I need a quick fix - help please

Discussion in 'General Electronics Chat' started by Gavhogg, Sep 23, 2008.

  1. Gavhogg

    Thread Starter New Member

    Sep 23, 2008
    Hi Folks

    I need help to reduce 28vdc down to 12vdc using resistors, but I'm not sure of the formula.

    Can anyone help?

    Many Thanks

  2. beenthere

    Retired Moderator

    Apr 20, 2004
    E = IR. There is no fixed formula for your application, as you don't indicate what current you wish to flow. Using fixed resistors means that the actual voltage changes with the load. Or you could get a 7812 regulator.
  3. scubasteve_911

    AAC Fanatic!

    Dec 27, 2007
    Vout = Vin ( R2 / (R1 + R2))

    Vin -> R1 -> R2 -> GND

    Vout / Vin = (R2 / (R1 + R2))
    12/28 = R2 / (R1 + R2)

    One equation with two variables, bad news. As beenthere says, you need to specify current. Any current flowing out via 12V will cause the voltage to change. If the current out from the divider is much less than the current flowing through R1 and R2, then it can work. You will have a lot of power loss as you bring the resistances down to accomplish this. This is only practical for high impedance inputs, although the temperature can effect the circuit.