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

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.

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.

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. 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 are 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.