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Thread: Basic electronics question...

  1. #1
    Variable Bitrate
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    Basic electronics question...

    Ok, so I'm not the smartest guy in the world, but I don't think I'm retarded, and I just want somebody to verify this. I am in the process of building a new case for my carpc. (Plexiglass case fabrication) anyways I'm installing some LED's into them right now and I'm going to use one of the 5V lines from the floppy power connector... now I screwed up earlier and just connected and LED directly to it.. thus burning it out. Now what I think I need to do is just go down to radio shack and pick up a 3V resistor.... am I right? just solder the 3V resister between the hotwire and the (+) portion of the LED?

    Let me know thanx in advance
    New CarPC on the way again I think. I miss my old one!!!

  2. #2
    Raw Wave
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    3V resistor? nonono.

    What you need is to limit the current going into the LED. You can do this by adding a resistor in series with the LED.

    Heres a typical setup :

    2V voltage drop on LED (depending on colour/type)
    15mA current to pass (depending how bright you want, dont exceed the limit)
    5V supply

    Check the datasheet for accurate values.

    (5-2) / 0.015 = 220 ohms (the nearest E12 series)

    The colour code (RED/RED/BROWN/GOLD) +-5% tolerance.

    So put a 220 ohms 1/4 watts resistor in series with the LED...observe the A/K polarity of the LED, the flat side goes on the negative side of the supply.

    You can get a 5V LED...probably easier for you

  3. #3
    Variable Bitrate
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    Any idea where? I've looked locally and all the LED's I've found online thus far are maxed out at 3V. by the way thanks for the quick response!!
    New CarPC on the way again I think. I miss my old one!!!

  4. #4
    Raw Wave
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    Dont know in US but Ill give you a UK site :

    http://www.maplin.co.uk/products/mod...737&Products=2

    The 3V LED you are talking about is the forward voltage, thats not the supply voltage. Any LED can be made to operate at any voltage (within reason) by adding the series resistor.

  5. #5
    jol
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    www.lsdiodes.com has leds, bright ones too. and almost free shipping!

  6. #6
    Raw Wave
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    Cant see 5V LED in there jol

  7. #7
    Raw Wave Confused's Avatar
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    Check out the LED calculator, which will tell you what size resistor you will need for the LED and supply voltage


    Garry
    Co-Developer of A.I.M.E.E
    www.aimee.cc

  8. #8
    cvi
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    Connect two LEDs in series and connect it to the 5v supply. Thats the easiest way to do it without resistors. The leds should be same specs to use this, but you dont need resistors.

    cvi

  9. #9
    Low Bitrate crosseye's Avatar
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    Quote Originally Posted by cvi
    Connect two LEDs in series and connect it to the 5v supply. Thats the easiest way to do it without resistors.
    CVI, first of all, why would you connect two LEDs in series? Resistors are WAY cheaper than LEDs. Also, connecting a resistor in series with an LED is just as easy as connecting two LEDs in series.

    Second of all, as far as I know, connecting two LEDs in series is not going to do anything to protect either LED. Kirchoff's Current Law implies that the current is the same through any number of components in the same loop (in this case, both LEDs in series). With two LEDs in series there is no significant impedence to limit the current. The only thing that's going to happen is the voltage drop across both LEDs is going to be equal to the sum of voltage drops across both LEDs in series.

    My recommendation, throw a 230ohm-1Kohm resistor in series with the LED. If the brightness is not suitable, reduce the value of the resistor to increase brightness in the LED.

  10. #10
    Raw Wave
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    Wait up guys, this is getting to complicated now


    Ive already done the hard bits, even show whats the colour code of the resistor should be and even explain how to connect it all up, it cant be any easier than that

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