Variable power supply using LM317

Here is an LM317 Adjustable power supply circuit. If you are a beginner in electronics.

You want a good variable power supply. This may be the best project for you.

It can supply the output voltage 1.2V to 30V at the max current of 1.5A.

New update Please read below this article.

Variable power supply using LM317

Variable power supply using LM317, 1.2V to 30V at 1A

This is the first DC power supply in my life that made to use in many projects. It is ideal for those who want to adjust voltage from 1.25V to 30V and currents up to 1A.

Which is sufficient for normal use. For example, It is a power supply instead of a one 1.5V AA battery.

When you want to listen to music from a 30 watts amplifier that required a voltage of 24V 1A, it can be done easily.

Before we commonly used the transistor regulator that is very difficult, large, and probably more expensive ICs.

LM317 Power supply circuit 1.2 to 30V 1A

LM317 Variable Power Supply Circuit diagram

But this circuit can be created with a single IC is lm317 based variable power supply.

The LM317 or LM117 series of adjustable 3-terminal positive voltage regulators is capable of supplying in excess of 1.5A over a 1.2V to the 37V output range,

And has many special features that I like are :

  • Output Voltage Tolerance 1%
  • Line Regulation 0.01%
  • Load Regulation 0.3%
  • Prevent the deposition temperature.
  • Short-circuit protection.
  • Ripple is eliminated with a ratio of 80dB
  • Maximum input voltage 40V

How it works

Followed circuits below.

Here is the step by step process:

First, the transformer T1 is changed an AC 220V down as AC 24V to the bridge diode rectifier D1(1N4001) to D4(1N4001).

There is DC voltage into the filter capacitor C1 equal to DC35V.

The output voltage from IC1 Depending on the Voltage Adj pin of the IC, or to adjust the VR1.

The VR1 is controlled the output DC voltage 1.25V  to 30V (32V) or 37V maximum voltage at 1.5A max all range.

Note: If you want to start at zero volts (0V) Look Here

Let’s set output voltage with:

Calculate the LM317 output voltage

Also: LM338 Adjustable Power Supply 5A and 10A

And we can calculate output voltage equal to:

Vout = 1.25 x {1+ (Rp/R1)

  • Vref = 1.25V
  • Typically R1 is 220Ω or 240Ω as a datasheet. I use the 220Ω.
  • Normally as a datasheet, I see them use VR= 5K (Potentiometer) But I have VR-10K only since it easy to use.Rp = {(VR1 x R2) / (VR1 + R2)}

Then we test it, Suppose, rotate VR1 to lowest resistance cause Rp = 0Ω. put it in the formula above:

Vout = 1.25 x {1+(0/220)}
= 1.25V

But, when adjusting VR1 to a maximum resistance of VR1 and R2 are parallel together.

Rp = 5.46K = 5460Ω.

Instead of low VR

Test it in formula above:

Vout = 1.25 x {1+(5460/220)}
= 32.2V

Then the capacitor C3 is Better performance filter of IC1.
The diode D5 and D6 ( both is 1N4007) is the protector from external voltage to reverse to makes the damage to the IC1.


How it builds

Then, We will assemble all the equipment onto the PCB. See the PCB layout and component layout and full content.

LM317 PCB copper layout

Actual-size of Single-sided Copper PCB layout

Component layout of LM317 power supply circuit
Components layout

Fine voltage adjusting

Many begin friends tell me this project difficult to adjust voltage output. so I add potentiometer 1K and parallel 1K-resistor together. then connects them to VR1 as Figure below.

add function thoroughly adjust voltage output
connecting two potentiomers

You will see that we can adjust the voltage at VR2 (new) is 4volts since resistance sum of 500Ω approximately.

For example, I set voltage is 9V with rotate VR1 is 8.00V and rotate VR2 easily to control the output voltage of 9.00V.

View in the video below

I assemble in the universal box to use easily.

Apply the transformer

I have old a transformer 12V CT 12V output. It should have a total voltage of 24V.

But I measure it as 30.9V too much voltage. It may cause overvoltage DCV as 30.9V x 1.414 = 43.7V.

Which can be damaged to IC1 by too much current.

Modify-transformer-to-21v

So I modify another transformer 12V CT 12V and 0V 6V 9V 12V output into 21-volts.
as Figure below

This circuit perfectly works, as the video below. I can adjust voltage output is 1.25V to 27V since I use the 21V output transformer.

If you can adjust 24V or 12V CT 12V. It causes output up to 30V. But IC overheat when a short circuit or overload.

I test circuit with the 12V 8W lamp as a load. A steady (DC) Voltage will not be transferred from 12V.

Add LED Voltmeter display

DC 0-30V 3 Wire Green LED Display Digital Voltage Voltmeter Panel


We can add LED voltmeter to show a voltage level of output.
Mr. Ali Mohammed, ask me how to use 3 wires voltmeter, red, black, and yellow.

Block diagram of adding a voltmeter to the first power supply-min

It is a good idea. It’s accurate and more convenient.


