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Thread: Are you good at math and decibals?




  1. #1

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    Can you figure this out? I need to figure out what ohm of a resistor I need for my project. I will be using a a 12 volt supply line from a PSU. My device I am powering up needs 7.5 volts that will draw 666ma.

    So I guess the calculation would be....

    ( 12v - 7.5 ) / ( 666mA ) = (Resistor in Ohms)


    Can you help me before I blow my whole project?

  2. #2
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    Don't look at me, I almost failed maths for about 3 years... :)
    Cameron "Mr.Tweak" Wilmot
    Managing Director
    Tweak Town Pty Ltd

  3. #3
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    *whips out trusty dick smith catalogue*

    i *think* you will want an 11 Ohm resistor

  4. #4
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    I've just spent the whole semester doing physics formulas

    Ohm's Law: Voltage = Current x Resistance

    So you want to find resistance, which is: Resistance = Voltage / Current

    Which is: 7.5V / 0.666A = 11.261 Ohms.

    However, to get the voltage down to 7.5V from 12V you will need a step-down converter, or else use the +12 rail and ground it into the +5V, this will give you a potential difference of about 7V. I know it's not 7.5V, but it's close.

    What is the project may I ask? :?:
    What came first - Insanity or Society?

  5. #5
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    WOOHOO!! I GOT IT RIGHT :D

  6. #6
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    yeah 11 ohms is close enough.

    that extra .261 is negligible.

  7. #7

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    Albinus, I know this trick will work for fans...but what about for an LCD screen? I am making a "in-house" file server out of spare parts. I have a 5" lcd I am going to use so I wont have to have a monitor next to it.

    Anyway, if the 7 volt trick will work would you think it would cause problems later down the road?

  8. #8
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    twinkle twinkle little star
    power = I squared R
    (thats how we remembered the formula in engineering)
    ie

    P = I^2R

  9. #9
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    Voltage = 1.5V, Amp = 0.015 A. Therefore, to calculate resistance needed for a 12V line-in;

    Ohm = (Mains Voltage - LED Voltage) / LED Amperes, so;
    Ohm = (12 Volts - 1.5 Volts) / 0.015 A
    = 700 Ohms or 750 Ohms

    Voltage = 1.5V, Amp = 0.015 A. Therefore, to calculate resistance needed for a 5V line-in;

    Ohm = (Mains Voltage - LED Voltage) / LED Amperes, so;
    Ohm = (5 Volts - 1.5 Volts) / 0.015 A
    = 233.33 Ohms or 240 Ohms

    I had to look it up in one of my Electrical Engineering Books from College

  10. #10
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