Resistor needed for 1W or 3W LEDs?

Discussion in 'The Projects Forum' started by deepak4you, Mar 11, 2015.

  1. deepak4you

    Thread Starter New Member

    May 1, 2010
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    Hello to all E-Gurus,

    Does thia approach of having a current limiting resistor, work even for high power LEDs like a 1W or 3W LED?

    I ask this because I have tried this in the past with power leds and after some hours od operation, the LED blows off as if it were supplied with too much power (or the LEDs just drew a lot of power). Not to mention, even the power supply takes a hit.

    Another thing is that if it were as simple as having a current limiting resiator to drive LEDs, then why comw up with CC/CV LED drivers?

    I even tried making a simple current limiter using bias/voltage control to a power BJT or power MOSFET for controlling current to power LEDs. Even with that, the LEDs die sooner than their stipulated life.

    I think this approach works for small LEDs with very low power leds but the dynamics change completely when we use high power LEDs.

    Thoughts?

    Regards,
    Deepak
     
  2. ScottWang

    Moderator

    Aug 23, 2012
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    This post was separated from another thread -- Resistor needed for LEDs?
    Because you hijack that thread, so I make your own thread.
     
  3. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    CC method can be used for all LED's.
    If your CC setting is high it will die no matter what type of CC circuit you use

    PS..High Power ones should you CC circuits as it's current is quite high for standard resistors.
    It just waste too much power as heat
     
  4. deepak4you

    Thread Starter New Member

    May 1, 2010
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    0
    @ScottWang: thanx. I pasted the question since the original thread was about powering leds with resistors. Separate thread is also good.

    @R!f@@: I had tried my setup with high wattage resistors and also with CC boards from China and India. Result was the same for some reason. To the extent that i even tried to power the LEDs at a little lower than nominal voltage, but still did not work. I think I might have been unlucky with all boards I tried.
     
  5. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    It's not tht I believe. U need to measure the current and adjust to the value the LED requires
     
  6. ScottWang

    Moderator

    Aug 23, 2012
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    In this site if you asking something in the other user's thread not to solve the problem for (T)hread (S)tarter that we called hijack, if you have any questions to ask then you just create your own thread as this thread.
     
  7. BobTPH

    Active Member

    Jun 5, 2013
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    Are you using a heat sink?

    Bob
     
    R!f@@ and mcgyvr like this.
  8. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    I bet you guessed it..
     
  9. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Yes an LED always needs some form of current limiting..
    A resistor is the most common way for low power LEDs..
    "LED Drivers" or "Constant Current" power supplies are typically used with "high power" LEDs..
    Mostly because of efficiency..
    High wattage resistors will be required with "high power" LEDs.. And thats really just a big heater..

    How about an example..
    LED Vf=3V
    LED current = 1000mA
    Power supply of 5V
    R = (5-3)/1
    R = 2 ohms
    Now calculate for the resistors wattage rating..
    W = I^2*R so 1x2 = 2Watts
    then multiply the wattage x2 (this is a commonly used safety factor... 2 to 3x calculated wattage to keep the resistor from overheating)
    So you need a 2 ohm minimum 4W resistor....

    Common 1/4, 1/8W resistors would go up in flames.

    What if Power supply = 12V
    Then R = 9 ohms
    and W = 9W (times 2 safety) = 18W resistor.. Thats HUGE..


    LED drivers common with "high power" leds like Meanwell LDD or LPF or similar could be up to 97% efficient or more..

    And you ABSOLUTELY need a heatsink with any "high power" LEDs.. even 1W and less..
    I love using old Duron computer heatsinks with 50W LEDs.. Perfect size/and cheap..
     
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