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Resistance calculation

Discussion in 'General Electronics Discussion' started by selva, May 6, 2010.

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  1. selva

    selva

    25
    0
    Apr 26, 2010
    Hi ,

    Pls don't mind for this simple question. i got confused need some help.

    The output from battery is 3.3V and 4250mA, i have to reduce it to 2.5V and 11uA which is a input to an IC. I have to connect some resistor to reduce the voltage and current from the battery.
    can anyone plz help in calculating the resistor value to be connected.

    Thanks in advance.

    Warm regards,
    Selva
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,473
    2,819
    Jan 21, 2010
    unless the IC has a fixed and rather accurate 11 uA current requirement, a simple resistor won't do it.

    You would probably be better off with a 2.5V voltage regulator. You could use an adjustable voltage regulator similar to a LM317 (preferably in a small package) but its quiescent current will exceed the 11uA many times over.

    For such a low current, you may be able to get away with a voltage reference chip such as an LM285-2.5
     
  3. selva

    selva

    25
    0
    Apr 26, 2010
    Its not that Steve, i am not gonna implement in physical, i just want to know how the calculations are made to get in to an particular resistor in specific point of a circuit,i don't want to be accurate for that current lets make it around by 11uA to 20uA or 25uA.

    If i have misunderstood something wrong excuse me and inform me that.

    thanks
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,473
    2,819
    Jan 21, 2010
    In that case, you simply need the appropriate resistor to drop 0.8V at 11uA. See here.
     
  5. NickS

    NickS

    367
    0
    Apr 6, 2010
    Vout = R2 / (R1+R2) * Vin
     

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  6. 55pilot

    55pilot

    434
    3
    Feb 23, 2010
    That assumes I-Out is zero. That is not the case here.

    The full equation is:

    Vout = R2 / (R1+R2) * Vin - Iout*(R1*R2)/(R1+R2)

    ---55p
     
    Last edited: May 6, 2010
  7. NickS

    NickS

    367
    0
    Apr 6, 2010
    Right you are if he taps off at Vout 55p.

    I was assuming that R2 was his device, like I thought Steve had directed. So I was thinking he would approximate his IC as R2 (by knowing it requires 11uA @2.5V) and then he could solve for just the in line resistor R1.

    But looking at it now that was not so obvious. I should have been more specific
     
  8. 55pilot

    55pilot

    434
    3
    Feb 23, 2010
    I agree with you there. It is obvious that it is not obvious what is being asked, especially to the OP.

    Did I make myself obscure enough? :D

    ---55p
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,473
    2,819
    Jan 21, 2010
    I am certainly not certain that I am in uncertain agreement on that point.
     
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