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IR LED off of 24V rail - Best way to do it?

Discussion in 'LEDs and Optoelectronics' started by Geonovast, Feb 26, 2015.

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  1. Geonovast

    Geonovast

    16
    2
    Aug 26, 2012
    I have a situation in a piece of equipment where power to a necessary IR LED has malfunctioned. Every other function on this equipment works fine, just for some reason it stopped powering the LED. The only DC power available inside the unit is 24 V straight off the PS.

    Using a linear circuit, I have confirmed that powering the LED by bypassing the main board works, unfortunately (obviously) it puts off a lot of heat. I'd like to put a small board inside the unit, with a switching supply to power just the IR LED.

    After some poking around, I've determined this circuit will work, and I can assemble for less that $1.50. This is pretty much straight from the LM2576 data sheet, just with my load attached. Data sheet also calls for a much larger inductor, as the regulator is rated at 3A, but I'm only going to be using ~80mA. So I'm planning on using an EC36-101K, which is rated for about 275mA.

    Is there a better way to do this, or anything wrong with doing it this way?
     

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  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,267
    Nov 28, 2011
    That looks reasonable. If the 24V rail is fairly constant, and efficiency isn't a concern, I would probably just use a big series resistor and let it get hot. You can get fully assembled buck regulators on eBay for under $2. Search on eBay for LM2596. If you get an adjustable one, you can use a lower series resistor and waste even less power. For example if the LED's VF is 1.9V (which I calculated based on your 39Ω resistor) then you could set the regulator output to 3.1V and use a 15Ω series resistor.
     
  3. Geonovast

    Geonovast

    16
    2
    Aug 26, 2012
    Thanks for your reply! I did the eBay search, and sure enough, there's a lot of ten of those assembled units for $12. Probably the better way to go.

    I had considered a single series resistor, but efficiency is a concern. This will be in a smallish enclosed area that already requires one fan, so I need to keep heat to a minimum.
     
    KrisBlueNZ likes this.
  4. BobK

    BobK

    7,671
    1,681
    Jan 5, 2010
    If you are running an IR LED, which needs about 1.3V typically, why convert down to 5V? Why not something smaller?

    Bob
     
  5. Geonovast

    Geonovast

    16
    2
    Aug 26, 2012
    The 5V version of the 2576 was the only version available from where I was getting the parts (also the only buck converter they had), and only $0.58 a piece. Sourcing parts elsewhere increased cost significantly, so glad I was able to get the pre-assembled units for even less than I had counted on making my own!
     
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