Power Supply Circuit-40 Volts-2A

LM317 is a widely used linear voltage regulator. It has 3 pins; input, output and adjustment. LM317 are energy wasters and always require a heat sink to keep the temperature of the circuit low. But due to its simplicity LM317 find a place in a wide range of power supply units.

This project aims at making a power supply circuit using the above-mentioned voltage regulator.


This is a power supply circuit diagram of 40Volts, that is able to deliver 2A current. This circuit is suitable for power amplifiers. Transistor TIP 42 A is used as a pass transistor to increase the current capabilities. Voltage regulator LM 317 is the heart of this circuit. Here the out put voltage is set to 40 V by R5 & R8. If you want any other out put voltage use a 10 K pot instead of R5 and adjust it.

Parts List

R1_______________3R9 1 or 2W Resistor
R2______________   22R  1/4W Resistor
R3_______________6K8 1/4W Resistor
R4_____________     220R  1/4W Resistor
R5_______________4K7 1/2W Resistor

C1____________        3300µF  50V Electrolytic Capacitor (or 4700µF 50V)
C2,C5__________       100nF  63V Polyester Capacitors
C3______________   10µF  63V Electrolytic Capacitor
C4_____________      220µF  50V Electrolytic Capacitor

D1_____                         Diode bridge  4A
D2___________          1N4002  Diode
D3______________  LED  Any  color
U1___________          LM317
Q1____________       TIP42A  Transistor
T1_____________     230V Primary, 40V  2A ,Secondary,
Mains transformer.


If diode bridge is not available, make one using four 1N 4007 Diodes.

Power Supply Circuit Diagram.



  1. The LM317 is specified “Maximum input voltage 40 volts”

    The 50 volt ac input will produce 75 volts input when no load, not clever.

  2. seetharaman

    Hi Leon TIP42 can handle a maximum of 4amps hence you may have to use 10nos in parallel

  3. Leon Remp

    Hi Sir i am working on a project that requires 36 volts and 30 to 35 amp. Can the tip43 be used in parrallel the same as 2n2955 to boostcurrent output. I have Tip42 on hand.

    Thanks in advanance Leon

  4. Thanks you very much.

    May I know why we need D2 to protect LM317 against C3?

    Why do we need both C1 and C2 and not just use 1 capacitor instead? (same for C4 and C5)

  5. Hi, may I know how the transistor Q1 affects the circuit? And what is the diode D2 for?



    • admin

      Q1 is a pass transistor which conducts the extra current. LM317 can handle only 1A. Diode D2 is a protection diode. Protection diodes are required for LM317 for output voltages above 25V and capacitance values above 10uF. Here D2 protects LM317 against C3.

  6. Edgardo Diolola

    Dear Seetharaman,
    Thank you very much for the information,
    More power to you.

  7. seetharaman

    Hi Edgardo,
    you are correct. there is a mistake in the writeup.
    R5 is the current limitting resistance for the LED.
    output can be varied with R3(10K pot).

  8. Edgardo Diolola

    Here the out put voltage is set to 40 V by R5 & R8.If you want any other out put voltage use a 10 K pot instead of R5 and adjust it.

    Dear Admin,
    On the above quote, I am confused, I can not find R8, and R5 is a series resistor for LED (D3).
    Can you make clarification on this circuit?, I would like to build one for this.
    Thanks in advance.