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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. (http://www.mp3car.com/vbulletin/showthread.php?t=19858) 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
    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

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    • #3
      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!!!

      Comment


      • #4
        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.

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        • #5
          www.lsdiodes.com has leds, bright ones too. and almost free shipping!
          -
          My cars
          -

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          • #6
            Cant see 5V LED in there jol

            Comment


            • #7
              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

              Comment


              • #8
                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

                Comment


                • #9
                  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.

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                  • #10
                    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

                    Comment


                    • #11
                      yea it can
                      it could be done for you...
                      Originally posted by menudude
                      thank you all for your help minus the useless post by sjlucky...

                      Comment


                      • #12
                        http://superbrightleds.com/

                        As the name implies, they have very bright LEDs

                        --Dima

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                        • #13
                          since we are talking about LEDs here, i need some help locating online store that carries 3mm Red(high bright) 12V LEDs.
                          thx
                          Danny.

                          "If today was perfect, there would be no need for tomorrow."
                          My Car Project® ‹== !10.4" Allbrite, touchscreen, epia, 150opus, gps, wireless, sony remote [old] 128x64 Crystalfontz +double din [new]

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                          • #14
                            Sorry, i wont speak up again. BUT...
                            * He needs more leds connected. (read post)
                            * He asked for a 3 Volt resistor.
                            Since there are no such thing, the easiest to do are leds in serial. Power coming from the computer psu would give 5V, that makes 2,5 V for each led.
                            * He dont have to mess with calculators or make a fool of himself asking for a 3 Volt resistor.
                            * It will work.

                            Comment


                            • #15
                              Originally posted by cvi
                              Sorry, i wont speak up again. BUT...
                              * He needs more leds connected. (read post)
                              * He asked for a 3 Volt resistor.
                              Since there are no such thing, the easiest to do are leds in serial. Power coming from the computer psu would give 5V, that makes 2,5 V for each led.
                              * He dont have to mess with calculators or make a fool of himself asking for a 3 Volt resistor.
                              * It will work.
                              I'm sorry to say, it won't work. An LED when conducting does not drop 2.5 volts. LEDs are misleading. They are CURRENT SPECIFIC not Voltage specific. The resistors aren't really used to reduce the voltage, they are used to limit the current. An LED will operate just fine with only 0.7 volts applied. The Spec on the LED of 3V is a MAX operating voltage, not a voltage requirement. Use the reistors. Or, if you wish, some LEDs are "pre-resistorized" to run TTL (5V) levels without an external resistor.
                              2006 Chevy Colorado: VIA M10000 EDEN, 1 GB RAM, 80 GB 2.5" Seagate HDD, USB Slim Slot DVD/RW, Holux GPS, MobileVU 10.4" LCD (touch not working yet), VOOMPC Case (blue), 70W DC-DC supply.

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