Here is the LM338 Adjustable DC power supply circuit, 1.2V to 30V.  It can provide a current maximum to 5A and 10A. If you have used  LM317 or LM350. They are similar, so easy to use with a few components. But LM338 have higher a current than LM317

The LM338 are adjustable 3-terminal positive voltage regulators capable of supplying in excess of 5A over a 1.2V to 32V output range.

They are exceptionally easy to use and require only 2 resistors to set the output voltage.

The careful circuit design has resulted in outstanding load and line regulation comparable to many commercial power supplies.

LM338 features

• 7A Maximum output current
• 5A output current
• Adjustable output 1.2V to 37V
• Line regulation typically 0.005% /V
• Line regulation typically 0.1%
• Thermal regulation
• Current limit constant with temperature

Pinout of LM338K To-03 and LM338T TO-220

LM338 Basic circuit Voltage Calculator

Look at a basic circuit. We use only 2 resistors can set the constant output voltage.

Vout = 1.25V x {1+R2/R1} + Iadj x R2

Some said Iadj is very low current(approx 50uA only).
So, we may chop them up. It is shorter and easy to calculate.

Vout = 1.25V x {1+R2/R1}

Which is better?

For example:
You use R1 = 270 ohms and R2= 390 ohms. It causes output is 3.06V

Is it easy? If you have voltages choice with most resistors. In local stores near you.

look at the list:

