In this article, we have discussed the LM338 Adjustable DC power supply circuit with a voltage of 1.2V to 30V.  It provides maximum current of 5A to 10A. If you had the chance to use LM317 or LM350, you know that they are similar to each other. Hence they are easy to use with a few components. The difference is that LM338 Power Supply has higher a current than LM317. You can look at the datasheet below to check out more specs.

LM338 _ Datasheet _ Adjustable Power Supply 5A and 10A

LM338 Datasheet and Pinout

The LM138/LM238/LM338 is an adjustable 3-terminal positive voltage regulator which is capable of supplying excess current of 5A over a 1.2V to 32V output range.

They are remarkably easy to use and require just 2 resistors to set the output voltage.

The circuit is designed carefully and has resulted in outstanding load and line regulation comparable to many other commercial power supplies.

The LM338 or LM138 family is available in a standard 3-lead transistor package.

LM338 Power Supply 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

LM338T, LM338K Pinout
Pinout of LM338K To-03 and LM338T TO-220

Schematic Diagram

Look at the Schematic Diagram showing the inside of LM338 Power Supply .

Schematic Diagram of LM338

It contains a lot of transistors, Zener diodes, resistors, and capacitors. It’s hard to learn all about it but let’s give it a try.

LM338 Power Supply Basic circuit Voltage Calculator

Look at the diagram of a basic circuit. We have used only 2 resistors which can set the constant output voltage.

LM338 Basic circuit Voltage Calculator

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

It might be said that Iadj has very low current (approx 50uA only).
So, we only need to cut it in small parts. This makes it shorter and easier to calculate.

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

Which concept is better?

For example:
You use R1 = 270 ohms and R2= 390 ohms. It provides an output of 3.06V

Its quite simple if you have a lot of voltage devices with most resistors available in local stores near you.

Look at the Resistors list (without calculating):

If you don’t have time to calculate, or you are just too lazy to dot them, see below. I have made things simpler for you. You must choose the right resistor according to the voltage required.

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 a 20V 5A power supply. You can find 20.00V: R1 = 220Ω, R2 = 3.3K.

Protection Diodes

Of course no one wants to cause the IC damage, right? As it is very expensive. Read below to keep it in excellent working condition.

In the circuit diagram given above we have used the external capacitors with any IC regulator. Sometimes, we need to attach protection diodes so that low current can be prevented to enter the parts of the regulator.

When the capacitors (like 20uF) discharge, it will cause a low internal series resistance which will deliver 20A spikes when it becomes short.

Although this surge is short, it still has enough energy to
damage the parts of IC.

LM338 Regulator with protection Diodes

Look at the circuit diagram.

We have connected the output capacitor (C1) to the regulator. This makes
the input become short. Then the output capacitor will start to discharge into the output of the regulator.

We will use D1, D2 1N4002 so that it can absorb this current spike in order to protect the regulator circuits.

The discharge current depends upon 3 factors.

  • Value of the capacitor
  • The output voltage of the regulator
  • The rate of decrease of VIN.

In LM138 this discharge path occurs through a large junction. It can hence tolerate a 25A surge without any problems.

This might not be true for other types of positive regulators.

Note: For output capacitors with 100uF or lesser value with output of 15V or less, there is no need to use diodes.

The bypass capacitor (C2) on the adjustment terminal will discharge through the low current junction.

This discharge occurs when there is a shortage in the input or output. Inside the LM138 is a 50X resistor which limits the peak discharge current.

No protection is required for output voltage of 25V or less and 10 mF capacitance.

So inside the circuit there is an LM138 with protection diodes which are added to be used with outputs greater than 25V and high values of output capacitance.

It’s quite easy, right?

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

We can use many ways to modify the LM317 Variable Regulator 0-30V 1A. It can be done by adding the power transistor MJ2955 in a circuit.  It can also be done by adding an Adjustable Voltage and current regulator IC power supply.

You can also build the Variable DC regulator 0-30V 5A circuit too. All these methods will be rather cumbersome and will be wasting too much of your money.

However, we can also build this circuit easily and cheaply by only using the packages IC No. LM338. They are similar to the LM317 IC number, but can supply up to 5A, like the circuit shown in the Figure.

How this circuit works?

The transformer T1 converts the AC 220V to 24 V, which will rectify the current using the bridge diode rectifier BD1 – 10A 400V. The filter capacitor C1 is equal to 35 volts until DCV comes out.

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

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

Adjustable power supply 1.2-30V 5A using LM338

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 completely, so that the IC LM338K can be installed with a large heat sink. All devices must have poles. Caution is required while connecting the circuit, especially with the electrolytic capacitor.


Figure 2 The PCB layout and components layout


The IC number is high priced so you may use the LM317 and transistor, so that the current demand can be met.
Click HERE >>> Best DC power supply 3Amp to adjust 1.2V-20V & 3V-6V-9V-12V add transistor 2N3055 parallel from 3A to 5A easily.

Related: Dual 15V Power Supply Schematic with PCB, +15V -15V 1A

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

If you want Variable Regulated Power Supply with high current of more than 10A and up, I would recommend using this circuit.  Its quite an easy build, you just have to use LM338 and LM107 again.

If the normal LM338 has current of about 5A then you must use 2 pcs. It will create more current of up to 10A. The VR1 will provide an output voltage of 1.2V to 20V which will cover the usual usage. This idea can protect all errors with only two LM338.

1-20V, 10A Adjustable DC Power Supply using LM338

See other LM338 Power Supply Circuits 

I really want you to get the most out of your learning. The LM338 has many uses. We can utilize it in many circuits as follows. Of course, we have tried to focus on simple circuits as our main topic.

0 to 22V Adjustable voltage regulator

How can you start the output voltage from “zero”? Normal it will begin at 1.2V. But other negative voltage can be used to offset this voltage to zero. Here we will use the LM113 Zener regulator IC, 1.2V.

0 to 22V Regulator using LM338

Precision Current Limiter

This is an easy to use constant current regulator. This device will limit the output current by adjusting R1.

Iout = Vref / R1

5A current Regulator circuit

The current will remain constant to 5A. We have used only one resistor to control the output current.
The output current = Vref / R1.

R1 = 0.24 ohms at 2 watts.

Here we must use the right power of Resistor too.

5A Current Regulator using LM338

Adjustable Current Regulator circuit

If you have the requirement to adjust output current, you must adjust R2 to set the current from 0A to 5A.

This circuit will use LM117 to set the current at Adj of LM338.

If you want to see examples of projects that use LM338 connected to multiple connections in parallel which will boost the current supply, you must explore this website more!

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