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Query regarding Resistors and cables and such

  • 31-07-2010 8:35pm
    #1
    BrokenArrows
    Registered Users, Registered Users 2 Posts: 7,501 ✭✭✭


    Right I have a small 12v DC fan .05A
    I want to drop the voltage from 12v to 7v.

    I plan on using a 140ohm resistor with a 1watt ratting.

    The fan i have has two wires one red, one black which connect to the circuit board with a clip.

    Where do i attach the resistor to get this to work correctly?


    Thanks.


Comments

  • #2
    RoundyMooney
    Closed Accounts Posts: 10,910 ✭✭✭✭RoundyMooney


    BrokenArrows wrote: »
    Right I have a small 12v DC fan .05A
    I want to drop the voltage from 12v to 7v.

    I plan on using a 140ohm resistor with a 1watt ratting.

    The fan i have has two wires one red, one black which connect to the circuit board with a clip.

    Where do i attach the resistor to get this to work correctly?


    Thanks.

    You need to drop 5v across the resistor, so a 100ohm will do fine. That will dissipate a quarter of a watt, but best to use a half watt resistor to be on the safe side.

    Just break the red wire (the positive), and connect the resistor in series with it, i.e., between the positive and the fan board.


  • #3
    BrokenArrows
    Registered Users, Registered Users 2 Posts: 7,501 ✭✭✭BrokenArrows


    RoundyMooney wrote: »
    You need to drop 5v across the resistor, so a 100ohm will do fine. That will dissipate a quarter of a watt, but best to use a half watt resistor to be on the safe side.

    Just break the red wire (the positive), and connect the resistor in series with it, i.e., between the positive and the fan board.

    excellent. Ill give this a shot.

    on a side note does anyone know where i can get a good tutorial on how Thermistors work.


  • #4
    M cebee
    Closed Accounts Posts: 4,431 ✭✭✭M cebee


    http://akapuma.info/test/ntc.pdf

    i just happened to be reading this recently-might be of interest


  • #5
    FarmerGreen
    Registered Users, Registered Users 2 Posts: 1,134 ✭✭✭FarmerGreen


    Just one small proplem with this is the fan will draw less current at 7v,
    which mucks up the calculation a bit.
    A better solution would be to use a zener to drop the excess 5v.
    Maplin do one....

    QF46A - BZV85C 5v1 1.3w


  • #6
    RoundyMooney
    Closed Accounts Posts: 10,910 ✭✭✭✭RoundyMooney


    Very true :)


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