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Component Size Criticality

Discussion in 'General Electronics Discussion' started by modern_messiah, Jul 14, 2013.

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  1. modern_messiah

    modern_messiah

    29
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    Dec 1, 2010
    Hi all,

    Hopefully the simplest question I'll ever ask...

    How critical are the sizes of passive components such as resistors and capacitors in a circuit? As far as I am aware, the most important thing after the actual value of the part, is the power rating.

    I refer specifically to this circuit:

    https://www.dropbox.com/s/t45y1j8wc49w2ue/3XML Circuit.png (I can't seem to embed Dropbox links)

    As you can see resistors R1 and R2 are 2512 in size. This is far bigger than I want. I can get a 0.1ohm, 1W resistor in a 1218 package. Is there any reason why I can't use the smaller part?

    Any advice is greatly appreciated.

    Thanks,

    - Matt
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,838
    Jan 21, 2010
    OK, you have 2A going through a 0.1 ohm resistor..

    That's 0.4W dissipated.

    A 1W resistor is a wise choice.

    A 1/2W resistor may not be sufficient because it might fail at high ambient temperatures (the power rating of components needs to be de-rated as ambient temperature rises)

    Now, what size. Well, it doesn't really matter. However the smaller component will likely get hotter (but maybe it's constructed of materials that make this OK).

    If I were really concerned I might check the datasheets to make sure that the dissipation figure for the smaller one is not based on having it connected to large pads for example.

    1218 and 2512 are really not that different in size. (40% difference).

    How many of these are you building?

    Is the saving of a tiny piece of board real-estate such a big deal?

    Is the smaller part cheaper, or more expensive? If it's not more expensive in this case then I'd be looking a little deeper. It's not like component manufacturers to make devices larger and more expensive than they need to be without getting some benefit.
     
  3. modern_messiah

    modern_messiah

    29
    0
    Dec 1, 2010
    Thanks for the reply.

    Board space is at a premium. So much so that I'm thinking this circuit may not fit regardless. Not with proper thermal considerations taken into account at least.

    I haven't delved deep enough into any datasheets yet, and I don't plan on building many (2 or 3) at all. That is if I even do it now. The smaller part was half the price.

    - Matt
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,838
    Jan 21, 2010
    If board space is at a real premium you could always mount the devices on their edge.

    It will make the board less resistance to damage from flexing/vibration but it will take less area and increase surface area for cooling.

    It will be a little harder to do, but if you're only doing 2 or 3 then the effort will be minimal.

    And do delve into the datsheets, you learn stuff. Maybe the smaller parts are some fantastic newer cheaper style. Or maybe there is some tradeoff. It's always good to know.
     
  5. modern_messiah

    modern_messiah

    29
    0
    Dec 1, 2010
    I do delve into datasheets, but a lot of the time it's spanglish to me. I guess the more I read the more it'll make sense.

    I'm only doing this because 2 modules I built that were designed by someone else are not working. They use PICs and my PICKIT3 programmer is being odd. I might try and resolve that problem first.
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,838
    Jan 21, 2010
    I've just seen the thread. I wouldn't be worrying about the resistors.
     
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