burnt resistors on PCB changing them to higher wattage

Discussion in 'General Electronics Chat' started by relicmarks, May 20, 2008.

  1. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    When repairing PCB of different designs or modules

    If you find alot of 1/4 watt resistors burnt , some repair tech. will change the 1/4 resistor to 1/2 watt resistors so they won't get burnt up again so they can handle more current, but now the 1/2 watt resistors can handle all these extra current and voltage but now further down the chain the IC's melt or other 1/4 watt resistors burn

    Its weird how where the 1/4 resistors are at in the schematic that have more current

    From looking at a schematic how would you know which 1/4 resistors are going to get MORE CURRENT than others?

    1/4 watt resistor list
    1.) Resistors to ground? have more current?
    2.) Resistors that sink current?
    3.)

    Which resistors in a network or design can STRESS the 1/4 watt resistor?
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Nothing mysterious, P=IE, or P=E*E/R. If it is a low value resistor it needs looking at. You calculate the amount of voltage across it and calculate the wattage it will be dissipating.

    A general rule of thumb is you overrate a component by 100%. If the part is going to be dissipating 1/8W, then a 1/4W will work. If it dissipates 1/4W you need 1/2W, et cetera. While I've been known to violate this never on a permanent design.
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
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    If a circuit has been properly designed and has operated for some time before resistors burn up, there is likely to be some reason why the current has increased enough to burn those resistors. Without a schematic to look at, that is about the best I can say.

    Simply replacing the resistors with higher wattage units seems unlikely to find/solve the initial problem.
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    Truth! The burnt resistor is an important clue troubleshooting circuits, it almost always points directly where to look for a problem.
     
  5. thingmaker3

    Retired Moderator

    May 16, 2005
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    If there is too much current going through the resistor (regardless of power rating) then there is too much voltage across same resistor. Why is there too much voltage? Without solving this, one is tending the symptoms instead of curing the cause.
     
  6. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    How do i find the increase of current is coming from? or what is causing it?

    Why would low value resistors increase current?

    Is it best to have low value resistor as 1/2 watts because they have more current across them?
     
  7. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    What do u mean by this IC's sharing a common input voltage?

    How can a bunch of IC"s share the same common input voltage? only in a splitter or mixer would they be sharing inputs

    Why would sharing a common input voltage create "increase of current"? through components ?
     
  8. thingmaker3

    Retired Moderator

    May 16, 2005
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    I=E/R

    P=I*E=E^2/R=I^2*R

    No one in this topic has said anything about "IC's sharing a common input voltage." Did you intend to respond to a different thread? If so, I (or another moderator) can move the post for you.
     
  9. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    No thats what someone told me that can "incease current" is if IC's share a common input voltages
     
  10. Wendy

    Moderator

    Mar 24, 2008
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    Think in terms of shorts, things like caps, or shorted semiconductors. The clue is the voltage levels, if the resistor is burnt then it has had too much current through it. If the resistor is intact then you can measure the voltage. If you have a schematic you should be able to figure out what the voltage should be, appoximately. Too much current means too much voltage. The formula's are pretty simple, but it does take practice applying them.

    Resistors don't usually burn up of their own accord though, something else made them burn up.

    Something that hasn't been discussed, is this a specific problem? Maybe something you could share a schematic of?
     
  11. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    current carrying and current limiting capacity between 1/2 watt and 1/4watt.

    1,) the 1/2 watt resistor is not going to current limit but be able to take the increase of current from the design flaw


    I don't know why in certain areas on the board only some resistors burn up , all i can think of it that there is points where current sources ADD or voltage source ADD to have more current

    Or since the board uses IC's op-amps these "increase the current" so maybe thats where the increase of current comes from and the resistor wattage rates can't handle it?
     
  12. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Do you have a specific circuit in mind? If you do, post the schematic.
     
  13. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    NO just in general theory and electronic design knowledge
     
  14. thingmaker3

    Retired Moderator

    May 16, 2005
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    Try an internet search on "six step troubleshooting method" for an efficient & effective technique.
     
  15. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Here's my lecture notes from a lecture in 2000.
     
  16. thingmaker3

    Retired Moderator

    May 16, 2005
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    That one is a little different than I was taught, but should work well enough for most of today's "throw away" boards.

    I was taught:
    1) Observe all symptoms
    2) List possible faults logically responsible for symptoms observed
    3) Localize fault to subsystem (sometimes accomplished by step 2)
    4) Localize fault to circuit
    5) Localize fault to component
    6) Failure analysis

    Obviously, the degree to which one troubleshoots will be determined by one's situation. Many modern service techs will simply swap out a bad board for a good one, and send the good one off for someone else to work on. Or they'll just contribute the board to their local landfill.

    Regardless of the degree to which one troubleshoots, an understanding of basic concepts is absolutely vital. Familiarity with the system and its subsystems and circuits goes a long way toward making the process easier.

    It also helps if the fault is just a fault - rather than a double fault or a triple fault.:rolleyes:
     
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