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Attaching a LED to the comp power supply

  1. Jun 17, 2007 #1
    I bought three LEDs, three 1k ohm resistors, and some wire. When I bought the LEDs, I made the person show me that they worked. He connected the negative end of the LED to the yellow wire, and the positive part to the resistor, which he then connected to the black wire. The LED lit up.

    As far as I know, the yellow wire is the 12v wire, and the black is the ground wire.

    At home, I spliced the wire into three wires, and connected the three diodes in parallel in the same manner. The diodes didnt work though. Also, my comp refused to start up (kept on getting rebooted again and again).

    What could I have done wrong? I dont even know if the diodes work anymore. I may have connected them incorrectly once, cause nothing happened the first time I did do it the right way. I dont have a lot of equipment to test the diodes (almost none in fact). Could I test them in some way?
  2. jcsd
  3. Jun 17, 2007 #2


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    I may be misinterpreting you on this part about the three wire thing. Could you post a small schematic on how you hooked up the LEDs? Did you use the black wire in your circuit? Also include resistor values, VF, and IF for the LEDs.

    Last edited: Jun 17, 2007
  4. Jun 17, 2007 #3
    I connected one end of the wire to the yellow wire from the PSU and split the other end into three parts. Each of these parts I connected to the negative terminal of the LED.

    I then connected another wire to the black wire coming from the PSU and again split the other end into three parts. These parts I connected to 1 kilo ohm resistor each. Then, I connected the resistors to the positive terminal of the LED.

    I think the LED voltage is about 3 volts and thats why I used a 1k ohm resistor to step down the voltage.
  5. Jun 17, 2007 #4


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    Are you sure it isn't positive to yellow (12V) and negative to ground (black) ?

    I get a picture that you're making three individual LED circuits here (yellow wire -> negative LED terminal -> positive LED terminal -> 1K resistor -> black wire) ?
    You're reverse biasing the LED here. In other words, it won't work. It should have been something like this [yellow wire -> positive LED terminal -> negative LED terminal -> 1K resistor -> black wire]
    You're using the 1K to control the amount of current that flows through the LED (current limiting resistor).
    Last edited: Jun 17, 2007
  6. Jun 17, 2007 #5


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    If your computer keeps rebooting, it probably means you'll pulling too much current from the power supply, and it's switching off to protect itself. Reverse-biasing your diodes will do nothing at all -- the diodes will conduct essentially zero current, won't light up, and won't affect the power supply at all.

    If they're connected properly (forward-biased), the three diodes will consume something on the order of 30 milliamps of current, which again won't affect the power supply.

    In short, the "symptoms" you're witnessing, like the computer restarting, will not happen no matter how you've wired the diodes.

    From the sound of it, you've somehow formed a short that has connected the yellow (12V) supply to ground. You might want to be very careful with your wiring to make sure nothing's touching anything else. I give money that you've got some wires twisted together, and these wires are touching the case. Since the case is grounded, you are shorting the power supply through the case.

    - Warren
  7. Jun 18, 2007 #6
    Oh man. :surprised I never thought of that. Thank you. I'll try that.
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