Voltage Regular Problem LM338T

Thread Starter

nonamehero

Joined Oct 31, 2009
1
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.

http://www.national.com/ds/LM/LM138.pdf
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
 

SgtWookie

Joined Jul 17, 2007
22,230
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.
 
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