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Fluctuating Voltages

Discussion in 'Electronic Repair' started by onyxx, Sep 29, 2005.

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  1. onyxx

    onyxx Guest

    Hello, I am working on a SMPS from a PC monitor. Voltages across the
    circuit are all wrong and not only that but they are fluctuating as
    well. I started by checking the AC voltage from the outlet through the
    monitor power cable, it is a steady 221Vac (I am in South America,
    things are a little more dangerous here..lol) but when i plug in the
    monitor, ac voltage starts fluctuating from 215Vac to 221Vac and all
    voltages in between. I can trace this problem from the ac plug, all the
    way to the SMPS controller, bridge rectifier and main filter cap as
    well. So the problem occurs both on the ac and dc sides of the PS.
    Another thing that is happening is that if i measure voltage on the
    chopper resistor´s Drain i get over a thousand volts!!!!! DMM goes
    blank....I've compare the MOSFET to one that is working and this one
    seems fine.
    I thought that the chopper transformer was shorted but I´ve replaced
    it with a good one and the same thing happens. Can any one help me?

    Thank you...
     
  2. James Sweet

    James Sweet Guest

    Hello, I am working on a SMPS from a PC monitor. Voltages across the
    circuit are all wrong and not only that but they are fluctuating as
    well. I started by checking the AC voltage from the outlet through the
    monitor power cable, it is a steady 221Vac (I am in South America,
    things are a little more dangerous here..lol) but when i plug in the
    monitor, ac voltage starts fluctuating from 215Vac to 221Vac and all
    voltages in between. I can trace this problem from the ac plug, all the
    way to the SMPS controller, bridge rectifier and main filter cap as
    well. So the problem occurs both on the ac and dc sides of the PS.
    Another thing that is happening is that if i measure voltage on the
    chopper resistor´s Drain i get over a thousand volts!!!!! DMM goes
    blank....I've compare the MOSFET to one that is working and this one
    seems fine.
    I thought that the chopper transformer was shorted but I´ve replaced
    it with a good one and the same thing happens. Can any one help me?

    The high voltage at the transistor is normal, it's caused by the inductive
    kick of the transformer, I've never actually seen a switching transformer
    fail so that's never really been a suspect.

    How are the capacitors on the output side? The power supply should easily
    compensate for the fluctuating input voltage.
     
  3. onyxx

    onyxx Guest

    Hello and thanks. You say that the high voltage is normal...but is it
    normal if it stays like that always? If it is caused by the inductive
    kick of the trans...should it not just be a temporary thing?
    And yes, all caps SEEM fine, all i have to check them is my little
    piece of $&·% DMM.
     
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