Block diagram of adding a voltmeter to the first power supply

In the circuit diagram, it needs an external DC power supply. We have to build a 9V DC regulator for it.
Please read this idea: Diy digital voltmeter panel meter

We connect a bridge diode (D1 through D4) to SEC (0 and 12V) of a transformer. Then we connect the measure voltage wire in yellow (+) to the output LM317 power supply. And Ground to (-).
Just this we can already read voltage output.

If you use other AC voltage such as 24 volts. You must change:

  • C1 = 1,000uF 50V Electrolytic capacitor
  • R1 = 1K 0.5W resistor

This so saves and easy circuit.

Why does it not work and FAQ

  • C2 — you can use a 0.1μF electrolytic capacitor instead of 0.1uF 63V or 50V ceramic or mylar type. But we need to be careful to be correct lead.
  • Transformer size—You should use 2A transformer to full current up to the 1.5A output. However, 1A transformer also works well lower current.
  • WVDC All capacitors, You can use a voltage of  50V. Specifically, Electrolytic capacitor!
  • Why R1 is coal?—If Diode-D5 is the wrong terminal. It causes an input high voltage across LM317. Then, it comes to R1 to VR1 and R2 to ground. So, They get high current and burned.

Please check all diode terminal in a right way only.

Diode wrong terminal, R1 burned
Connects Diode wrong, R1 burned
  • If you put the D6 polarity incorrect, The VR10K will burn.
  • You can solder the components on Perforated or universal PCB Board.
  • Why use C3-470uF? It is a filter capacitor. You can use a 1uF tantalum capacitor the same in the datasheet.   But I use this because I have it. It also works well.
  • Why is the output 1.5A?—The current is not constant at 1.5A throughout all voltage ranges.
Current and voltage relationship graph-of-LM317

If you need high current more. Please look at:
LM317 2N3055  high current regulator.

  • Electronic devices with polarity Must be put correctly.  For example, Diodes, Electrolytic capacitors, LM317, etc.
Electronic devices with polarity Must be put correctly

Use an LM317 as 0 to 30V adjustable regulator

There are many ways to do 0 to a 30V adjustable regulator.  But this is easiest with helping with two diodes.
When the current flows the diodes. It always has a voltage across it of 0.65V to 0.7V.

If we connect the two diodes in series. They have 1.3V across them. In normal LM317, start voltage at 1.2V. But this voltage is in both diodes. So, the output starts at 0V.

LM317 as 0 to 30V adjustable regulator using diodes

But it has a disadvantage. The current is slightly reduced by the resistance in diodes

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Sample circuit of LM317 power supply Others

Besides this circuit, We also have other interesting circuits. Choose a simple first.

LM317 adjustable voltage regulator 1.2V to 10V

This is also LM317 based variable power supply and low noise, adjustable voltage output: 1.25V to 10VDC (0-12V) from 12V battery source, so simple circuit

If you have a 12V battery. But you have a load to use the voltage 1.5V to 10V at 0.75A. You should reduce the noise as well.

Low noise variable power supply using LM317,1.25V to 10VDC

In this circuit, it will converter DC low voltage, 12V  to be out of 1.25V to 10VDC.  by can give current topmost get about 1.5A.

You should use an IC number LM317K(on TO-03). Because it has powered more than LM317T (on TO-220).

While it works. It is so hot too. So, need a heat sink that large-sized.

Others parts function

  • The R4 use to adjust the output voltage level.
  • The C1-470uF 25V (electrolytic capacitors)  acts like a miniature battery that supplies power during the spike.
  • C3-0.1uF 63V (Ceramic Capacitor or Mylar Capacitor)  reduces  noise
  • C2-22uF 25V to decrease all noise well. The others detail, Read in the circuit.

Simplest LM317 Adjustment power supply, 1.25-15V

This is the simplest LM317 based variable power supply. We can adjust the output voltage of 1.25V to 15V. The output current level of each voltage is different.

For example: If you adjust the voltage of 12V, the current level will be 0.5A. When you set the voltage of 15V, it causes the output current of 0.2A.

Adjustment-power-supply-values-1.25-15V-Max-current-0.5-amps


Simplest LM317 Adjustment power supply, 1.25-15V

In the circuit diagram, When the voltage from 220V AC main reach transformer. It reduces voltage AC220V into 18VAC.

Then, this low AC voltage flows to a full-wave rectifier, D1, D2.

Next, DC voltage flows into C1.  It is a filter capacitor to smooth and increase the DC voltage of 20V as an unregulated voltage.

After that, the unregulated voltage flows to the DC Regulator circuit. Which uses an LM317, R1, and VR1.

This circuit provides a constant voltage to the load. Which we can adjust many voltage levels of 1.2V to 15V by adjusting VR1.

By the way, C2 is a 0.1μF capacitor to filter out the transient noise which can be induced into the supply by stray magnetic fields.

More great LM317 Power supply circuits

In addition, You maybe not like this. But you may modify these circuits work well too. Below.

And now you can see

3A Adjustable Regulators using LM350T

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Apichet Garaipoom

I love the electronic circuits. I will collect a lot of circuit electronics to teach my son and are useful for everyone.
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