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Calculating Maximum Permissible Current

  1. Aug 13, 2009 #1
    1. The problem statement, all variables and given/known data

    B) What is the maximum permissible current which can flow through a 1 watt resistor colour coded Brown, Black Orange, Gold ?

    C ) what is the highest voltage that can be applied to the resistor in part ( B ) ?


    2. Relevant equations

    P = I^2 * R



    3. The attempt at a solution

    P = 1w
    R = 10,000

    I = sqrt(P/R)
    I = sqrt(1/10,000)
    I = sqrt 0.00001
    I = 0.01 Ma

    Variance Factor :

    Gold = 5%

    0.01 * 1.05 = 0.0105Ma

    Maximum Current = 0.0105Ma

    Can someone please double check this for me ? i'm struggling abit with this problem. Thank you!!
     
  2. jcsd
  3. Aug 13, 2009 #2
    1. Recheck sqrt(1/10000) = sqrt(0.00001).

    2. Why are you expressing current in mA?

    3. Yes the tolerance is 5% but that should not be used to calculate maximum current. (The 5% refers to the resistance, so the resistance can vary from 9500 ohms to 10500 ohms. This means that in order to dissipate 1 W, the 9500 ohm resistor would require sqrt(1/9500) amps or 10.26 mA and the 10500 ohm resistor would require sqrt(1/10500) amps or 9.76 mA.) Note that a 5% change in resistance results in approximately 2.5% change in current.
     
  4. Aug 13, 2009 #3
    oh okay,

    1.so sqrt 0.00001 = 3.16

    2. Would it be 3.16 amps then ?

    3. Okay so is it correct to say that to calculate maximum current, i need to calculate the maximum resistance, using the tolerance of in this case 5 percent,. Then repeat the previous formulae I = sqrt(P/R) with the new resistance.

    I = sqrt 1/10500
    I = sqrt 9.523
    I = 9.76 Ma

    Thank you for your help. I often get mixed up with the units and make very basic mistakes, something i need to work on thanks..
     
  5. Aug 13, 2009 #4
    1. recalculate sqrt(1/10000) and watch your decimal places.

    2. No.
     
  6. Aug 13, 2009 #5
    Also to calculate the maximum voltage, would it be correct to say ...

    V=IR
    V = 9.76*9500 ( 9500 because that is the lowest resistance point )
    V = 0.00976*9500
    V = 92.72 volts

    Thank you
     
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