PSU with FAN8754 won't torn on

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  • rievax_60
    Badcaps Veteran
    • May 2012
    • 897
    • australia

    #41
    Re: PSU with FAN8754 won't torn on

    The data sheet for the FAN8754 mentions that it has built in protection for open regulation loop. You can't depend on it to save the power supply every time.

    Comment

    • dick_barton
      Badcaps Legend
      • Aug 2015
      • 6642
      • Wales

      #42
      Re: PSU with FAN8754 won't torn on

      I've simulated the opto-isolator circuit attached. The opto-isolator is not activated until the 24V output supply comes up at which point it is switched on. Anything less than 24V at the output and the opto-isolator is off.

      Edit
      D3 acting as the internal opto-isolator led.
      Attached Files
      Last edited by dick_barton; 09-28-2016, 07:27 AM.
      Willing to help but I'm no expert.

      Comment

      • rievax_60
        Badcaps Veteran
        • May 2012
        • 897
        • australia

        #43
        Re: PSU with FAN8754 won't torn on

        I'm suspecting that there is a fault causing the opto to turn on when the output is only 3v.

        Comment

        • budm
          Badcaps Legend
          • Feb 2010
          • 40746
          • USA

          #44
          Re: PSU with FAN8754 won't torn on

          Originally posted by fejlong
          Resistance between pin 1 and 2 of IC U101, with no power applied, is 100 Ohms.
          Voltage drop on optcoupler pins 1 and 2 is 1.05V
          With power on, led voltage of the optocoupler between pin 1 and 2 is 1.05V .
          So NO power applied to the board and you measured the resistance between pin 2 (FB) and GND and you get 100 Ohms reading which is not normal, it should be in thousands of Ohms.
          As you can see in the diagram, there is 100 Ohms resistor R103 connected to pin 2, the other end of the resistor is connected to the MOSFET Q101 the Source of the MOSFET is then connected to GND, so you should check and see if that MOSFET is shorted out or not.
          Last edited by budm; 09-28-2016, 09:07 AM.
          Never stop learning
          Basic LCD TV and Monitor troubleshooting guides.
          http://www.badcaps.net/forum/showthr...956#post305956

          Voltage Regulator (LDO) testing:
          http://www.badcaps.net/forum/showthr...999#post300999

          Inverter testing using old CFL:
          http://www.badcaps.net/forum/showthr...er+testing+cfl

          Tear down pictures : Hit the ">" Show Albums and stories" on the left side
          http://s807.photobucket.com/user/budm/library/

          TV Factory reset codes listing:
          http://www.badcaps.net/forum/showthread.php?t=24809

          Comment

          • dick_barton
            Badcaps Legend
            • Aug 2015
            • 6642
            • Wales

            #45
            Re: PSU with FAN8754 won't torn on

            Have you disconnected the main board from the power supply and used a 1K resistor to connect 3.3V (standby voltage) to P_On and then checked the 24V supply?
            Willing to help but I'm no expert.

            Comment

            • fejlong
              Senior Member
              • Dec 2013
              • 55
              • United Kingdom

              #46
              Re: PSU with FAN8754 won't torn on

              Hey, I'm back. I just had a day off and had time to play with this PSU.

              I did played with the whole day,and I was really frustrated, but the success arrived after about 12 hours of thinkering and trial, but at the end worth it.
              I tried finding the 431 problem, with a circuit built on a proto board, similar to the one which regulates the voltage. No luck
              Inspecting, measuring resistors, caps, opto's, either removing them, and with new spare parts. Not working.
              At the end I measures the voltage coming out from the 431 cathode, and driving it with an external laboratory PSU adjusting from 0-32V. The voltage on the cathode was cutting at 2.8V . After trying again, it was cutting at 4.5V . Finally I decided to desolder the 431 SMD type and solder a new one back. And voila, it works.

              Thanks for all of you guys trying to help, especially for mr. rievax 60 "I'm suspecting that there is a fault causing the opto to turn on when the output is only 3v."
              you where right, the 431 was faulty and was turning on randomly.
              Another TV saved from being a waste for the planet.

              Comment

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