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Thread: adding LED's

  1. #1
    Member danthemanohhyea's Avatar
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    adding LED's

    hi everyone, i appoligize if this has been asked before, but this forum seems to be cleaned out often of old stuff..

    anyway, how could attach a single LED to my power supply? Would i need resistors or anything and if so what ones would i need? Thanks for any help you can give!

  2. #2
    Senior Member naptownman's Avatar
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    LED's have a forward voltage drop of about 1.6 or 1.7 volts. They also have a maximum current that they can pass before the heat dissipation gets too high and they fry. Let's assume for convenience that the current will be 20 milliamps. In order to use them to monitor a voltage on a power supply you will need a resistor.

    To calculate the rsistance needed, take the voltage you are going to power it from and subtract 1.7 volts. Divide this by the current in amps and you get resistance in Ohms. Or divide by miliamps and you get resistance in kOhms.

    For example, you want to monitor 5 volts. You take 5, subtract 1.7 and get a difference of 3.3. Divide this by 0.020 (30 milliamps expressed in amps) and you get 165 Ohms. This isn't a standard value but 168 Ohms is.

    The next thing you want to make sure of is the power rating of the resistor. Square the current flow (.02 x .02) and you get 0.0004. Multiply this times the resistance and you get 0.066 watts. So a 1/8 watt resistor will be more than adequate for this example. The formula for power is I (amps) squared times the resistance.

    For a 12 volt rail you would need 515 Ohms at 0.205 watts.

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    Senior Member naptownman's Avatar
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    Sorry for the confusion: Where I said "for example you want to monitor 5 volts" replace the reference to 30 milliamps to read 20 milliamps. 0.020 is 20 milliamps expressed as Amps. It's late and I had too many 3's with the voltage of 3.3 volts! I had three's on the brain.

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    Member danthemanohhyea's Avatar
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    i hate to be a pain the the rear end but i'm not too smart with all that electrical stuff, so much of what you said doesnt make a whole lot of sense to me. However what i got out of the end is that i need to get a 515 Ohm resistor with the LED when its hooked up to the 12V rail?

    Also, a while ago i swapped out the standard LED's in my case (power and HDD activity) for blue ones and just put them in. Do i need to put resistors on those as well to prevent damage to my power supply or something of that nature?

  5. #5
    Senior Member naptownman's Avatar
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    Go to a Radio shack and ask for a resistor around 515 ohms if you want to monitor 12 volts. 510 Ohms is the closest standard value. And don't forget LED's are polarized but you probably know this after changing them in your PC case.

    You don't need to add resistors when you change the LEDs in a case because the resistors are already mounted on the motherboard.

  6. #6
    Member danthemanohhyea's Avatar
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    yep i know that they're polarized, but which end would i solder the LED onto? or doesnt it matter

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    Guest leprechaun_40's Avatar
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    Not to get off the subject, but why do you want to do this? And, where are you putting this led?

  8. #8
    Member danthemanohhyea's Avatar
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    i cut a hole in my front bezel so that the added front fan gets air, and i painted in there so it would look nice and i want to light it up so you can see it. and instead of spending the money to get a cold cathode light, i want to just hook up an LED that i already have to light up the inside of this hole.

  9. #9
    Guest leprechaun_40's Avatar
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    Cool,, of course you could just get a fan with led's in it, ,less work that way

  10. #10
    Member danthemanohhyea's Avatar
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    well i did think of that.. but the fan i have in there now is a variable speed one and pushes quite a lot of air, and i'm happy with it.. plus, a LED fan would cost around.. $10 and i dont have that much and a resistor costs less than a dollar so yeah, i'm cheap

  11. #11
    Complete & Utter Member j.m@talk's Avatar
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    Quote Originally Posted by danthemanohhyea
    yep i know that they're polarized, but which end would i solder the LED onto? or doesnt it matter
    Makes no diff


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    Member Newbie2's Avatar
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    Quote Originally Posted by danthemanohhyea
    well i did think of that.. but the fan i have in there now is a variable speed one and pushes quite a lot of air, and i'm happy with it.. plus, a LED fan would cost around.. $10 and i dont have that much and a resistor costs less than a dollar so yeah, i'm cheap
    What is the brand and model of this variable speed fan?

  13. #13
    Complete & Utter Member j.m@talk's Avatar
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    Theres three ways of achieving this .....

    1.Some fans are "Smart" so they rotate faster the warmer the ambient temp becomes automatically. Click

    2. A fan with a high low switch (Cheap nasty idea)

    3. A fan speed controller & a standard dumb fan Click



  14. #14
    Member Newbie2's Avatar
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    You call standard fans dumb?

  15. #15
    Complete & Utter Member j.m@talk's Avatar
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    yeah


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