpemma Ecky thump

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    Right. The average digital multimeter has an input resistance of a few megohms on the voltage setting. On the 5V supply, put just 1M of meter resistance in series with the 330R board resistor and the meter will read 5 x 1,000,000/1,000,330 = 4.998V. Or near as dammit, 5V.

    With the drive activity LED it gets slightly more complicated as there's an ordinary diode and a transistor switch in the circuit so if either IDE channel is active the one LED lights up, but let's not go there. :)
     
  12. scoob8000

    scoob8000 Wheres my plasma cutter?

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    Cool

    That would explain why the HDD LED is slightly dimmer then the power LED. Not enough so that it bothers me though. :)

    Thanks for all the help!
     

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