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Definitive guide to SMT resistor dissipation?

Discussion in 'Electronic Components' started by Lostgallifreyan, Nov 17, 2007.

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  1. I tried a search on Google:
    1206 resistor power dissipation

    I thought I'd get lots of helpful reults. I got lots of UNhelpful results,
    and further searching made matters worse, not better!

    Is there a definitive guide to standards for parts of size 0603, 0805,
    1206, 1210? If so, can anyone help me with direct linkage? I guess the
    contruction materials affect this, and things are always changing, but any
    stable or currently accurate references will do.

    I'm used to axial metal film resistors where you got 1/8 W, 1/4 W, or 0.5W
    and 0.6W, and 1W and 2W. With SMT, finding references is harder than
    finding actual non-SMT resistors themselves!
     
  2. Sod it, I'll rephrase that. What are the best 1% tolerance resistors in SMT
    to replace 0.6W and 2W metal film? (0.5W is also acceptable).
     
  3. Better rephrase that too... I want metal film. Just not axial lead types.
     
  4. Usually 250mW at 70°C "Ta" derated to zero at or around 155°C. The
    exact definition of Ta is another question.
    All the reputable makers have data sheets.


    Best regards,
    Spehro Pefhany
     
  5. Thanks. 250 mW is what I've seen enough times to think it's reasonable to
    assume most will match that. Same with 1210 doing 500 mW, I think, but I
    haven't seen enough examples.

    I guess Ta means the actual temperature of the resistor, as opposed to
    ambient, which really could mean anything depending how proximate it was,
    or whether the air moved...
    I don't even know who they are, with SMT resistors, and specific maker's
    guides should never be relied on for general standards, which is why I
    asked the question about general guides. Asking in a place where people
    have direct experience of various standards is the best thing, rather than
    blundering on Google for days...

    Most resistors found on eBay might have a size declared but no hint of who
    made them.
     
  6. Ok, found one answer to this:
    http://www.rcd-comp.com/rcd/press/SM Guide Resistors page.pdf
    Might be useful to other people, it's nice... Any others out there?
     
  7. Fred Abse

    Fred Abse Guest

    The numbers are dimensions in 10-mil (thou) units:

    0603 is .06" x .03" (60 mil by 30 mil)
    0805 is likewise .080" x .050"
    etc. etc.

    Power dissipation depends to a large extent on how they are mounted,
    size of pad/trace, proximity, the type of resistor construction and
    material.

    Consult the manufacturers' data for recommended dissipation limits, it's
    freely available (Vishay, Yageo, etc).
     
  8. Thankyou. Vishay I discovered in the last half hour. :) Looked like a good
    reference capable of being extended to other firms parts, in many cases I
    saw.

    I noticed that the dissipation specs are conservative, so I guess it's
    possible to increase it in the case of one or two with a good anchorage on
    broad pads. I might just try a test to see what it takes to overheat them
    in an actual-size test mounting. Can't do that till I get them though,
    hence the questions...

    Is there any SMT resistor that is ideal to substitute for an 0.6W metal
    film type? Something standardised enough that it won't matter exactly who I
    buy it from?
     
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