Fixing an ATX PSU, found charred resistors

Discussion in 'General Electronics Chat' started by NoelSof, Jan 8, 2016.

  1. NoelSof

    Thread Starter New Member

    Sep 22, 2015
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    Hi,
    I'm fixing this PC power supply which has a bunch of swollen electrolytics on the output. I have them all replaced with new ones, and it's working good now.
    I notice these bleeder resistors (or dummy loads?) on the output. Some of them, in the 5V rail, is low enough in resistance value, 20 Ohms - 2 Watts rated. Is this resistor really necessary to be in there? It's pretty hot to the touch when the power supply is on. If you can see in the picture, it's charred and has blackened the board around it. Also the capacitor near it seems to bulge more than the others.
    So can I just remove all those resistors, or at least replace them with higher value ones? what value would it be? I am aware that some SPMS requires a minimum load on the output to work correctly, but this power supply is to be used on what it's intended to, a PC.
    Thank you for your advice.
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Welcome to AAC
    Those resistors are needed for regulation.
    You can replace them with same resistance but higher wattage to reduce heat.
    The heat will heat the cap quicker than usual as that is indeed a poor design
     
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
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  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
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  5. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Looks totally fine to me..
    I see no "charring/blackening"
     
  6. NoelSof

    Thread Starter New Member

    Sep 22, 2015
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    Here's a better angle
     
  7. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    T'hat is a very old power supply. The metal powder in the big torroidal inductor may have started to oxidize and not work as efficdently as before. That means, the iron core saturates at lower current and the ability of the supply to meet originally specified currents may have fallen off. The supply will run hotter and not maintain voltage regulation.

    For 20 to $40, you can buy a brand new, cooler running, higher frequency, lighter weight power supply that will give you years of safe power.

    On the other hand, you can also try to confirm that it is charring - it looks more like oily mold fur. Try wiping it off with a damp cloth.
     
  8. NoelSof

    Thread Starter New Member

    Sep 22, 2015
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    Thanks you guys.
    Yeah, this PSU is a cheap one, but it's also the one that lasts up until now among my other PSUs that have gone rest in peace
    So I've moved the resistors to somewhere away from the caps, like in the picture. I hope that's ok.
     
  9. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    It maybe cheap, but its the easiest to modify to variable, uses a lm324 and Tl494 chips, much better than the At series of chips.
     
  10. #12

    Expert

    Nov 30, 2010
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    All I see is normal dust.
    You are going to find out when you plug it in.:D
    It's probably OK. The output is not inside the high frequency loop, so distance (and therefore inductance) shouldn't be an issue with a load resistor.
     
  11. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    From his last post there is some slight discoloration (blackening).. But open up any ATX power supply and you will see similar if not worse..
    Its "normal".. but a good preventative measure to address them too while you are in there..
     
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