why resistor is open in this circuit

Discussion in 'The Projects Forum' started by Benengineer, Feb 23, 2016.

  1. Benengineer

    Thread Starter Member

    Feb 6, 2016
    101
    2
    Hello, can anyone help me with resistor issue in this circuit
    Please see resistor R149, the value is about 1K. However, when I turn on the power, the resistor is open when I measure it with multi-meter. Does anyone know the reason?
    The other circuit for LINE, LINE_AFT_SHUNT and NEUTRAL is below.


    Thank you for your help.
     
    Last edited: Feb 24, 2016
  2. #12

    Expert

    Nov 30, 2010
    16,298
    6,810
    You are not supposed to try to measure ohms with the power on.
    Turning the power on doesn't magically make the resistor turn into infinity ohms and turning the power off doesn't magically make the resistor 1K again.
     
  3. Benengineer

    Thread Starter Member

    Feb 6, 2016
    101
    2
    Thank you. I mean its value is Mohms, not 1K when the power is off. I guess the esd damage the resistor. Am I right?
     
  4. #12

    Expert

    Nov 30, 2010
    16,298
    6,810
    Resistors are mostly immune to electrostatic discharge, but if it measures as too much resistance with the power off, it's wrong. More likely is you have a mistake in something else.
     
  5. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Lift the resistor out at one end and then measure it, with the power off.
     
  6. Benengineer

    Thread Starter Member

    Feb 6, 2016
    101
    2
    It is totally open now
     
  7. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    So its open circuit, over 1meg ohms?
     
  8. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Then its destroyed.. or your meter can't measure it.. or its not the resistor you thought it was.. or you don't know how to use your meter properly..
     
    absf and #12 like this.
  9. Benengineer

    Thread Starter Member

    Feb 6, 2016
    101
    2
    thank you, I am checking it now.
     
  10. Benengineer

    Thread Starter Member

    Feb 6, 2016
    101
    2
    Yes.
     
  11. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Case closed....
     
  12. RichardO

    Well-Known Member

    May 4, 2013
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    It seems to me the next questions should be: "Why did the resistor fail?". I am guesing there is something wrong in the "LINE_AFT_SHUNT" circuit.

    edit:
    And the next question is: "What else has failed?"
     
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  13. #12

    Expert

    Nov 30, 2010
    16,298
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    It's hard to kill a 1K resistor unless you burn it to death. I'm with mcgyver in post #8. There simply has to be some other mistake going on.
     
  14. RichardO

    Well-Known Member

    May 4, 2013
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    Does the resistor look burned?
     
  15. ian field

    Distinguished Member

    Oct 27, 2012
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    My first ever DMM was provided by someone doing that.

    A Hung Chang obscure model that I rescued from the bin, the resistance and ACV ranges were dead.

    Nothing to lose I opened it up - a complex looking LSI chip and a socketed op-amp - take a wild guess at what I tried next.

    There was a preset pot on the board, so I borrowed a meter with a recent calibration certificate and made sure the restored ranges were at least somewhere close to where they should be.
     
  16. Benengineer

    Thread Starter Member

    Feb 6, 2016
    101
    2
    The resistor is damage
     
  17. ian field

    Distinguished Member

    Oct 27, 2012
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    Could be anything downstream - recently I had a TV with breakdown in the common mode choke, that probably wouldn't have shown up on DC checks.

    Take out the chopper transistor(s) - if it still goes bang; its probably the bridge rectifier.
     
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