Multiple Leds, consistently lit project

Discussion in 'The Projects Forum' started by module, Apr 13, 2009.

  1. module

    Thread Starter New Member

    Apr 13, 2009
    I would like to produce an array of leds that stays lit as long as the power supply is running. Sounds easy enough.

    I've procured a glorious bag of leds from the internet auction consortium.

    I have my computer power supply with a +12v rail @ 28A. (to be honest, not a clue what I'm saying here, I just know it is knowledge that's probably required)

    My bag of leds sits here taunting me, because I'm scared to death I'm going to fry something. Blame it on old age, when I was 13, this would not have been an issue.

    So the bag of leds gives me one descriptor: input voltage +1.3V-1.6VDC.

    I'm going to make a few assumptions:
    8 * 1.5V = 12V
    with this, I assume I can make children in groups of 8 and hook them to a parenting wire to hookup to the power supply lines without resistors. I've attached a picture with this concept, but in the grand scheme of things, there would be around 10 children all connected to the parent wire.

    Is this going to work?

    Thanks for preventing any potential fires, lost electrical equipment, or new orifices on my body!
  2. Audioguru

    New Member

    Dec 20, 2007
    Red LEDs are 1.8V to 2.2V each.
    Since your LEDs are labelled only 1.3V to only 1.6V then they must be the infrared kind that are not visible.

    An LED is not a resistor like an incandescent light bulb that is dim when the voltage is low and is brighter when the voltage is high.

    An LED is a diode that turns off when the voltage is a little low and burns out when the voltage is a little high.

    An LED has a range of voltage. It might be 1.3V or it might be 1.6V.
    If you connect eight 1.3V LEDs in series (they use 10.4V) and connect them to a 12V supply then their current will be extremely high and they will blow up.
    If you connect eight 1.6V LEDs in series (they use 12.8V) and connect them to a 12V supply then they won'y light or they will be very dim.

    LEDs need something to limit the current like a resistor in series.
  3. Wendy


    Mar 24, 2008
  4. module

    Thread Starter New Member

    Apr 13, 2009

    They are indeed infrared. Since my package only lists +1.3-1.6VDC, how would I go about testing which voltages are safe to run at?

    Or is the +1.3 an indicator that 1.3 is what it runs at and the - is something else?
  5. Wendy


    Mar 24, 2008
    Read the article, LEDs are not voltage devices, they are current devices. Resistors are not optional, and there is a limit how many you can put in a chain. Resistors are cheap though, about 3¢ each.
  6. SgtWookie


    Jul 17, 2007
    They certainly didn't give you much to go on!

    Usually, a manufacturer will give you at least a typical Vf @ current; that is, what the voltage will be across the LED when it is forward biased at a specific current.

    First, you need to determine what the actual Vf is at some known current. Since you don't have a specification, I suggest that caution will suit you well - better to be cautious than fry a bag of LEDs.

    Let's start with a suggested current of 15mA. This should be safe for most LEDs.

    You will need a current limiting resistor for the test, but it's value will need to be calculated.

    Since a Vf of 1.3v to 1.6v has been mentioned, we'll start by subtracting the average of the two voltages from your supply voltage.
    (1.3+1.6)/2 = 1.45v.
    You say your supply is 12v.
    12v - 1.45v = 10.55v.

    Now the question is, what value of resistance is needed to cause 15mA current to flow when the voltage across the resistor is 10.55v?
    The answer can be found in Ohm's Law, which states:
    R = E/I, or Resistance = Voltage / Current.
    R = 10.55v / 15mA = 10.55v / 0.015a = 703.333... Ohms.
    The closest standard values are 680 Ohms and 750 Ohms. Chart:
    However, you could use a pair of 1.4k resistors in parallel to arrive at 700 Ohms.

    So, you hook up one LED in series with the two 1.4k resistors in parallel across your 12v power supply, and then using a digital multimeter, measure the voltage across the LED.

    Do that with a good number of the LEDs, and calculate the average Vf.

    Then we'll have some real numbers to work with.
  7. module

    Thread Starter New Member

    Apr 13, 2009
    Sounds like I'm taking a trip to radioshack tomorrow! Thanks for the information, I'll post how it goes tomorrow.
  8. module

    Thread Starter New Member

    Apr 13, 2009
    So I got brave...

    I alligator clipped up a led and hooked it up to a 1.5v battery. I then proceeded to rip apart a webcam and remove the IR filter lens. It shined bright! So, I soldered 6 together, and joy, 9 volts powered them up.

    Of course with this new-found bravery, I started tearing into a molex plug in my computer's power supply. I found out very quickly why you should wait a few hours after unplugging until you play with them. I also had to perform emergency services to my now gaping open finger, thanks to a razor blade + molex adapter situation gone wrong.

    Anyways, long story short, 6 arrays of 8 IR leds hooked up to power supply = success. The multimeter helped me decide it was going to be ok. Who would have ever thought a $10 device could tell me so much?