PCB Burning from Hot resistors

Discussion in 'General Electronics Chat' started by RodneyB, Dec 12, 2014.

  1. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    I have attached a picture of a PCB that I have which is burning from the heat generated by the resistors. Is there any way of protecting the PCB from burning.

    I was thinking about putting aluminium foil under the resistors but needed conformation and then that also comes with a set of its own problems.
     
  2. bertus

    Administrator

    Apr 5, 2008
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  3. #12

    Expert

    Nov 30, 2010
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    You can also buy resistors that don't run so hot per watt, or just buy over rated resistors like 5W rated in a 3W position. I recognize that particular shape as a resistor that can survive higher temperatures...but the circuit board can't!
     
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  4. bertus

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  5. #12

    Expert

    Nov 30, 2010
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    Yes. There are several ways to get bulkier resistors, some round, some square. What I wanted to emphasize is the ends being larger than the resistor body and that light grey background color indicating the, "run hot" resistors.
     
  6. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Resistors should always be sized at 2 to 3 times the required wattage and then keep them off the board by 1/8" or more.
    That should be more than sufficient.
     
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  7. alfacliff

    Well-Known Member

    Dec 13, 2013
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    if thbere are no pc runs under the resistors, drill some holes in the fiberglass for ventilation. also, if you have replacement resistors, mount them on longer leads to space them above the board.
     
  8. sheldons

    Active Member

    Oct 26, 2011
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    All good advice but as the circuit is just powering leds there should never be that much heat generated resistor wise to burn the board in that way if the resistor value/wattage has been properly worked out with reference to the supply voltage available. ....are these ran direct from mains ? What is the mystery supply voltage used ??? You could fit slightly higher wattage resistors and space them away from the board a little.....
     
  9. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    If you look carefully at the picture the annular rings for the resistors also show discoloration. This is due to the leads themselves conducting heat out of the resistor into the board. Spacing the body of the part will only help a little with that issue.

    The pads should be greatly increased in size to large copper masses to aid in heat dissipation. These pads should be of the "thermal via" type that isolated the pad a bit from the mass so you can still solder to it.
     
  10. MrChips

    Moderator

    Oct 2, 2009
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    I think the issues are very obvious:
    1. PCB is charred
    2. Resistors not rated for proper power dissipation
    3. Solder joints will deteriorate
    You are powering the LEDs from AC mains, correct?

    That is a violation of the Terms of Service (TOS) of All About Circuits and further discussion along those lines will result in this thread being closed.
     
  11. BobTPH

    Active Member

    Jun 5, 2013
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    I doubt that it is powered from mains as the resistors appear to be 100 Ohms.

    Bob
     
  12. sheldons

    Active Member

    Oct 26, 2011
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    There's diodes there too(1N4007 ?)......still shouldn't be running hot as they are only driving leds....so....what is the supply voltage used???...
     
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