LED confusion

Discussion in 'The Projects Forum' started by RodneyB, Sep 5, 2013.

  1. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    13
    I am at my wits end I built a Blue LED spot light using as per the attaché diagram.

    I ran a 24 volt supply to a regulator which dropped the voltage from 24 to 13.9 volts It is a 6 amp supply @13.9 Volts regulator.

    for 3 days solid the light was on and then for no reason that I know the LED's stopped working

    PLEASE can some one lell me what I am doing wrong!
     
  2. JingleJoe

    Member

    Jul 23, 2011
    185
    10
    Your circuit seems sound, just did a few calculations which I'm sure you did to arrive at that resistor value :)

    Have you tested the LEDs individually out of circuit? have you done the standard fault finding test: check the input and check the output and see if it's what you expect. The input in this case may be your 24V or 13.9V regulated voltage, the output would be the voltage accross each LED.
     
  3. wayneh

    Expert

    Sep 9, 2010
    12,154
    3,060
    Yes, a few minutes with a voltmeter will reveal the problem. Probably blew a fuse?
     
  4. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    Do you know how are the V/I of LED?

    Assuming that the V_LED = 3V.
    3v x 4 = 12V
    13.9V - 12V = 1.9V
    I = 1.9V/47Ω=40mA.

    If the V_LED = 12V, V_R = 24V - 12V = 12V.
    The R should be like this:
    R =12V/40mA = 300 Ω
    If using 80% of 40mA = 32mA
    Then R=12V/32mA=375Ω, you may using 390Ω.
    If V_LED isn't not 3V, then you can calculate the value as above.
     
    Last edited: Sep 5, 2013
  5. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
    633
    13
    Thank you for all the help. I will check and report back my findings
     
  6. wayneh

    Expert

    Sep 9, 2010
    12,154
    3,060
    It's unlikely the LEDs are the problem. Start at the wall socket and work towards the LEDs, not the other way around.
     
  7. bance

    Member

    Aug 11, 2012
    315
    34
    If you haven't got a heat sink on the regulator, I would think that is where the problem is. You're dropping lots of voltage across it, and that goes in heat loss (power dissipation)

    HTH Steve
     
  8. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,771
    971
    just ditch the regulator..and recalculate the resistors for 24V..
    more in each series string will help with resistor wattage..
     
    bance likes this.
  9. bance

    Member

    Aug 11, 2012
    315
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    +1 @ mcgyvr Clearly a more simple solution!
     
  10. #12

    Expert

    Nov 30, 2010
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    6,829
    330 ohms, 1/2 watt, for (5) 3.2V LEDs and 24 volts to produce 24.2 ma.
     
  11. Ramussons

    Active Member

    May 3, 2013
    557
    92
    Have 2 Chains of 8 LEDs and a very small resistor. Generate less heat

    Ramesh
     
  12. #12

    Expert

    Nov 30, 2010
    16,345
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    Two chains of 8 LEDs does not make 20 LEDs.
    8 LEDs times 3.2 volts is more than the 24 volts available.
     
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