Resistors for a LED Light String

Discussion in 'General Electronics Chat' started by SBURDETT, Jun 1, 2013.

  1. SBURDETT

    Thread Starter New Member

    Jun 1, 2013
    1
    0
    Hello

    I was looking for some help with a LED light String in order to determine the correct resistors to use, per LED. My string consists of LEDs in a parallel circuit driven by an adapter converting 120v AC to 6v DC. Using Ohm law, i came up with the results below. If anyone can assist with reviewing my calcuations, that would be great. Thanks

    RED LED = 2.3volt Forward Voltage
    Resistor = 6volts – 2.3 / (20mA (0.020) x 14) = 13ohm.
    Use 15ohm (Brown/Green/Black)
    Wattage = 6volts –2.3 X (20mA (0.020) x 14) = 1.03 watts.
    Use Resistor with 1.7 watt rating or higher.

    AMBER LED= 2.3volt Forward Voltage
    Resistor = 6volts – 2.3 / (20mA (0.020) x 11) = 16.8ohm.
    Use 18ohm (Brown/Gray/Black)
    Wattage = 6volts –2.3 X (20mA (0.020) x 11) = 0.80 watts.
    Use Resistor with 1.35 watt rating or higher.

    BLUE LED= 3.6volt Forward Voltage
    Resistor = 6volts – 3.6 / (20mA (0.020) x 13) = 9.23ohm.
    Use 10ohm (Brown/Black/Black)
    Wattage = 6volts –3.6 X (20mA (0.020) x 13) = 0.624 watts.
    Use Resistor with 1.0 watt rating or higher.

    GREEN LED= 3.6volt Forward Voltage
    Resistor = 6volts – 3.6 / (20mA (0.020) x 12) = 10ohm.
    Use 12ohm (Brown/Red/Black)
    Wattage = 6volts –3.6 X (20mA (0.020) x 12) = 0.576 watts.
    Use Resistor with 1.0 watt rating or higher.
     
  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    5,003
    745
    Your better to put a separate resistor for each led in series, and put all the (leds/resistors) in parallel across the 6V supply,
    Leds

    putting leds in parallel across each other is no good as one led may have a lower forward volt drop than the other, and will stop the others from working.

    I would use 10 ohms for the Blue/Green leds and 15 ohms for the Red/Yellow leds all 1Watt, that will give you a total current from your supply of about 2 amps
     
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