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Need help to replace a resistor

ldarby
ldarby over 2 years ago

Hopefully this isn't a dumb question but this resistor is cracked and slightly burnt.  I measured it, it's open circuit.   It's from my bosch washing machine which is about 12 years old.  I think it's brown, black, silver, gold which is 0.1 Ohm (can't be grey, that's 1 giga ohm), but I've found several other photos showing this resistor on this pcb is brown black brown, 100 ohm, and people saying it's 100 ohm as well, and no one saying it's 0.1 ohm.  It's 'Ako 731799-06 BSH 9000449412'  if you would like search yourself.  Could it really be 100 ohm, and the burn slowly chared ring 3 to look grey/silver over time?  If it's supposed to be 100 ohm, and I replace it with 0.1, I'm sure it'll blow up...

Next question, since this failure seems to be very common, would it make sense to replace it with a higher power resistor e.g. 5W or 10W?

Thanks.


image

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  • shabaz
    shabaz over 2 years ago in reply to ldarby +4
    Unfortunately not enough detail from the photos, the connection snakes off from the top side of the board, but I can't see where due to angle. Anyway, it's a resistor directly off the Live connection…
  • ldarby
    ldarby over 2 years ago in reply to anniel747 +2
    Yeah, oops, maybe was possible. There was a much larger crack (not shown in above photos), I tried opening up that crack but that made the whole shell pop off and took all the wire with it... lesson l…
  • ldarby
    ldarby over 2 years ago in reply to shabaz +2
    Thanks very much for that, the chip left of center is a full bridge rectifier, it connects to that, then the cap is 10uF, 400V, and that connects to the D (Drain) pin. I couldn't see any inductors though…
  • shabaz
    shabaz over 2 years ago

    Hi,

    If you can share a high-res photo of the surrounding circuitry, on both sides of the board, then it is possible to make a better guess. It's very feasible that discoloration could have occurred. It also seems unlikely to be a 0.1 ohm resistor, but it's speculation unless more detail is provided.

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  • bobthemallett
    bobthemallett over 2 years ago

    Oh yeah, that's a 100R 5% (this type I doubt is available in 0R1), looks 3W or maybe 2W (smaller) (but check the size - dims are a VERY useful clue!). When a Power Resistor has smoked up it's often a FET gone closed circuit so check. If all going fine then should be a good enough rating but if you can find a 5W and get it to fit, can't hurt! Bob

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  • robogary
    robogary over 2 years ago

    An unlikely cause, the resistor maybe was sized too close to tolerance or had an inherent defect and caused it to smoke (I had a boss tell me once only really dumb engineers undersize resistors), but ..........significantly more probable is there is a failure downstream of the resistor.

    The downstream failure could be rock solid, or be intermittent like a chaffed wire than touches ground.

    Upsizing a resistor sometimes works but probably doesnt fix the root cause. 

    It also depends on what the machine is, resistors are sometime sacrificed in a design to burn out on a failure to save a significantly more expensive component, like a vacuum tube.

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  • dougw
    dougw over 2 years ago

    Could it be a burnt black ring? Making it a 10 ohm resistor...

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  • anniel747
    anniel747 over 2 years ago
    [deleted]
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  • Jan Cumps
    Jan Cumps over 2 years ago

    That resistor still looks good. They get hot and discolour by default. Can you measure it.

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  • ldarby
    ldarby over 2 years ago in reply to shabaz

    Thanks shabaz, here are the images:

    imageimage image

    I've circled where it's connected to, which connects to the main power switch on the front panel.  Earlier I measured 3V AC, 0V DC  on that when it was connected to mains and powered off, and 0V AV, 0V DC when switched on, hope this helps?

    Also robogary yes in 2nd pic I've circled a chip that probably also failed, the conformal coating had bubbles in it, it's a LNK304GN.  In fact these 2 components fail so often they're sold together: https://www.aliexpress.com/i/1005001758260213.html (which shows 100R).

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  • anniel747
    anniel747 over 2 years ago in reply to Jan Cumps
    [deleted]
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  • Jan Cumps
    Jan Cumps over 2 years ago in reply to anniel747

    then it's dead indeed :)

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  • ldarby
    ldarby over 2 years ago in reply to anniel747

    Thanks anniel747 , I just tried cutting it open but it basically shattered, but I've managed to measure 3-6 R on short lengths of the wire, so I'm sure it would be 100R total, not 0.1!

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