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Applying mutiple resistors to create an 18k

  1. Jun 17, 2011 #1
    Hello. my robot requires an 18k resistor at one point. my problem is that I don't have one and my radioshack has never heard of it.

    anyway, I was wondering on how to create one. I know that adding in a series circuit it's just R1+R2... and I think that parallel circuits add with the reciprocal.

    so how can one get an 18k. Someone told me that I could make a parallel circuit with a 22k and some 1ks, but I don't think that would work. I know that I could create one in a serial circuit, but that might take a lot of resistors.

    thanks for the help.
     
  2. jcsd
  3. Jun 17, 2011 #2

    uart

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    18k is very much a standard value. In higher tolerance parts many more values may be available, but even the most basic 10% parts should be available in decade multiples of the following.

    1, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2
     
  4. Jun 17, 2011 #3

    MATLABdude

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    5% resistors follow the E24 series: there are 24 resistor values in a decade. That's fancy speak for there being 24 logarithmically-spaced values from 10 to 91 ohms (and 24 from 100 to 910 and so forth).

    You can calculate the value of two parallel resistances as follows:
    [tex]\frac{1}{R_{parallel}}=\left(\frac{1}{R_{1}}+\frac{1}{R_{2}}\right)[/tex] or
    [tex]R_{parallel}=\left(\frac{1}{R_{1}}+\frac{1}{R_{2}}\right)^{-1}[/tex]

    Three resistors in parallel add the same way:
    [tex]\frac{1}{R_{parallel}}=\left(\frac{1}{R_{1}}+\frac{1}{R_{2}}+\frac{1}{R_{3}}\right)[/tex]

    A handy thing to remember is that two equal resistors in parallel have an equivalent resistance of half their value (three in parallel having a third, and so on). So if you don't have an 18k, you can put two 36k resistors in parallel (which should be more plentiful--hit search to do the calculation):
    http://www.google.com/webhp?q=(1/36+1/36)^-1
     
  5. Jun 17, 2011 #4

    uart

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    Yep, and both the E12 and the E24 series include 18k. So it's pretty hard to believe that radioshack has never heard of it.
     
  6. Jun 17, 2011 #5

    MATLABdude

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    Maybe the sales guy was just trying to sell some batteries or an extended warranty?

    In any case, to the OP, I recommend finding a real electronics supply / hobby shop in your area (assuming one still exists).
     
  7. Jun 17, 2011 #6
    thanks for your help. I see how I can do this now.
     
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