Inverter Feedback Problem

Discussion in 'General Electronics Chat' started by hazim, May 7, 2014.

  1. hazim

    Thread Starter Active Member

    Jan 3, 2008
    419
    13
    Dear,

    I have two UPS's (inverter with charger) that uses IC SG3525 to generate the PWM signal... The inverter's input is 12V battery and output is 220VAC.

    The problem is that the output voltage is not controlled by feedback, meaning that when the battery voltage is say 12.8V the output is 285V as tested, and as the voltage drops the output voltage decreases. Also, as the load increase the output voltage decrease.

    I know the buyer very well, he told me he have about 200pcs, so if I found a solution for this problem I could help him very much.

    The PCB should be studied, but before, as I know there should be a feedback winding in the transformer, which I can't find in the inverter. The picture clarify it better.

    So do you think there is any solution for this problem?

    Regards,
    Hazim

    [​IMG]
     
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    why not a small transformer and rectifier to provide feedback? connected to output of inverter to sense load on system.
     
  3. hazim

    Thread Starter Active Member

    Jan 3, 2008
    419
    13
    ok, but where to connect the rectifier's output??
     
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,998
    745
    You need to look at the circuit to see if it uses an opto-coupler, or direct voltage feedback,on the feed back loop to the error amp on pins 1&2, to see if you can modify it,

    datasheet
     
    Last edited: May 7, 2014
    hazim likes this.
  5. hazim

    Thread Starter Active Member

    Jan 3, 2008
    419
    13
    Dear,

    The circuit uses the opto-coupler 4N35 as clear in the picture, it's fed using a 1KΩ resistor after a 4-diode bridge, so I don't think the voltage on the bridge output is high voltage... Reverse engineering the PCB is hard and will take long time, so if the pictures are not enough, ask me about the components connections and I will help you.

    Note that I flip rotated one of the two pictures so they became on the same directions, easier to figure out.

    [​IMG]

    [​IMG]
     
  6. hazim

    Thread Starter Active Member

    Jan 3, 2008
    419
    13
    No any help?
     
  7. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    It may be worth your while to ship one to one of the gurus on here. If you could find one who liked a challenge.:)
     
    hazim likes this.
  8. hazim

    Thread Starter Active Member

    Jan 3, 2008
    419
    13
    I like challenge but I have no time at all. I believe I can find a solution, so I will spend my holiday trying to solve this problem... Thanks anyway :)
     
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