# LM317 2N3055 3A Variable power supply

Many years ago, I built a first Variable power supply using LM317. Now, I have still used it. But if you need to use a 3A adjustable voltage regulator.

This below circuit is suitable for you with 5 important reasons.

First, it is a high-quality power supply.

Second, the output current up to 3A.

Third, adjust the output voltage of 1.25V to 20V.

Fourth, adjust the voltage in the step of 3V, 6V, 9V, 12V.

Finally, use a normal component, LM317, 2N3055 and more.

So, it very easy to do.

## Working of 3A Adjustable voltage regulator circuit

In the 3A adjustable voltage regulator circuit above.

First of all, get an AC main to the circuit. It comes to T1 through S1 and F1.

• S1—power switch to on-off.
• F1—0.5A fuse to protect the circuit when overcurrent.
• T1—the step-down transformer converts AC-main to the low ACV, 18V at 3A current.

Then, the AC18V comes to a diode bridge-BD1, to rectifies ACV to DCV, pulsed DC.

Next, the capacitor-C1 filters the pulsed DC to the stable DC, unregulated.

While some current flows R6 and LED1. The LED1 shows power on. And the R6 limits the current to save LED1

Then, the unregulated voltage comes to an input of IC1 through R1. The IC1 is main, DC variable voltage regulator IC, the famous LM317. It will make the output voltage is very steady.

## Current boosting circuit for LM317

Above I said the circuit can power the 3A output current. But as we know LM317 can power out of 1.5A. What can we do? To increase it up! We help it with the transistors.

And R1 limits current to IC1. The voltage across R1 causes a current biased to Q2, driver transistor. R2 is a current limiting resistor of Q2.

When Q2 conducts, the high current will flow its collector and emitter to base of Q1, the power transistor.

The Q1 is running because it gets a current biased, There is a voltage across R3.

Thus, the high power current will flow through the collector and emitter to the output, in the max current.

If you need a high current of the output. It depends on 3 important things:

### ‘Keep reading: 0-50V variable power supply, at 3A’ »

The circuit has two option you can choose.
Fine mode—When we switch the S2-selector to the fine mode. We can rotate the variable resistor-VR1 to change the output voltage, about 1.25V to 20V.

If you do not understand clearly. Please back to My First Variable Power Supply 1.2V to 30V 1A

Stepping Mode—When we switch S2 to the step mode. We often use the same does not change For example 9V, 12V to uses it instead of a battery. Thus this option is so good. You can rotate S3 to select voltage out of 3V, 6V, 9V, and 12V.

Which you can add voltage range with some resistor at the S3.

This feature like this circuit: LM317 Linear power supply Regulator selector 1.5V,3V,4.5V,5V,6V,9V 1.5A

The function of components
C2-filter capacitor eliminates the power spike voltage before comes to LM317.

C6-capacitor filter DC voltage to smooth better, low noise.

#### Components lists

Semiconductors
IC1: LM317T, 3 terminal positive adjustable regulator. Quantity: 1.
Q1: 2N3055, 15A 60V NPN transistor. Quantity: 1.
Q2: TIP32, 4A___60V PNP transistor. Quantity: 1.

Electrolytic capacitors

C1—6,800uF 35V. Quantity: 1.
C3—33uF 35V. Quantity: 1.
C2—0.01uF 50V—Ceramic Capacitor. Quantity: 1.
R1—15 ohms, 1W Resistor. Quantity: 1.

Resistor 0.5W, tolerance: 5%
R2: 150 ohms. Quantity: 1.
R3: 470 ohms. Quantity: 1.
R4: 150 ohms. Quantity: 1.
R5: 15K. Quantity: 1.
R6, R11: 1K. Quantity: 2.
R8: 220 ohms. Quantity: 1.
R9: 560 ohms. Quantity: 1.
R10: 33 ohms. Quantity: 1.
R12: 1.2K. Quantity: 1.
R12: 220 ohms. Quantity: 1.
T1: 3A, 18V transformer. Quantity: 1.
LED1: color as you need. Quantity: 1.
S1: Power switch. Quantity: 1.
S2: SPDT switch. Quantity: 1.
S3: Selector switch—see text. Quantity: 1.
PCB, Heatsink, and others…

## Build 3A adjustable voltage regulator

This project has a few parts. I may solder the components onto the perforated board. Place position equipment according to the circuit. As Figure 2 the PCB layout. And assemble all components as Figure 3.

The Q1 should be held on a large heat sink. It is very hot for using.

Switches are connected correctly.

