dimmer CKT.

Discussion in 'General Electronics Chat' started by sumit123, Oct 28, 2013.

  1. sumit123

    Thread Starter New Member

    Oct 28, 2013
    11
    0
    Hi,

    We have SMPS 5V,26A but we need to use the voltage from 3V to 4V(use as a dimmer ckt.).We tried to drop the voltage from inbuilt potentiometer but it goes maximum 4Vdc.
    Kindly help us to resolve the issue.How we can drop it?
     
    Last edited by a moderator: Oct 28, 2013
  2. Ramussons

    Active Member

    May 3, 2013
    557
    92
    You will need to adjust the output voltage feedback of the PWM control. And the under voltage cutoff too.

    Can you get a schematic?

    What are the Heat sinked transistors?

    What are the IC's used?

    Ramesh
     
  3. sumit123

    Thread Starter New Member

    Oct 28, 2013
    11
    0
    Sorry we don't have schematic.

    the heat sink diode is SBL3040PT (2NO'S)
     
  4. Ramussons

    Active Member

    May 3, 2013
    557
    92
    What about the control IC's? Make an effort to trace the voltage feedback line.

    Else, use a Hi Power Linear Regulator.

    Ramesh
     
  5. #12

    Expert

    Nov 30, 2010
    16,347
    6,835
    There is no way to defeat the regulator externally. You can only adjust it on the circuit board. I have done this. I had a 6 volt smps and wanted 6.3 volts for vacuum tube filaments. Easy as changing one resistor...right after you read the schematic and know where to do the adjustment.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    There is your clue; the "inbuilt potentiometer" adjusts the feedback loop to the internal regulator. Either the potentiometer's value needs to change, or a fixed resistor next to the potentiometer needs to be replaced with a different value. You need to see what is attached to that potentiometer, and draw a schematic.
     
  7. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    784
    If you substitute an ATX PSU, that may not be strictly true.

    The 3.3V output usually has a sense line wire crimped into one of the 3.3V wires to sense the voltage a little closer to the actual load.

    BEWARE though; if you let the sense wire float the PSU will go bang!!!

    You also have to put a load on the 5V rail for everything to work correctly - but its in the right ballpark for the 26A the OP mentioned.

    If a lower current capability is acceptable - the surplus market used to be awash with small open-frame switchers, pick one with a TL431 error amp circuit and the voltage sensing resistors are easy to trace out.
     
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