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SMPS Power Supply weird problem.

Discussion in 'Power Electronics' started by RHeTTRoNiCS, Apr 24, 2013.

  1. RHeTTRoNiCS

    RHeTTRoNiCS

    35
    0
    Mar 28, 2013
    Hello Guys..here i am again...now t-shooting again an smps...this smps when u power initially, it'll give u good output and AC fan spin correctly. But when u turn-off and power on again, the fan spin very slow and the ouput gets only 1.5Vdc only (expected 5Vdc). Ok here's another thing, when i discharged the high voltage dc capacitor and power-on it'll goes back to good condition...Thus, u need to discharge the caps if u want to power-up the second time to maintain good operation....otherwise, it'll gives u bad output and low spin fan :confused:

    Hope someone can help...thanks a lot.
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,174
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    Jan 21, 2010
    Do you have a schematic and pictures of the board(s)?
     
  3. RHeTTRoNiCS

    RHeTTRoNiCS

    35
    0
    Mar 28, 2013
    It's an smps for computer (cpu).
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,174
    2,689
    Jan 21, 2010
    Buy a new one. They're cheap compared with the cost of the rest of your computer and the data you may have stored on it.

    If you want to play with it afterwards to use it for something else, go for it.
     
  5. RHeTTRoNiCS

    RHeTTRoNiCS

    35
    0
    Mar 28, 2013
    Yes actually we bought a new one already...but it was installed right away, forgetting to use it for while as reference for t-shooting the faulty one...Now the end user don't want to pull it out again :-(
    Anyways, i'll tried to find some tips from other forums....and continue exploring the unit :)
     
  6. RHeTTRoNiCS

    RHeTTRoNiCS

    35
    0
    Mar 28, 2013
    Hello Guys...have some good news on this topic and wanted to share...Got some tips from other forums....anyways, the culprit was the power mosfet...might be switching getting slower for some reason :-( ???
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,174
    2,689
    Jan 21, 2010
    That's interesting.

    I'd love to know the thinking behind that. (I'm not disputing the answer, just at how it was arrived at)
     
  8. Rleo6965

    Rleo6965

    585
    9
    Jan 22, 2012
    If problem re-occur. Check for defective capacitor and resistor changed value.
     
  9. RHeTTRoNiCS

    RHeTTRoNiCS

    35
    0
    Mar 28, 2013

    Actually that's a sort of wild thinking of mine...the thing i know to discharge this big caps is when the mosfet switching to ground...:confused: (not sure)...but the only thing i pretty sure is that when i replaced the mosfets the units goes well.
     
  10. RHeTTRoNiCS

    RHeTTRoNiCS

    35
    0
    Mar 28, 2013
    Yes all big caps i'd changed before but not helping...ohh, i never check the resistors :rolleyes:

    Let's wait for the unit to comeback :D
     
  11. Rleo6965

    Rleo6965

    585
    9
    Jan 22, 2012
    Much better if you have spare time. Build an ESR capacitor tester. It's a big help for troubleshooting in electronics. Especially smps circuits.
     
  12. mechtronics

    mechtronics

    153
    2
    Aug 7, 2011
    yes it could be the resistor when they over heat there resistance tends to increase or decrease which can cause slow discharge or opposite through the capacitor.
     
  13. RHeTTRoNiCS

    RHeTTRoNiCS

    35
    0
    Mar 28, 2013


    Yes, we have LCR/ESR meter BK precission 889b model...but the problem is i seldom use it because as far as i know, measuring the parts in-circuit will give u inaccurate reading...you need to remove the part and measure it manually...Is there any tips u can share on this? thanks
     
  14. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,174
    2,689
    Jan 21, 2010
    Most ESR measurements can be taken in-circuit.

    1) the voltages applies are typically much lower than needed to forward bias a semiconductor junction (100mV is typical)

    2) the capacitors should have the lowest impedance in the circuit, so surrounding resistors and inductors etc should not have a huge effect.

    The remaining major issue is when there are several capacitors in parallel.
     
  15. Rleo6965

    Rleo6965

    585
    9
    Jan 22, 2012
    Depends on the circuit design of ESR tester. Googled ESR Tester from Internet and build it. It's using high frequency step down transformer. It's high impedance and not loading in circuit capacitor testing.
     
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