help please

Discussion in 'The Projects Forum' started by steve02, Jul 4, 2011.

  1. steve02

    Thread Starter New Member

    Jul 2, 2011
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    hi i have 36 white leds forward current 1000ma forward voltage 3.9 power supply is 24volts what is best way to wire these up thanks any help would be appreciated steve
     
  2. #12

    Expert

    Nov 30, 2010
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    The simple way is to put 5 LEDs in series with a 4.7 ohm, 10 watt resistor and repeat this 7 or 8 times, but the wasted power is huge. This really cries out for a switch-mode power supply like this one:http://www.romanblack.com/smps/a05.htm
    (Last one on the page.)

    This is so large that it is stretching my limits. 36 amps of LEDs!
    I have to go think for a few minutes. Somebody else might come along and help while my head is spinning.
     
  3. debjit625

    Well-Known Member

    Apr 17, 2010
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    186
    Put 6 LEDs in series total voltage drop by the 6 LEDs will be (3.9 * 6 = 23.4) then use a LM317 (adjustable voltage regulator) and adjust it for 23.4 volt.Do it 6 times for your 36 LEDs.

    Good Luck
     
  4. iONic

    AAC Fanatic!

    Nov 16, 2007
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    Is a 317 going to regulate 24V to 23.4V, there's quite a bit of dropout? And regulating the voltage is not what the OP need, but rather he needs to regulate the current.

    Plus, we are not sure his supply can offer at least 6A.
     
    Last edited: Jul 4, 2011
  5. SgtWookie

    Expert

    Jul 17, 2007
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    This won't work at all.

    For one thing, an LM317 has a dropout voltage of at least 1.7v from IN to OUT at that current, so that leaves 24v-1.7v=22.3v; already not enough.

    The 2nd item is that you do not regulate LED current by controlling the source voltage alone.

    As diodes heat up, their Vf decreases, which increases their forward current. This situation is known as "thermal runaway", and frequently causes severe damage to components.
     
  6. #12

    Expert

    Nov 30, 2010
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    We have to assume the power supply can provide 6 amps or this thing won't work at all.
    I do have some fear that the 24 volts is coming from (2) 12 volt batteries, and the voltage will get lower as the lights are used. This would complicate the circuitry.
    A 317 chip will not work with only .6 volts.
    Regulating the voltage will not power the LEDs without the smoke getting out.

    ps, wookie was typing while I was typing. That's why we both said the same thing.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Not knowing the expertise of our original poster, nor whether the 24v is supplied by batteries or a power supply of some sort, I'll suggest a single 1000mA BuckPuck per 5-led string:
    http://www.ledsupply.com/03023-d-n-1000.php
    They're $15/ea plus shipping. However, they will be far more efficient than using a simple resistor, and much easier to wire up than trying to build a number of buck-type regulators themselves.

    The LEDs will require heat-sinking, as they will be dissipating nearly 4 Watts of power each; 19.5 Watts per string. This is a non-trivial amount of heat to dissipate. If the heat sinking is not sufficient, they will wind up with molten blobs of plastic instead of LEDs.
     
    #12 likes this.
  8. steve02

    Thread Starter New Member

    Jul 2, 2011
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    hi 24volts will be from pc power supply hope this helps
     
  9. iONic

    AAC Fanatic!

    Nov 16, 2007
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    Sgt. Wookie's suggestion with the BuckPuck, although expensive, is the best option so far. We really could use an exclamation of what your end-game is, what is your power source and its current capacity and even what your intentions are with this vast amount of light!!
     
  10. castley

    Member

    Jul 17, 2011
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    0
    Try 3 strings of 6 in series connected in parallel
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    I missed the "PC power supply" statement in reply #8.
    While a typical ATX form factor PC supply can output quite a few Amperes from the +12v supply, the -12v is quite anemic, usually less than 1A.

    So, you'd have to use shorter strings on the +12v supply alone.
     
  12. KJ6EAD

    Senior Member

    Apr 30, 2011
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  13. SgtWookie

    Expert

    Jul 17, 2007
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    I don't like that calculator, as it gives poor recommendations under many circumstances.

    Here's an example situation:

    [​IMG]

    Now if you use 11.9999999v instead of 12v with the rest of the inputs the same except for voltage, it'll come up with far better answers.

    Anyone using that "wizard" stands a good chance of winding up with a pile of molten plastic instead of functional LEDs.

    I tried to convince the site owner to correct their code during many E-mail exchanges, but they were unwilling to do so. As a consequence, I try to warn people against using that site.
     
  14. KJ6EAD

    Senior Member

    Apr 30, 2011
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    I still use it to save time but I know not to trust it without reservation. I like that it can accept large numbers of LEDs and gives the power figures; most don't. If I refer beginners to it in the future I'll add a warning. They should be practicing their Ohms Law anyway I suppose.
     
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