Simple LED Circut Power Supply Selection

Discussion in 'The Projects Forum' started by Narinik, Mar 23, 2010.

  1. Narinik

    Thread Starter New Member

    Mar 23, 2010
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    0
    Ive been searching the internet for a while now, and this site keeps coming up so I decided to post.Ive been digging around using the search and haven't found quite what im looking for either.

    The leds I have are ultra bright blues and greens 20ma, 2.8 forward voltage as far as I can measure. They were packaged with 470ohm resistors as a kit for 12v sources, that's how i got the 20ma.

    I have a few old cell phone wall plugs that are 12v and 500-600ma I would like to use to light my desk.
    Im planning on getting up to a 4x8 array of leds eventually, 4 rows of blue, 4 rows of green with a 4 way switch connecting the colors. All off, green on, blue on, blue and green on.

    I want to make sure my desk wont light on fire if I leave the lights on for extended periods of time. Im often away for 3 days at a time and forgetful.
    Will I be ok using the cell phone wall plug with the array suggested?
    Can I build half of the array, only the blues, with the same result?
    Will 25 gauge wire be ok as well?

    Thanks in advance
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Have you read this?

    LEDs, 555s, Flashers, and Light Chasers

    As long as you aren't overdriving the cell phone wall wart you should be OK. A general rule of thumb is 50% of the max power capacity.
     
  3. Narinik

    Thread Starter New Member

    Mar 23, 2010
    2
    0
    Ive looked through it, i have a basic understanding of electronics but i didnt see anything on wall warts specifically. wasnt sure if i needed a current limiter or not since im using the wall wort.

    It says on the back output 12v dc 600ma, i dont think ill be getting anywhere near 300ma, maybe 160ma. do i need to heatsink anything? im planning on just mounting the leds in a piece of scrap acrylic, soldering them and heatshrinking them.

    will long pieces of wire matter? 3-6 ft? 25 gauge?
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    It is a matter of Ohm's Law, most cell phone power supplies are switching power supplies, that is to say, they are regulated. If it is regulated it is a simple issue, the voltage is constant, so it is easy to work around.

    Have you measured the output of the wall warts? Check them with no load, then add a 100 ma load (again, use Ohm's Law).

    Even if a wall wart isn't regulated, you can still use it. Either add a regulator (constant voltage or constant current), or approximate the current you will be pulling and measure the voltage.

    Most of the cell phone power supplies I've ever messed with are 5VDC.
     
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