adjustable power supply using LM317T

This circuit provides various output voltages like 3v,5v,6v,9v,12v which are used in another circuits.It also supplies output current in excess of 1.5A. The 230V AC mains is stepped down to 15V AC by step-down transformer.The secondary winding of the transformer should support a current of 2 amperes.the bridge rectifier consisting four diodes(D1,D2,D3,D4) converts the AC to pulsating DC with a peak voltage of 21V.LED1 lights up to indicate the availability of output from rectifier.The current limiting through the LED is achieved by using resistor R1(2.2K). The current through the LED is limited to below 10 mA. The capacitor C1(470u,6v) converts the pulsating output of bridge rectifier into constant DC voltage.capacitor C2 (0.1u) eliminates the high frequency noise.
LM317T is used as voltage regulator.It is a positive voltage regulator and capable of supplying currents in excess of 1.5A over 1.2V to 37V output voltage range.The five way rotary switch S2 is used to select different output voltages.The output voltage is given by Vo =1.25(1+Rx÷R2).where Rx is the resistance connected between Adj pin of regulator and ground.
When the rotary switch is in off position the value of Rx is R3+R4=1900 ohms and the output voltage is 12V.In various other positions Rx is 1900ohms parallel with the resistance selected by rotary switch. Resistors with 1% tolerence are recommended.

output voltage Vs Equivalent series resistance Rx

output voltage                               12V    9V      6V      5V        3V


series equivalent resistor Rx (ohms) 1900  1353   838     655       323

The LM317 regulator is to be fitted with heat sink between the regulator and PCB to provide the best heat transfer.

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components used:

R1-2.2K

R2-200 ohms

R3-100 ohms

R4-1.8K

R5-4.7K

R6-1.5K

R7-1K

R8-390 ohms

diodes:

D1,D3,D3,D4-1N5008

D5-1N4007

capacitors:

C1-470u,63V

C2-0.1u

C3-10u,25V

IC1-LM317T

fuse(F1)-2A

s1-on/off  switch

led

s2-rotrary switch

T1-230V AC primary to 15V-0-15V, 2A secondary transformer