LM317 Voltage Regulator

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LM317 is a DC positive adjustable voltage regulator.

consist of three pins. input, output, and adjustment

  1. the input voltage between 3 to 40 volts
  2. Output voltage range from 1 point 25 to 37 volts
  3. and adjustment controls the output voltage
  • the output voltage is stable and a change in the input voltage did not affect the output voltage. "but note that; the input voltage must be greater than the output voltage by 1.25 volt"
  • the maximum output current is 1.5 amper

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Step 1: Circuit

this is a simple circuit diagram of the voltage regulator

the value of the output voltage depends on the value of resistor R1 and R2 and input voltage

he result of the output voltage can be determined by this equation

Vout=Vref (1+R1/R2)


Vref is the difference in voltage between the OUT pin and the Adjustment pin is equal to 1.25 Volt

for example:-

we have 12v dc and we need 5v for charging mobile and the recommended value for resistor R2 is (240 Ω), what resistor we should use for resistor R1

solution:-

we have (Vref=1.25v, R2=240Ω, Vout=5v)

from the above equation

5v = 1.25(1+R1/240Ω)

5v/1.25v = 1+ R1/240

4=1+ R1/240Ω

R2= (4-1)x 240Ω

R2= 720Ω



Step 2: Simple Power Supply

we can change the resistor R2 and using the variable resistor, in this way we can make a simple power supply,

- use the 35 volts from an input voltage

- and 10KΩ variable resistor we can give any voltage

- By rotating the variable resistor we can change the output voltage.

and most voltage usage by the devices is:-

3v, 3.3v, 5v, 8v, 9v, 12v, 14.5v, 18v, 19.5v, 24v, 30v


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