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SMT caps question

Discussion in 'Electronic Components' started by Ken_B, Aug 30, 2004.

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  1. Ken_B

    Ken_B Guest

    I haven't worked with SMT's much, and have been trying to learn about the
    codes. However I can find no information on the small caps that have no
    printing on them whatsoever. The only variation I can detect between them is
    slight variations in color, and some variation in thickness.

    How does one determine the value of these caps?

    Thanks.
     
  2. Roger Gt

    Roger Gt Guest

    : I haven't worked with SMT's much, and have been trying to learn
    about the
    : codes. However I can find no information on the small caps that
    have no
    : printing on them whatsoever. The only variation I can detect
    between them is
    : slight variations in color, and some variation in thickness.
    :
    : How does one determine the value of these caps?
    :
    : Thanks.

    A meter?

    Some are marked, but in code, all are best read by a meter.
     
  3. Read the label printed on the reel.

    Best regards,
    Spehro Pefhany
     
  4. The small ceramic caps don't have any codes on them at all. The only
    way to tell the value is to measure it.
    Once it leaves the bag or reel it came in you loose all track of the
    value. A good reason not to have more than one type open and on the
    bench at any one time while hand soldering!

    Dave :)
     
  5. Ken_B

    Ken_B Guest

    So if one of those caps is bad, I'm out of luck without a schematic.

    I'm not a technician, just a hobbyist, and I'm trying to repair a device
    with an open smt cap. Does the variation in shade of color mean anything?

    I'm thinking I could infer the capacitance of the open one by measuring one
    that looks identical.

    Thanks for the replies.
     

  6. No, you cannot. My parts bins are full of SMT caps that cover a six decade
    range of capacitances and that all look exactly identical. Variation in
    shade probably means the caps came from different manufacturers or different
    manufacturing batches. Which might mean different values or different
    dielectrics; or it might not.

    You might be able to infer the capacitance from figuring out what the
    capacitor is doing in the circuit, even without a schematic. For instance,
    if it's between a supply pin of an IC and ground, it's a bypass capacitor,
    and 100nF is a likely value. If it's in a value-critical part of the
    circuit, though (e.g., something that determines frequency), you're out of
    luck. Unless it's in a stereo, in which case you might be able to find a
    corresponding cap in the other channel.
     
  7. Tim Shoppa

    Tim Shoppa Guest

    Not necessarily... an 0603 cap could be anything from 0.1pF to 4.7uF, a
    factor of almost ten million in capacitance.

    But figuring out what the cap does (bypass? tuned circuit? interstage
    coupling?) will give a better guess as to value(s) than just physical
    dimensions.

    Did the old (bad) cap crack in two, delaminate, blow up?

    And maybe it wasn't even a cap... there are unmarked SMT resistors, diodes,
    and fuses, too. (Diodes have a polarity but as long as they put the
    tape and reel in the right way, the polarity doesn't have to be marked.)

    Tim.
     
  8. Ken_B

    Ken_B Guest

    A nearby IC blew up. I measured the caps and resistors around it and all
    seemed ok except no reading on one cap (had a C number next to it).

    While they are miniaturizing everything, they should work on teeny-tiny
    markings that a person could read through a magnefying glass!

    No biggie, just some junk I thought I'd try to fix. Modern technology sure
    is making that harder to do, huh?
     
  9. Tim Shoppa

    Tim Shoppa Guest

    Testing components in-circuit isn't always reliable. While it's a bad
    thing that a "IC blew up" and it's possible that a cap failure is
    related (cause or effect), I still think that you have to understand what
    that capacitor is there for before you can start guessing values or
    reasons.
    12-layer PCB's don't make it any easier to trace the circuit :)

    Tim.
     
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