LED power question

Discussion in 'The Projects Forum' started by russpatterson, May 29, 2011.

  1. russpatterson

    Thread Starter Member

    Feb 1, 2010
    351
    16
    Hi,

    I'm doing a project that uses 8, 22' strings of 12V LED strings. I need to be able to switch 4 banks in order to do the 2 color fade effect I'm after.

    Each 22' string runs at 12V, 3.3A. I have 8, 60 watt power supplies. Is it ok to tie the +12V from two supplies together and switch two strings through one MOSFET?

    The MOSFET is a IRL3714PBF, which is rated for 36A.

    http://www.irf.com/product-info/datasheets/data/irl3714pbf.pdf

    Thanks!
    [​IMG]
     
  2. KJ6EAD

    Senior Member

    Apr 30, 2011
    1,425
    363
    Yes, and the way you're doing it in the schematic is best. Since you're switching the ground connection, the two supplies could even be different voltages without trouble.
     
  3. russpatterson

    Thread Starter Member

    Feb 1, 2010
    351
    16
    Thanks! So the supplies are in no danger of having a higher voltage come back through the wrong way?

    Good news. I didn't want to have to drive more MOSFET's with my PIC, much less solder them all up.
     
  4. russpatterson

    Thread Starter Member

    Feb 1, 2010
    351
    16
    What if I just connected all 8 power supplies up in parallel (all output grounds connected together, all output +12V connected together)?
     
  5. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    It is usually not a good idea to connect power supplies in parallel, as they will rarely share the load well unless they have been specifically designed for the job. This applies especially to supplies with output voltage regulation. What can happen is that some of the supplies may regulate at a slightly higher output than the others, causing the latter to shut down so that the load current falls unfairly on the higher voltage units.

    In any case where the output voltages of the supplies differ by larger amounts, damage may be done. Most supplies won't accept reverse output currents to allow dangerous circulating currents to develop, but some modern switching types just might - particularly where a MOSFET is used as a "driven" free-wheel element.

    On the whole, I would let each supply drive its own load, to avoid any possible unwanted interactions.
     
  6. russpatterson

    Thread Starter Member

    Feb 1, 2010
    351
    16
    @Adjuster, thanks for the reply. So do you think that I should not connect the two supplies as I showed in the schematic? I should instead drive two MOSFET's from my PIC I/O pin? I'm concerned that the 20mA from the PIC pin won't drive the two gates effectively. Then I'll get some slow switching and heat on the MOSFET.

    How about this idea. Since I'm fading between amber and green (all amber, slowly fades to all green) and the two duty cycles will add up to 100%, maybe I can connect two 22', 3.3A strings to one 60 watt supply since they should never both be 100% on at the the same time. 90/10, 50/50, 10/90, etc.

    Plus I only have 4 of those IRL3714's on hand and I need this done by Tuesday morning.

    Here's a link to the power supply in question.

    http://www.sloanled.com/pdf/DataSheet_Mod60-W.pdf

    Thanks!
     
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