Where to find carbon SMD resistor

Discussion in 'General Electronics Chat' started by Dritech, Feb 20, 2016.

  1. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Hi,

    Below is a quote from the LT1167 datasheet:

    "Adding an external 5k input resistor in series with each input allows DC input fault voltages up to ±100V and improves the ESD immunity to 8kV (contact) and 15kV (air discharge), which is the IEC 1000-4-2 level 4 specification. The input resistors should be carbon and not metal film or carbon film."

    Where can I find SMD carbon resistors please. I only managed to find carbon film resistors.

    Thanks in advance.
     
    Last edited: Feb 20, 2016
  2. Papabravo

    Expert

    Feb 24, 2006
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    They may not exist in the SMD form factor. They are very nearly obsolete in thru-hold form as well. Hamfests and flea markets might be your salvation.
     
  3. bertus

    Administrator

    Apr 5, 2008
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  4. Lestraveled

    Well-Known Member

    May 19, 2014
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    I think what Linear Technologies is looking for is a non-inductive resistor. Inductance is the only parameter that carbon composition resistors have over film resistors. DigiKey stocks some carbon comp chip resistors but not many. They do make non-inductive film resistors.
     
  5. ian field

    Distinguished Member

    Oct 27, 2012
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    These days; carbon composition resistors are very rarely used - except sometimes for RF work where parasitic inductance would be a problem.

    AFAIK: most SMD resistors are basically just a slice of ceramic coated on one side with carbon or metal film, and end contacts plated on.

    Anything under M-Ohm will be just that and no more inductive than a PCB track the same proportions. Very high resistance values may involve a zig-zag film track on the ceramic which may incur some parasitic inductance - but I'm fairly sure the manufacturer will minimise that as much as possible.
     
  6. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Thanks for the replies. Will the resistors in the link below work for this application? Unfortunately they are not SMD, but that should be fine.

    RS carbon resistor
     
  7. Lestraveled

    Well-Known Member

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    Those are carbon composition resistors.
     
  8. ian field

    Distinguished Member

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    Composition are OK for dissipating transients, which is probably why the datasheets specifies them. They're not great for continuous dissipation because they drift.

    Film resistors are pretty much the opposite, they handle continuous dissipation pretty well but can fail open if subjected to transients.

    The application probably only needs "the letter of the law" as laid down in the datasheet, if the equipment is used in an electrically harsh environment.
     
  9. Lestraveled

    Well-Known Member

    May 19, 2014
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    @ian field
    In this application, accuracy is not needed. 5K verses the 1000 gigi-ohm input resistance will have no measurable impact. An inductive resistor would create voltage errors from transients in a noisy environment.
     
  10. SLK001

    Well-Known Member

    Nov 29, 2011
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    I agree with Lestraveled that the problem with the carbon film and metal film resistors is that they are spiral cut to the proper resistances. This "inductor" would make any transient pulse worse. I would bet that a regular SMT resistor would work quite well in this application.

    You could always contact the engineers at Linear and ask them for clarification. Specifying a virtually obsolete part to use with their part seems most "unwise".
     
  11. Lestraveled

    Well-Known Member

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    There are non-inductive film resistors. They are not cheap.
     
  12. ian field

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    At that range of values an SMD film resistor wouldn't be any more inductive than composition - maybe even less if its shorter relative to its width.

    It was merely a recap of the relative merits of the two types.

    The datasheet has to anticipate worst case scenario - in an average environment, the resistor type may not be *ALL THAT* critical.

    Composition resistors are very forgiving of transients that far exceed their voltage rating, whereas spikes can chimble away at film resistors until they become high or open.

    Chances are; the chip would fail long before the limiting factor became significant with either type of resistor.
     
  13. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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  14. ian field

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    Over what time period was that test conducted?

    Transients of 8kV as previously mentioned exceed the resistors voltage rating by quite a few orders of magnitude. For a narrow transient; the dissipation is well within spec, but each transient chips away a little of the film. Carbon composition is much more resilient to this type of abuse.

    As I stated previously: the datasheet has to take into account "worst case scenario" film resistors might be adequate in a mild environment.

    At the end of the day: The manufacturer only guarantees the part reliability/performance if you operate it within the specifications laid out in the datasheet. But very often those specifications are a little over cautious and you can get away with certain things.
     
  15. Lestraveled

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