If you are a beginner must check and check, before entering the power, should review several times. To prevent damage!

In particular, Position legs of diodes, electrolytic capacitors, transistors, IC1.

If something is not wrong, you will have the full power 3A, which is talented enough to, show you how. “I make with myself, have a proud and active needs, in save”.

Figure 2 The PCB layout of the LM317 adjustable voltage regulator circuit

Figure 3 assemble all components layout to the PCB.

#### Be careful polarity component

Some components have the polarity. For example, the electrolytic capacitors, diodes, LM317, LM337, etc.  If you incorrect them. Your circuit does not work. Event them damage.

## Also, LM317 adjustable voltage regulator

You will also these projects: 1.2V-25V at 3Amp Adjustable Regulators using LM350T instead. By remove both transistors as MR OHM 1970 suggest us.

You may be not like it. Because of large and expensive. See below circuits better!

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MR OHM 1970

You can use LM350T and then you don’t need Q1 Q2 and it’s External Components,I Built this circuit Years ago with LM350T and it works Just Fine!!! This is a Great Design,,,, My idea is just a Suggestion,,More simpler for beginners(using the LM350T!!!)

Aung Soe Oo

Transistor Q2 2SB435 and also LM350 not available in my place. If you could tell me replacement part no. for Q2, really appreciated because I just wanted to build your circuit by myself. Thank you.

MR OHM 1970

@Aung Soe Oo,,,Try E/BAY I’ve Seen LM350 there. And Then You don’t Need Q2( or Q1!!!) I like To Keep Things Simple,,Even Though This Is a GREAT DESIGN We Can Eliminate The Transistors And The associated Resistors …Just a Suggestion Here!!!

cma-electronic

yes this circuit is very good and b435=bd244

krisana utai

Good Idea,Good circuit “Top”

serunkuma shem

pretty good idea

Don

Hi,
Sorry, my electronic is basic.
My question:
How can we have 3 Amps, if LM317 outputis only 1.5 Amps????
Thanks

gilbert

how can i make this power supply if the available transformer is 24V output only

SUBHAJIT MONDAL

very nice

agung Nugroho

Dear sir, if I want to build 30 A adjustable like on that schematic, what should I change. Please I need as soon as possible. I have tip2955 and tip3055.

John Sabalboro

Sir Im from the Philippines can you make a simple and cheapest diagram of Power Supply 3v, 4.5v, 6v, 7.5v, 9v, 12v and 15v with 2 Ampere? Your response is very much appreciated. Thank you

chayapon

Hi, John Sabalboro
This circuit work well.

prsanna

I’ve a PARALLEL CONNECTED 20V DC SUPPLY LED LIGHTING SYSTEM BUT I DON’T HAVE 20V POWER SUPPLY.IS THERE ANY POSSIBLE WAY TO DECREASE THE VOLTAGE OF THAT LIGHTING SYSTEM TO 10V… THANK YOU IN ADVANCE

Canal

Thanks for this great project, i try to build one for me..

Abdulhabib

Hi Sir,
I want to connect 24 V DC supply, is hit possible to work?

jay-em

sir what is my transformer is 16volts only?

jay-em

sir, what if my transformer is 16 volts?

kamran khan

we need 6volt fixed dc power suply for project….

renante

hi sir, im from Philippines I build this circuit but its not running well. when I tried to change the connection of emitter and collector of 2N3055. to negative line its already working. is there something wrong with my circuit?

karthik

hello friends
Can any one explain what is the purpose of R5:15k resistor in the above circuit

ANOOP MYSORE

R5 acts in parallel to the potentiometer or the step adjust resisters. At any time, if the output is connected to a load and you switch between step adjust to fine adjust, for a moment, the adjust pin will be floating. This shouldn’t happen. May be that’s why R5 is needed. It doesn’t affect the resistance as its very high and in parallel to the other resistors.

Yvon Kamanda Nakam

Hello Sir, could you send me the diagrams of the stabilized power supply of plus and minus – + 12, – + 15volt

shrikant sahu

C/o prakash sahu, sahu provison stores
Dr. dikshit gali, nandi chowk, tikrapara

shrikant sahu

can this circuit be used to charge as car battery charger (12V 30A) Lead acid battery too? if yes what would be rated voltage & current for charging a discharged battery?

M A N

Hi

This circuit is very useful and cheap
Thanks

Anne Mary

hello sir how can i change 3A to 5A output

Ashok Punde

TIP32____4A___60V PNP transistor. but it is available with 3A…..how?