Voltage Regular Problem LM338T

Discussion in 'General Electronics Chat' started by nonamehero, Oct 31, 2009.

  1. nonamehero

    Thread Starter New Member

    Oct 31, 2009
    I am currently trying to use LM338T to build a voltage regulator from my battery to the motor.
    The battery provides around 6V, and i m trying to drop it down to 5 volt.
    And I tried the circuit from this datasheet.

    Got my output voltage to be 5 V when my input voltage is 6 V.
    However when i connect in my motor, the output voltage drops to 4.5 volts.

    why is this happening, Is there anyway to fix this?
    Should I use a different voltage regulator?
    thx in ahead for any suggestions
  2. SgtWookie


    Jul 17, 2007
    The LM138/LM338 has a "dropout" voltage that varies over current and temperature.
    See the "Dropout Voltage" graph at the bottom of page 3 of the datasheet you linked to.

    "Dropout" is the difference in voltage between the input and the output of the regulator.

    The nominal dropout with a 1A load at room temperature is 1.7v.

    Using a linear regulator to decrease the voltage supplied to a motor is really inefficient. Power is dissipated as heat in the regulator.