LM317 with pass transistor circuit regulators

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

This circuit is using LM317 with a pass transistor circuit for 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

LM317 2N3055 Variable power supply circuit
LM317, 3A adjustable voltage regulator circuit diagram
Output: 1.2V to 20V, and 3V,6V,9V,12V

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 this becomes the pulsed DC.

We called the unregulated supply.

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 the 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 the 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:

  • First, the current of the transformer.
  • Second, the current of the diode bridge.
  • Third, the capacitance of C1. It is enough.

Keep reading:

Adjusting the output voltage

The circuit has two options 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 see:
My first LM317 Variable Power Supply (1.2V to 30V at 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.

Read Also:

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

See the feature like this circuit:
1.5V, 3V, 4.5V, 5V, 6V, 9V LM317 Selector Supply

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

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

Electrolytic capacitors

C1: 6,800uF 35V
C3: 33uF 35V

C2: 0.01uF 50V, Ceramic Capacitor.
R1: 15 ohms, 1W Resistor.

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

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.

Learn more:
More Power supply circuits

How 741 OP-AMP Power supply Work

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

LM317 2N3055 Variable power supply PCB component

Figure 3:  assemble all components layout to the PCB.

Be careful polarity component

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

important polarity components of 3A power supply using LM317
The important polarity components of 3A power supply using LM317 and 2N3055

Also, LM317 adjustable voltage regulator

Also this project we can use LM350 to make 1.2V to 25V at 3A Adjustable Regulator instead. Thanks, MR OHM 1970 suggests to us just remove both transistors.

You may be not like it. Because of large and expensive.

See below circuits better!

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I always try to make Electronics Learning Easy.

31 thoughts on “LM317 with pass transistor circuit regulators”

  1. 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!!!)

  2. 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.

  3. @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!!!

  4. 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.

  5. 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


  7. 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?

  8. 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.

  9. 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?


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