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Fixing old Fluke DMM, why the carbon resistor?

Discussion in 'Hobby Electronics' started by [email protected], Sep 9, 2008.

  1. Guest

    I zapped an old Fluke 23 yesterday by inadvertantly applying about
    several KV which burnt a 1 Megohm 5% carbon composition resistor which
    is in series with an overvoltage MOV type thing with a split in it.
    This resistor is connected to the + input.

    I don't have the service manual or schematic, does anyone know if this
    needs to be replaced with a carbon, other resistors are metal film?
     
  2. The service manual is here (two parts):
    http://www.eserviceinfo.com/index.php?what=search2&searchstring=77
    23 is the same as the 73.

    Parts list shows it as a "cer" type, that would be ceramic.

    Dave.
     
  3. Den

    Den Guest


    I wonder if that is the same substrate as used in fusible resistors often
    used in similar sacrificial applications?
     
  4. Guest

  5. Guest

    It appears that the fusible resistors are explicitly stated, and this
    one, R2 is not one of them.
     
  6. Phil Allison

    Phil Allison Guest

    ** Nonsense.

    "cer" indicates a " cermet" aka " metal glaze " resistor.


    Got it, thanks Dave.


    ** But where ??


    ....... Phil
     
  7. Not at all. Could well be ceramic composition, like this one for
    example:
    http://www.ohmite.com/catalog/pdf/ox_oy_series.pdf
    Designed for high energy surges and the like.

    The actual part number is in the BOM if Geoff is interested to know
    what the exact one is.

    Dave.
     
  8. Phil Allison

    Phil Allison Guest

    "David L. Jones"
    "Phil Allison"

    **Restoring the crucial snipped line:

    " "cer" indicates a " cermet" aka " metal glaze " resistor. "

    ** Fraid it does - you OVER- SNIPPING ASREHOLE!!



    ** Bollocks.



    ...... Phil
     
  9. Franc Zabkar

    Franc Zabkar Guest

    R2 has a part number of RG1/2-105M-5%. and is described as ...

    Res, Cer, 1M, 1W, 5%

    FWIW, the following datasheet is for RG series "PRECISION NON MILITARY
    METAL GLAZE™ RESISTORS":
    http://www.irctt.com/pdf_files/RG.pdf

    The RG 1/2 types are rated at 1W, 5%, ±200 ppm/°C.

    The construction is described as "Metal Glaze thick film element fired
    at 1000°C to solid ceramic core".

    The Fluke manual lists the following resistors from the same
    manufacturer:

    R1 Res, MF, 1K, fusible, 5%, p/n FA8466
    R20 Res, WW, fusible, .36, 2W p/n SPF3605

    FWIW, IRC also have FA8025 series (FA8225, FA8325, FA8425), 2%, metal
    film, flameproof resistors on a ceramic rod:
    http://www.irctt.com/pdf_files/FA8025.pdf

    .... and SP20F series 1W, fusible, wirewound resistors:
    http://www.irctt.com/pdf_files/SP20.pdf

    - Franc Zabkar
     
  10. Phil Allison

    Phil Allison Guest

    "Franc Zabkar"
    "David L. Jones"
    "Phil Allison"

    ** QED.

    Good research - Franc !





    ..... Phil
     
  11. Eeyore

    Eeyore Guest

    My thinking too. Carbon resistors can cause really interesting fires. Get a
    flameproof / fusible one.

    Graham
     
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