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resistor question on 'aircasting' project

Discussion in 'General Electronics Discussion' started by HNeil, Jan 17, 2013.

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  1. HNeil

    HNeil

    2
    0
    Jan 17, 2013
    Hi,
    I am beginning to build an AirCasting Air Monitor (a great project: http://aircasting.org/ ) and am following the extensive instructions here: http://www.habitatmap.org/habitatmap_docs/HowToBuildAnAirCastingAirMonitor.pdf .

    My question is about sourcing the 27 ohm resistor. I simply can’t tell which one I need –I have asked the people who wrote the instructions but I have not heard back and don’t want to pester them, it could be a simple question. The difference between them seems to be in watts, which I don’t know enough about to be able to choose without extensive research.

    Can anyone advise me on which of these I should order? Or perhaps some other alternative? ( I’ve tried to find it at 5 different radio shacks and am resorting to online).

    The first two look like the one in the instructions PDF (page 8 and in the steps photos after) but have different specs even though they seem to be using identical jpgs (wouldn’t the color code change if they are different?):

    http://www.radioshack.com/product/index.jsp?productId=12573183 This is .27 not 27 ohms.

    http://www.radioshack.com/product/index.jsp?productId=12554423 This one might be it?

    Other possibilities:

    http://www.parts-express.com/pe/showdetl.cfm?partnumber=002-27 Looks like wrong colors.

    http://www.radioshack.com/product/index.jsp?productId=12554446 Photo helpfully in black and white.

    I’d appreciate any advice, I am looking forward to building this monitor!
    Thanks much—
    Heidi

    PS this is my first time reading/posting to the forum, it looks like a terrific resource.
     
  2. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    I would think that any 27 ohm resistor would work in this circuit. The resistors run in a low powered circuit and the power is likely to be low. I have not calculated the power.

    A 27 ohm resistor will have the colours:-
    Red 2
    Violet 7
    Black 0 (number of zeros)

    A quarter or half watt resistor should do. Small resistors are not labelled with the power rating. You can get an estimate of the power rating from the size.

    W=V*V/R or I*I*R or I*V
     
  3. HNeil

    HNeil

    2
    0
    Jan 17, 2013
    Thank you! That's very helpful, much appreciated.
     
  4. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    651
    May 8, 2012
    There was a time when 1/2 Watt resistors were the most common wattage found in most circuits. Today 1/4 Watt are the most common and easiest to source.

    Chris
     
  5. BobK

    BobK

    7,682
    1,686
    Jan 5, 2010
    Unless you are doing SMT.

    Bob
     
  6. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    651
    May 8, 2012
    You bring up a good point. What is the most common SMT wattage? If I were a betting man I'd put odds on something near 1/8W but I'm guessing.

    Chris
     
  7. CocaCola

    CocaCola

    3,635
    5
    Apr 7, 2012
    1/8W is the most common SMD rating but you really need to check the datasheets as sometimes you will find 1/4W or even some other odd value, never assume a rating based on the package size basically...
     
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