Power supply help

Discussion in 'General Electronics Chat' started by Enkidu, Jul 29, 2009.

  1. Enkidu

    Thread Starter New Member

    Jul 23, 2009
    3
    0
    Hello Everybody,

    I`m having problems with an SMPS I pulled from an LCD monitor. The supply has two outputs: +12V and +5V, and both indicate proper voltage when measured without a load. When I put a load on it (12V 0.3A fan), it goes into Auto-Restart mode and starts cycling twice a second or so. I tried replacing all elecrolytic capacitors, tested the two dual Schottky rectifiers (temporarily replaced them with others to be sure), but nothing has changed. It starts working with a smaller load (0.125A) and keeps working upto 0.3A, but then starts cycling again. Thought there might be a weird short somewhere on the secondary side, so I tried testing the feedback voltage, which should start the Auto-Restart mode at 4,5V, but it hasn`t even reached 1,5V. The supply is an LG p/n:6870T445D10, the switch controller is ICE 2ASO1. Any help and ideas are appreciated.
     
  2. petrolhead

    New Member

    Jul 26, 2009
    2
    0
    The supply was probably designed specifically for a certain range of loads and have its feedback loop designed as such that the 5v line needs at least a little bit of load for it to regulate properly. A load on the 12v line will cause the pwm controller to turn up the juice but as theres no load on the 5v the ovp will probably be kicking in killing the supply. These supplies have their feedback loops weighted to be more voltage stable for one of the outputs than the other, and the load on one affects the output on the other. They probably went forntighter control of the 5v to give the logic and control circuits an easier time than the less fussy video and audio drive circuits. Try it with a dummy load on the 5v line.
    Sorry about the spelling my finhers are a bit big for typing a long post on an iphone
     
  3. Enkidu

    Thread Starter New Member

    Jul 23, 2009
    3
    0
    Thanks, I`ll give it a try, when I get the time.
     
  4. Enkidu

    Thread Starter New Member

    Jul 23, 2009
    3
    0
    Both outputs can handle about 0.3 amps. But they can handle it simultaneously. Could there be something on the primary side to account for this ?
     
Loading...