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marking schemes for SMD resistors

Discussion in 'Electronic Design' started by Winfield Hill, Apr 15, 2006.

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  1. The conventional marking scheme for SMD 5% resistors is two
    digits plus an exponent of 10, e.g., 120 would be 12 ohms,
    and 122 would be 1.2k, etc. But, do _any_ manufacturers use
    a direct method, so 120 means 120 ohms and say 1k2 = 1.2k?
  2. James Waldby

    James Waldby Guest

    Among 0805's I've got on hand in the 1 to 10 ohm range,
    x.x seems to be a little more common than xRx, but both occur
    repeatedly. Eg, 1.1, 2R7, 3.3, etc. Besides those, I
    haven't seen any 5% SMD resistors or data sheets with direct

  3. Na.
    It always is the number of significant digits
    plus the exponent. For some reason, character
    were avoided.

  4. Guest

    That reason would probably be that e.g." 120K" would require one more
    character than "124". And the printed text is already small enough.
    More characters mean an even smaller font.

  5. James Waldby wrote...
    For resistors under 10 ohms, yes. Nothing above that?

    I ask because I've been analyzing my technician's washing-
    machine VFD motor-control circuit board. This has an 0805
    resistor labeled 120, which must be near 120 ohms, because
    if it was 12 ohms the circuit not only wouldn't work, it'd
    go up in smoke. Actually, it did go up in smoke, burning
    out the "120" resistor (and many other components), so I
    can't measure it. The circuit board has mostly US-made or
    familiar parts, but also a few foreign-looking parts. The
    other 0805 resistors have conventional labeling, a 33-ohm
    part says 330. But this 120 fellow simply can't be 12 ohms!

    Further complicating the issue is that this 120 resistor was
    thrown away when my technician replaced it, but both he and I
    clearly remember it said 120, I had even written it down. To
    my memory it looked just like the others. But hey, it really
    couldn't have been 12 ohms! :-<>
  6. Fred Bloggs

    Fred Bloggs Guest

    Which brand? A universal motor, transmissionless drive?
  7. colin

    colin Guest

    Ive come accros some marked 1R0, 1210 case size, ive never seen 1k0 etc
    though, maybe its just the low values. the coding system I came acros on
    some 1% 0603 is rather anoying, you have to look up a 2 digit code to find
    the 3 digit value and look up the letter to find the multiplier value.

    Colin =^.^=

  8. The "R" is used in place of a decimal point to make sure the value
    can be read. I wrote some javascripts to decode the three digit 5%
    values and another for the the four digit 1% and .1% values for
    production and stockroom workers. I'll put them on my website if anyone
    is interested. I also have one for SMD capacitors with the number/letter
    ID codes if anyone needs it.

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  9. Michael A. Terrell wrote...
    That sounds both useful and interesting, Michael. I like reading
    javascript code. :) Put it up for us, with a link from here.
  10. colin

    colin Guest

    Ive only just got into using javascript for some elementry calculations, I
    got fed up calculating reactances etc ever since i lost my walchart from an
    early issue of everyday electronics, and a single web page is far easier
    than creating a whole c++ application wich I would otherwise have done, it
    was just a smallish table of hand entered values, but I will try and make it
    so moving the cursor over the table gives all the inbetween values.

    I also just did a page to list all the frequencies available from a PLL
    within a range of N and R divisor values. only drwaback is it seems to get
    suddenly get very slow once a certain number of array entries is reached
    although this can be minimised.

    Incidently the problem I find with the 2 digit + 1 letter 0603 1% codes is
    they are extremly hard to distinguish (I need to use a microscope to
    distuinguish the last letter) its so easy to read 10C (12.4kohm) as 100ohms

    I dont know if this is common but I always used to surprise people when I
    could read colour resistor codes instantly, (especialy when I was working as
    a software engineer)

    Colin =^.^=
  11. joseph2k

    joseph2k Guest

    Gosh, the "typographic" resistor value markings date back to Apollo times
    about 40 years ago. they haven't changed much either. i was surprising
    people by reading the much more varied capacitor color codes.
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