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Discussion in 'Misc Electronics' started by curt, Dec 13, 2003.

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  1. curt

    curt Guest

    I understand the formula for current is:I=E/R
    and I notice some products say the current for 240v is i/2 that of
    120v.
    Am I asleep or why am I seeing the current increase with voltage
    increase (or do I have the formula wrong??
    Thanks,
    curt
     
  2. CWatters

    CWatters Guest

    No but I think you might be confusing two different sets of equations....

    1) If you have a fixed value resistor then the current flowing in the
    resistor is given by the equation: I = V/R Notice that if you double the
    voltage V you get double the current I.

    2) The power (Watts) consumed by a piece of electric equipment is given by
    the equation W = I x V. Lets say you were designing two different toasters -
    one designed to work on 240V and one designed to work on 120V. You wouldn't
    want the one running on 120V to take twice as long would you? So you design
    them both to use the same amount of power. If one operates on half the
    voltage it needs twice the current to produce the same power. eg V x I =
    (V/2) x (Ix2)

    Look at it another way. A fridge needs to burn a ceratain amount of power to
    keep the contents cold. If you feed the fridge twice the voltage it only
    needs half the current to keep the power the same.
     
  3. Eugene

    Eugene Guest

    NO: it's right, there are 2(pi)turns unaccounted for in the formula... and
    then squared!
     
  4. If a light is 120W for example, the light will take 1A at 120VAC or
    ..5A at 240VAC. It's the power that stays the same as the voltage
    changes. So the resistance for the 240VAC light is four times the
    resistance of a 120VAC light.

    Understand, rubber band?


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  5. [snip]
    Kind of a sorry example. The fridge could run at four times the
    power, but for one fourth the time, since it's a duty cycle type of
    appliance.

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    Just when you thought you had all this figured out, the gov't
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  6. John G

    John G Guest

    "Watson A.Name - Watt Sun, Dark Remover"
     
  7. You need to apply the right formula to your observation:

    Power = Voltage * Current
    The formula is correct, but does not apply to your product observation
    in this case, as the product is NOT a pure resistance.

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  8. CWatters

    CWatters Guest


    Ok ok I didn't actually mean the _same_ fridge.

    But if you want an argument.... I'm sure fridge pumps must have an optimum
    operating duty cycle at which the fridge overall is most efficient. In which
    case the fridge should be redesigned to operate close to that point
    regardless of the voltage that it's designed to run on.

    I note you didn't propose a better example?

    A toaster perhaps? Who wants to wait 4 times longer for their toast?
     
  9. CWatters

    CWatters Guest

    Oops sorry I see you did provide a better example. The light is better you
    win.


     
  10. Or Pop Tarts. :eek:)



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    ###Got a Question about ELECTRONICS? Check HERE First:###
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    goes directly to the trash unless you add NOSPAM in the
    Subject: line with other stuff. alondra101 <at> hotmail.com
    Don't be ripped off by the big book dealers. Go to the URL
    that will give you a choice and save you money(up to half).
    http://www.everybookstore.com You'll be glad you did!
    Just when you thought you had all this figured out, the gov't
    changed it: http://physics.nist.gov/cuu/Units/binary.html
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  11.  
  12. Pure resistance? What's that mean? I thinkk you mean the resistance
    of the light is non-linear.

    Pure? As opposed to unpure? Maybe you need to filter the light to
    make sure it's pure.. ;-)



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    ###Got a Question about ELECTRONICS? Check HERE First:###
    http://users.pandora.be/educypedia/electronics/databank.htm
    My email address is whitelisted. *All* email sent to it
    goes directly to the trash unless you add NOSPAM in the
    Subject: line with other stuff. alondra101 <at> hotmail.com
    Don't be ripped off by the big book dealers. Go to the URL
    that will give you a choice and save you money(up to half).
    http://www.everybookstore.com You'll be glad you did!
    Just when you thought you had all this figured out, the gov't
    changed it: http://physics.nist.gov/cuu/Units/binary.html
    @@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@
     
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