1.43V : R1 = 470Ω, R2 = 68Ω
1.47V : R1 = 470Ω, R2 = 82Ω
1.47V : R1 = 390Ω, R2 = 68Ω
1.51V : R1 = 330Ω, R2 = 68Ω
1.51V : R1 = 390Ω, R2 = 82Ω
1.52V : R1 = 470Ω, R2 = 100Ω
1.53V : R1 = 390Ω, R2 = 82Ω
1.56V : R1 = 330Ω, R2 = 82Ω
1.57V : R1 = 270Ω, R2 = 68Ω
1.57V : R1 = 470Ω, R2 = 120Ω
1.57V : R1 = 390Ω, R2 = 100Ω
1.59V : R1 = 390Ω, R2 = 100Ω
1.60V : R1 = 240Ω, R2 = 68Ω
1.63V : R1 = 330Ω, R2 = 100Ω
1.63V : R1 = 270Ω, R2 = 82Ω
1.64V : R1 = 390Ω, R2 = 120Ω
1.64V : R1 = 220Ω, R2 = 68Ω
1.65V : R1 = 470Ω, R2 = 150Ω
1.66V : R1 = 390Ω, R2 = 120Ω
1.68V : R1 = 240Ω, R2 = 82Ω
1.71V : R1 = 330Ω, R2 = 120Ω
1.71V : R1 = 270Ω, R2 = 100Ω
1.72V : R1 = 220Ω, R2 = 82Ω
1.72V : R1 = 180Ω, R2 = 68Ω
1.73V : R1 = 470Ω, R2 = 180Ω
1.73V : R1 = 390Ω, R2 = 150Ω
1.76V : R1 = 390Ω, R2 = 150Ω
1.77V : R1 = 240Ω, R2 = 100Ω
1.81V : R1 = 270Ω, R2 = 120Ω
1.82V : R1 = 150Ω, R2 = 68Ω
1.82V : R1 = 330Ω, R2 = 150Ω
1.82V : R1 = 180Ω, R2 = 82Ω
1.83V : R1 = 390Ω, R2 = 180Ω
1.84V : R1 = 470Ω, R2 = 220Ω
1.86V : R1 = 390Ω, R2 = 180Ω
1.88V : R1 = 240Ω, R2 = 120Ω
1.89V : R1 = 470Ω, R2 = 240Ω
1.93V : R1 = 330Ω, R2 = 180Ω
1.93V : R1 = 150Ω, R2 = 82Ω
1.94V : R1 = 270Ω, R2 = 150Ω
1.96V : R1 = 390Ω, R2 = 220Ω
1.97V : R1 = 470Ω, R2 = 270Ω
1.99V : R1 = 390Ω, R2 = 220Ω
2.02V : R1 = 390Ω, R2 = 240Ω
2.03V : R1 = 240Ω, R2 = 150Ω
2.06V : R1 = 390Ω, R2 = 240Ω
2.08V : R1 = 330Ω, R2 = 220Ω
2.10V : R1 = 220Ω, R2 = 150Ω
2.12V : R1 = 390Ω, R2 = 270Ω
2.13V : R1 = 470Ω, R2 = 330Ω
2.16V : R1 = 330Ω, R2 = 240Ω
2.16V : R1 = 390Ω, R2 = 270Ω
2.19V : R1 = 240Ω, R2 = 180Ω
2.23V : R1 = 470Ω, R2 = 390Ω
2.25V : R1 = 150Ω, R2 = 120Ω
2.27V : R1 = 270Ω, R2 = 220Ω
2.27V : R1 = 330Ω, R2 = 270Ω
2.29V : R1 = 470Ω, R2 = 390Ω
2.29V : R1 = 180Ω, R2 = 150Ω
2.31V : R1 = 390Ω, R2 = 330Ω
2.36V : R1 = 270Ω, R2 = 240Ω
2.37V : R1 = 390Ω, R2 = 330Ω
2.40V : R1 = 240Ω, R2 = 220Ω
2.44V : R1 = 390Ω, R2 = 390Ω
2.50V : R1 = 470Ω, R2 = 470Ω
2.57V : R1 = 390Ω, R2 = 390Ω
2.61V : R1 = 220Ω, R2 = 240Ω
2.65V : R1 = 330Ω, R2 = 390Ω
2.66V : R1 = 240Ω, R2 = 270Ω
2.73V : R1 = 330Ω, R2 = 390Ω
2.74V : R1 = 470Ω, R2 = 560Ω
2.75V : R1 = 150Ω, R2 = 180Ω
2.76V : R1 = 390Ω, R2 = 470Ω
2.78V : R1 = 270Ω, R2 = 330Ω
2.78V : R1 = 220Ω, R2 = 270Ω
2.84V : R1 = 390Ω, R2 = 470Ω
2.92V : R1 = 180Ω, R2 = 240Ω
2.96V : R1 = 270Ω, R2 = 390Ω
2.97V : R1 = 240Ω, R2 = 330Ω
3.03V : R1 = 330Ω, R2 = 470Ω
3.05V : R1 = 390Ω, R2 = 560Ω
3.06V : R1 = 270Ω, R2 = 390Ω
3.06V : R1 = 470Ω, R2 = 680Ω
3.08V : R1 = 150Ω, R2 = 220Ω
3.13V : R1 = 220Ω, R2 = 330Ω
3.14V : R1 = 390Ω, R2 = 560Ω
3.18V : R1 = 240Ω, R2 = 390Ω
3.25V : R1 = 150Ω, R2 = 240Ω
3.28V : R1 = 240Ω, R2 = 390Ω
3.35V : R1 = 220Ω, R2 = 390Ω
3.37V : R1 = 330Ω, R2 = 560Ω
3.43V : R1 = 270Ω, R2 = 470Ω
3.43V : R1 = 390Ω, R2 = 680Ω
3.43V : R1 = 470Ω, R2 = 820Ω
3.47V : R1 = 220Ω, R2 = 390Ω
3.50V : R1 = 150Ω, R2 = 270Ω
3.54V : R1 = 180Ω, R2 = 330Ω
3.55V : R1 = 390Ω, R2 = 680Ω
3.70V : R1 = 240Ω, R2 = 470Ω
3.82V : R1 = 180Ω, R2 = 390Ω
3.83V : R1 = 330Ω, R2 = 680Ω
3.84V : R1 = 270Ω, R2 = 560Ω
3.88V : R1 = 390Ω, R2 = 820Ω
3.91V : R1 = 470Ω, R2 = 1K
3.92V : R1 = 220Ω, R2 = 470Ω
3.96V : R1 = 180Ω, R2 = 390Ω
4.00V : R1 = 150Ω, R2 = 330Ω
4.02V : R1 = 390Ω, R2 = 820Ω
4.17V : R1 = 240Ω, R2 = 560Ω
4.33V : R1 = 150Ω, R2 = 390Ω
4.36V : R1 = 330Ω, R2 = 820Ω
4.40V : R1 = 270Ω, R2 = 680Ω
4.43V : R1 = 220Ω, R2 = 560Ω
4.44V : R1 = 470Ω, R2 = 1.2K
4.46V : R1 = 390Ω, R2 = 1K
4.50V : R1 = 150Ω, R2 = 390Ω
4.51V : R1 = 180Ω, R2 = 470Ω
4.63V : R1 = 390Ω, R2 = 1K
4.79V : R1 = 240Ω, R2 = 680
5.04V : R1 = 330Ω, R2 = 1K
5.05V : R1 = 270Ω, R2 = 820Ω
5.10V : R1 = 390Ω, R2 = 1.2K
5.11V : R1 = 220Ω, R2 = 680Ω
5.14V : R1 = 180Ω, R2 = 560Ω
5.17V : R1 = 150Ω, R2 = 470Ω
5.24V : R1 = 470Ω, R2 = 1.5K
5.30V : R1 = 390Ω, R2 = 1.2K
5.52V : R1 = 240Ω, R2 = 820Ω
5.80V : R1 = 330Ω, R2 = 1.2K
5.88V : R1 = 270Ω, R2 = 1K
5.91V : R1 = 220Ω, R2 = 820Ω
5.92V : R1 = 150Ω, R2 = 560Ω
5.97V : R1 = 180Ω, R2 = 680Ω
6.04V : R1 = 470Ω, R2 = 1.8K
6.06V : R1 = 390Ω, R2 = 1.5K
6.32V : R1 = 390Ω, R2 = 1.5K
6.46V : R1 = 240Ω, R2 = 1K
6.81V : R1 = 270Ω, R2 = 1.2K
6.92V : R1 = 150Ω, R2 = 680Ω
6.93V : R1 = 330Ω, R2 = 1.5K
6.94V : R1 = 180Ω, R2 = 820Ω
7.02V : R1 = 390Ω, R2 = 1.8K
7.10V : R1 = 470Ω, R2 = 2.2K
7.33V : R1 = 390Ω, R2 = 1.8K
7.50V : R1 = 240Ω, R2 = 1.2K
8.07V : R1 = 330Ω, R2 = 1.8K
8.08V : R1 = 150Ω, R2 = 820Ω
8.19V : R1 = 270Ω, R2 = 1.5K
8.30V : R1 = 390Ω, R2 = 2.2K
8.43V : R1 = 470Ω, R2 = 2.7K
8.68V : R1 = 390Ω, R2 = 2.2K
9.06V : R1 = 240Ω, R2 = 1.5K
9.58V : R1 = 330Ω, R2 = 2.2K
9.77V : R1 = 220Ω, R2 = 1.5K
9.90V : R1 = 390Ω, R2 = 2.7K
10.03V : R1 = 470Ω, R2 = 3.3K
10.37V : R1 = 390Ω, R2 = 2.7K
10.63V : R1 = 240Ω, R2 = 1.8K
11.25V : R1 = 150Ω, R2 = 1.2K
11.44V : R1 = 270Ω, R2 = 2.2K
11.48V : R1 = 330Ω, R2 = 2.7K
11.67V : R1 = 180Ω, R2 = 1.5K
11.83V : R1 = 390Ω, R2 = 3.3K
12.40V : R1 = 390Ω, R2 = 3.3K
12.71V : R1 = 240Ω, R2 = 2.2K
13.75V : R1 = 330Ω, R2 = 3.3K
15.31V : R1 = 240Ω, R2 = 2.7K
16.25V : R1 = 150Ω, R2 = 1.8K
16.53V : R1 = 270Ω, R2 = 3.3K
16.59V : R1 = 220Ω, R2 = 2.7K
18.44V : R1 = 240Ω, R2 = 3.3K
19.58V : R1 = 150Ω, R2 = 2.2K
20.00V : R1 = 220Ω, R2 = 3.3K
23.75V : R1 = 150Ω, R2 = 2.7K
24.17V : R1 = 180Ω, R2 = 3.3K
28.75V : R1 = 150Ω, R2 = 3.3K

