Can a resistor in an old circuit show signs of damage by overheating?

Discussion in 'General Electronics Chat' started by MarkAB, Feb 27, 2016.

  1. MarkAB

    Thread Starter Member

    Feb 27, 2016
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    I have just repaired a burned out trace on the main PCB of a kitchen robot leading to a pair of serial connected resistors.
    The robot in now working but the resistors are overheating to a point that they smell. I do not think this is normal and I wonder if I should replace them or look at what is causing this to overheat.
    Can a resistor's value decrease as it deteriorates?

    KW715256.jpg
     
  2. #12

    Expert

    Nov 30, 2010
    16,257
    6,757
    yes.

    Resistors don't heat themselves. Current heats them. Replacing resistors won't fix this. Find the cause of the excess current.
     
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  3. SLK001

    Well-Known Member

    Nov 29, 2011
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    Yes.

    Your circuit's resistors may be under rated for the power they are dissipating. I would definely find out why this thing is overheating.

    What is a kitchen robot? An automatic bacon cooker or an automatic pancake maker?
     
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  4. takao21203

    Distinguished Member

    Apr 28, 2012
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    my guess its a tr less psu.

    Replace with USB adapter some are easy to remove from shell.
    There are such modules sold without shell.
    Even LED drivers do as the voltage wont rise infinitely you could patch the current as well.

    If your circuit needs more than 5v either patch the feedback loop or use dc dc converter booster.

    They use resistors because at 10,000 units its a considerable saving and failure rate is a bit higher probably as well, odors, you name it.
     
  5. MarkAB

    Thread Starter Member

    Feb 27, 2016
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    ????
    Any Ideas what could cause the excess current?
     
  6. #12

    Expert

    Nov 30, 2010
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    Any ideas where I left my crystal ball?
    We don't even know what a kitchen robot is. How could we know its internal circuitry?
     
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  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    I'd guess those fat resistors are droppers in a transformer-less supply. Check that diodes D2 and D3 haven't gone short-circuit.
     
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  8. wayneh

    Expert

    Sep 9, 2010
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    D3 looks odd in the photo, like it's translucent or reflecting something. Maybe burnt or maybe a camera anomaly. Anyway, check it.
     
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  9. MarkAB

    Thread Starter Member

    Feb 27, 2016
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    Sorry but that is not the actual PCB but a picture I found online of an identical one.

    Will check Diodes anyway and post picture of the actual one.

    Thanks for the help.
     
  10. WBahn

    Moderator

    Mar 31, 2012
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    The fact that you had PCB traces burning out in the first place means something is wrong. That something may or may not even be on this board -- you could have a short somewhere in this "robot". That may well have caused damage to this board beyond the burned trace such that, even if you fix the original problem, this board may now cause problems that might damage the robot in other ways. Best bet would be to check out the robot itself and then replace this board with a new one, if possible. Troubleshooting and repairing this board may not be practical.
     
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  11. MarkAB

    Thread Starter Member

    Feb 27, 2016
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    Hi guys just pulled out all the diodes and resistors and I am starting to think as WBahn said the problem could be elsewhere !
    The only thing is that there is nothing else but a motor and all this PCB does is handle some safety switches and spin a motor at different speed in one direction and the other.
    It is just a food mixer.
    Why could these resistors be heating up?

    kenwood km260.jpg
     
  12. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    All four of those big resistors look burnt and both the relays look to have suffered too.
    Is that glue, or spewed innards, on top of the big electrolytic cap below the resistors?
     
  13. MarkAB

    Thread Starter Member

    Feb 27, 2016
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    Removed and checked the resistors and they measured fine 2.14k,(both in series) well within the 5% parameter. and yes that is glue from the resistors. why do they glue them?
    The yellow one is a Carli MPX40 capacitor and I think its good, did a check on it but with a ohms meter. will pull out the relay but think its fine too.
    Motor looks good too.
     
  14. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    To provide physical support, since they're mounted on-end.
     
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  15. SLK001

    Well-Known Member

    Nov 29, 2011
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    Does the motor drag when running? Or, does it spin freely when spun by hand?

    Do you run your "robot" for long periods? How long does it take for the resistors to start smelling?

    Where is the picture of the back side of the board?
     
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  16. MarkAB

    Thread Starter Member

    Feb 27, 2016
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    Motor does not drag and yes it spins freely.
    I used to run it for 10 min as instruction manual states but now after 1 minute it starts to smell.

    front PCB.jpg

    And the rear

    Rear PCB.jpg
     
  17. SLK001

    Well-Known Member

    Nov 29, 2011
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    Did your big capacitor burn out? Check it carefully for a hole where plasma jetted out.
     
  18. MarkAB

    Thread Starter Member

    Feb 27, 2016
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    No that is just glue form the resistors support ):

    Thanx anyways
     
  19. SLK001

    Well-Known Member

    Nov 29, 2011
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    No, I mean check it in the area where the board is burned (when it was mounted). The capacitor is probably across the lines and may have broken down, which would have created a jet of plasma that would have shot out of a hole in the shell (or the bottom) and damaged the board. If you look at the actual damage, it starts under the capacitor.
     
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  20. MarkAB

    Thread Starter Member

    Feb 27, 2016
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    Do you mean the Carli MPX40 capacitor? it looks good, sealed and I have tried to check it with a normal digital meter but not to sure how to check it without a proper capacitor meter.
     
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