x100 LEDs - do I need a resistor @ 3.0v ?

Discussion in 'General Electronics Chat' started by spondootre, Oct 21, 2006.

  1. spondootre

    Thread Starter New Member

    Sep 21, 2006
    8
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    Hi,

    I am setting up a 100 LED lighting system, i'm going to use a computer PSU 3V rail to power them. I don't want to solder individual resistors to each LED so what size resistor would I need to run all 100 safely? i'm thinking about 0.5 Ohm, 10W wire wound? Or is this completely wrong? Or is there NO need for resistors when using this type of supply??

    LEDs are 75mA MAX , 3.0v supply.

    Malc
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    Going to be a shaky proposition. Your resistor is more like .4 ohms, but the dissipation is 22.5 watts. Use a minimum of 30 watts for safety - 50 watts wouldn't be bad.

    The problems are that the supply voltage is right at the operating voltage for the LED's. If the voltage goes below the LED's break-over level, then they all go out. If the supply sags a bit under the load, then your LED's are gonna blink. A voltage source with a higher output capable of 8 - 10 amps would be a better choice.

    Using a higher voltage allows you to arange the LED's in parallel strings. Might make wiring easier for an illumination layout. Plus, it one joint goes bad - or even one LED craps out, then only those LED's in the string go dark instead of all of them. Remember Christmas tree bulbs.
     
  3. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
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    An ATX supply would have 3.3V instead of 3V.
     
  4. spondootre

    Thread Starter New Member

    Sep 21, 2006
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    Thanks for the replies,

    Yes the ATX PSU is rated @ 3.3V (3.5V measured with a multimeter) The LEDs are actually slow colour change LEDs (blue, green, Red) when connected to the PSU the voltage fluctuates between 3.5v and 3.35v during the cycle. The 3.3v rail on this particular PSU is rated @ 24amps. I've emailed the supplier of these LEDs and he has assured me that as long as the LEDs are to be fed from a PSU within their operating voltage (3.0v - 3.6v) then no resistors are needed?! I've also been told on another forum that it's no good using one resitor for all 100 LEDs and I need to up the voltage to 5v or 12v and resistor each LED seperately............ so many views but which is correct? If the PSU is regulated to not go above 3.5v surely these LEDs will be fine? Granted if there was a spike etc then they would be taken out but under normal conditions isn't 3.3v (3.5v) ideal?
     
  5. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
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    Do you have the datasheet for the LED? Or link to the manufacturer & part number?
     
  6. spondootre

    Thread Starter New Member

    Sep 21, 2006
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  7. mrmeval

    Distinguished Member

    Jun 30, 2006
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    I didn't see your other post. It sounds as if those have current limiting in them already.

    You have a problem with running all of those on one string, if one or more LEDs burn out they will all be destroyed as the current rises if there is no current limit for them.

    There are LED drivers that can handle driving them. I used an LM334 and a transistor to drive a three 20ma LEDs.

    I think you can use an LM317 in current limit configuration though it may need an external transitor.
     
  8. BladeSabre

    Senior Member

    Aug 11, 2005
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    It is true that for normal LEDs, using one resistor for all of them is a bad idea. (As others here have said, if the supply voltage is high enough you can reduce the number of resistors by arranging the LEDs in strings, with one resistor per string.) The difference is that you're using colour change LEDs, which must have some additional circuitry in them to provide the colour change. Both views can be correct because you're talking about two different things.

    I was reading this page on the subject a few days ago:
    http://www.kpsec.freeuk.com/components/led.htm
    (The paragraphs on LEDs in parallel and on flashing LEDs may be relevant.)
     
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