For example, you need 20V 5A power supply. You look at 20.00V : R1 = 220Ω, R2 = 3.3K.

It is easy, right?

1.25V to 30V,  5A Variable power supply using LM338

We may have many ways such as:  to modify the LM317 Variable Regulator 0-30V 1A. By adding the power transistor MJ2955 in a circuit.  As following Adjustable Voltage and current regulator IC power supply. Or  You may build the  Variable dc regulator 0-30V 5A circuit,  as well. But these methods. Rather cumbersome and wasting too much money.

However, we can build this circuit easily and cheaply, By using the packages IC No. LM338 only one, Similar to the LM317 IC number, but it can supply up to 5A, like the circuit shown in Fig.

How this circuit works

The transformer T1 converts the AC 220V to 24 Vac, so be rectified the current by the bridge diode rectifier BD1 – 10A 400V. Until DCV has come out that the filter capacitor C1 is equal to 35 volts.

The IC1 is the heart of the operation of this circuit. By the voltage output value obtained from the IC depends on the voltage value at the Adj pin of IC1, or can be varied by adjusting the VR1.

However, the output voltage will be approximately equal to 1.25+1.25VR1/R1
The output voltage at the output pin of the IC1 is a more powerful filter with the capacitor C3.

Parts you will need

IC1: LM338K, LM338P
D1: Bridge Diode 10A
D2, D3: 1N4007, 1000V 1A diode
R1: 220Ω 0.5W resistors 5%
R2: 12K 0.5W Resistors 5%
VR1: 10K Potentiometer
C1,C3: 4700uF 50V Electrolytic
C2: 0.1uF 50V
LED 5mm
T1: Transformer, 24V 5A secondary

The Building

You must solder all devices in the PCB to completely, for the IC LM338K should install with a large heat sink. and all device has the poles. Caution connected the correct, especially electrolytic capacitor.

Figure 2 The PCB layout and components layout

1-20V, 10A Adjustable DC Power Supply

1.2V-20V 10A adjustable dc power supply  using LM338

If you want Variable Regulated Power Supply high current more than 10A up. I  would recommend this circuit.  Because build easy, use LM338 and LM107 again.

The normal LM338 has current about 5A. Then, must use 2 pcs. It causes more current up to  10A. The VR1 adjusts an output voltage of 1.2V to 20V to cover the usual usability. This idea can protect all errors with two LM338.

If you want to see examples of projects. Using LM338 to multiple connections in parallel. To boost the higher current.