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Electronics Mystery LED's and other fine questions

Discussion in 'Modding' started by onarahosting, 30 Aug 2005.

  1. onarahosting

    onarahosting What's a Dremel?

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    Hi all,
    I've done a search and found a heap of usefull stuff on LEDs. I am still trying to process a lot of it. I have a couple of questions:

    A couple of years ago I bought a heap of high millicandle Leds for a project and wired them all up. I remember blowing some of them before I discovered the voltage stuff. I ended up wiring them up in series so they would handle the 12 volts, i was powering off a 12 volt gell cell. I never used resistors on them. Now I have dug them out and i recall buying lots of red ones (probably 2.2volt) and i bought some white, blue and mutil coloured ones (they cycle between red, blue and green). Problems is I don't know which is which. I have a box full of clear Led's! :wallbash:

    Now I believe that they have different voltages (the reds and other colours), so how can I find out which is which without blowing the crap out of them?

    Question 2: when people add Leds to thier mods is it common practice to just pull power from existing led bits on the motherboard or do people use 12volt or 5 volt power leads from the PSU and use resistors? I'll be using a few Leds on my project and wondered what the common practice was.

    Thanks for your help.

    (edit) is this the sort of resistor I need for the 2.2volt leds - Resistr 1/4W 56R Grn-Blu-Blk)? (/edit)
     
    Last edited: 30 Aug 2005
  2. Nexxo

    Nexxo * Prefab Sprout – The King of Rock 'n' Roll

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    If you look closely at the head of the LED, you will be able to see if it has one die (small square thingy stuck on top of a "reflector dish" on the end of one of the legs) or several. The multi-colour LEDs have three dies and probably some electronics to regulate the colour cycling.

    I would test the colour by using a 1.5V supply. This usually makes them light up weakly, but enough to tell the colour. The red ones will definitely light up. If the LED does not light up however, then you know it has to be blue or white or multi-colour LEDs, which want up to 3.5V. Test those again but at 3V.

    Connecting them to the motherboard is highly impractical. Generally people connect LEDs to the PSU molexes. Either 5V or 12V will do --whatever happens to be available. You need a resistor for this. To calculate its value, use this LED calculator.

    To use it, enter the supply voltage (e.g. either 12V or 5V) and the "Diode Forward Voltage" which is basically the optimal ammount of Volts for that LED. For red LEDs that is generally 2.2V, for blue, white or purple LEDs it is 3.5V (sellers usually list this in the specs). LEDs generally want 20mA, so enter 20 under "Diode Rated Current". It should then tell you the value of the resistor you need.

    If you put LEDs in series, simply add their forward voltage levels together. If you put them in parallel however, you have to add their mA together.

    For more on the joy of LEDs, visit The LED Museum.
     
  3. onarahosting

    onarahosting What's a Dremel?

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    Thanks a lot for your help. I feel confident to set out on my journey of LED now :D
     
  4. madhatter

    madhatter What's a Dremel?

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    or you could get a power supply, start it at 0v, connect an led and slowly increase the voltage till you see light? incidentally, the lowest voltage you need to power one, it it's quanta. from the experiments i did, if i remember correctly, a green led would need the least power before it emmitted some photons...
     
  5. Nexxo

    Nexxo * Prefab Sprout – The King of Rock 'n' Roll

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    I chose 1.5V and 3V because that is easily achieved with a pair of batteries. I'm assuming that someone who is starting out in the very basics of LEDs, volts, resistors etc. won't have a nice adjustable power supply. Quanta in this case are pretty academic...
     
  6. cpemma

    cpemma Ecky thump

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    Once a LED gets enough volts to light, current shoots up very rapidly with any slight extra voltage, so that's a dangerous practice. Leds are best tested with a 5V or 6V supply with a resistor in series, any value between 240 ohm and 1k will do to show if it works and what colour you've got. A 9V PP3 with a 2k resistor can also be used, the high-value resistor will prevent damage if you connect the LED the wrong way round.

    If you've got a multimeter you can then measure the voltage across the LED to get its forward voltage and use the LED calculator for a more accurate resistor value at any supply voltage.

    The motherboard LED headers are already current-limited by an on-board resistor, so swapping to your own LEDs won't do any harm, but there may not be enough current available to light them up as brightly as you want.
     

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