Regulated 12v PSU running parallel banks of Leds in series. Potentiometer dimming??

Discussion in 'General Electronics Chat' started by SoulClinic, Oct 19, 2015.

  1. SoulClinic

    Thread Starter New Member

    Oct 19, 2015
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    Led wiring parallel series.gif

    Hi guys and thank you so much for your help in advance.
    Using the above diagram as a rough approach, i have decided on an attack plan..

    First I will mod an old PC Atx PSU as these have stable, regulated rails at 12v, 5v and 3.3v with lots of amps available
    As above in attachment, I will be running strings of 3watt leds (3.2 - 3.6v, 700mA) and I'm thinking I can , in theory, put a potentiometer between r1(obviously the correct levelling resistor) and d1 to control dimming? Would this be possible? Would each series string or "set" dim evenly or would only the last led on the string loose current/voltage? Or have I got the whole circuit type wrong?

    Sorry, but I'm somewhat new to all this.

    Cheers

    Tim
     
  2. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    A pot in series with R1 would provide even brightness control of D1,D2,D3. If you want to control the brightness of all LED strings from a single pot, PWM would be a better solution.
     
  3. SoulClinic

    Thread Starter New Member

    Oct 19, 2015
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    Thank you so much for your quick response.
    No, I was hoping to isolate and have independent control over each string but all sharing the same power supply? Am I right that the pot would go between r1 and d1? or could I put it before r1 so as to give more flexibility to pot positioning/access?
     
  4. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    It doesn't matter where, in the series circuit, the pot is located but make sure you use one that can dissipate enough power. There are better ways but they are a bit more complex.
     
  5. SoulClinic

    Thread Starter New Member

    Oct 19, 2015
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    Perfect, thanks again
     
  6. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    I smell pots burning up in their immediate future..
     
    blocco a spirale likes this.
  7. SoulClinic

    Thread Starter New Member

    Oct 19, 2015
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  8. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Since I assume you want this as cheap as possible there are these all over ebay that should work just fine...
    http://www.ebay.com/itm/DC12V-24V-8...367671?hash=item43e4332bf7:g:qpIAAOSw2XFUeE0G


    Personally when I build a dimmable high current LED setup I use AC/DC LED drivers like the meanwell LPF series which feature an input for a 100k potentiometer to allow dimming (or PWM or 0-10V analog).
    But I'd much rather spend $30 per string to know I have a reliable solution and I typically have 10-12 LEDs per string and multiple series strings..
    There are also the LDD series which would be used with your PC power supply and also allow you to not have the resistors.

    IMO anytime you go into the "high power" (1W or higher) LEDs then using resistors is just inefficient and a constant current supply is a much better solution.
     
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