0-30V 20A High current adjustable voltage regulator circuit

If you are looking for a high current Adjustable voltage regulator circuit. This may be a better choice for you.

It can give the output current 20A or 400watts and can adjust the voltage of 4 to 20V—or apply to 0 to 30V easily. It is good quality, excellent performance and durable with PCB.

For use in electronic telecommunication, High power radio transmitter, etc.

This project uses a few components. Because of use fours of LM338—-5A voltage regulator and IC- 741—popular op-amp—in linear power supply mode.

Try to build then you will like it!

How it works

The LM338K that we bring to use be DC voltage regulator circuit on the Floating type, The simple applications style of this IC As shown in Figure 1

Basic typical applications of LM338

How to use LM338 IC in basic

Figure 1 circuit, in normal conditions, the voltage between pin Adj and pin output is equal to a 1.25V stable that flow pass R1, R2 will have constant as well.

The output voltage be equal to the voltage at pin Adj + 1.25 volt or Calculated as follows

Vo = 1.25 (R1 + R2) / R1

High current with parallel LM338

In normal IC-LM338 Can supply up to 5 amps, but to load current maximum 20 amps, we will bring it to parallel.

What to watch out, when we connect many IC with parallel form, is the average current flowing through the circuit. Each equally.

The easiest way is to connect a resistor to the output pin of IC as shown in figure 2.

The value of the resistors-Rs used to it, it will be much less than the R1.

Based on the circuit, we can set.

IoRs = 1.25 – Vo(R1/(R1+R2) )

And from the work of circuits set of down, will be.

IiRs = 1.25 – Vo (R1 /(R1+R2) )

From these two equations, which are all the same, it is that Io = Ii.

Or simply, the current through the LM338 IC both are equal.

Connecting LM338 in parallel form

The connecting LM338 in parallel form

In practice, we do not circuits to use it. Since the voltage across drop Rs will change based on the current flowing through the load and Referenced voltage of IC. Also, different from each other.

External LM338 controls using uA741 op-amp

External LM338 controls using uA741

Therefore, we need to control external circuits. To control the voltage at pin adj, as shown in Figure 3.

From the circuit, we will see that at pin negative of IC to have a half voltage of output voltage. And at pin positive to have equal to Referenced voltage.

Which it is caused by a constant current flowing through the transistor to Rs and P1.

From the properties of the op-amp circuit to the regulated level output voltage that. Until having the same voltage at pin input.

So the voltage at pin base of a transistor- Q1 is equal to the voltage at pin negative of IC.

The voltage these, to make changes in resistance of the transistor, causing voltage in referenced point change.

The resistance of the transistor is inversely proportional to the output voltage, to compensate for the voltage loss of Rs. Due to the does not equal flow of these load current.

4-20V 20A High current adjustable power supply

High power DC regulator 4-20 volts 20 amps by LM338

  • From all principles above, we have circuits applications, as shown in Figure 4, if you want to add the IC-LM338, enabling them to be higher current.
  • For a transformer that can supply at least 30 amperes, and voltage secondary coil, should not be less than 18 volts.

To optimize the circuits for the capacitor-C2 should better use of 20000uF.

Read: How to use LM317 Datasheet and pinout 

How to make a high current adjustment voltage regulator

Parts list
IC1: LM741
IC2-IC5: LM338K or LM338P
Q1: BD140
D1: Bridge diode 35A
D2: 1N4148, 75V 150mA Diodes
R1: 150Ω Resistor 0.5W
R2: 100Ω Resistor 0.5W
R3, R4: 4.7K Resistors 1/2W
R5-R8: 0.3Ω Resistors 5W
C1: 0.01uF 200V, Polyester Capacitor
C2,C5: 4,700uF 50V, Electrolytic Capacitors
C3: 0.1uF 63V, Polyester Capacitor
C4: 10uF 25V Tantalum
C6: 47uF 35V, Electrolytic Capacitors


PCB of high power DC regulator-4-20-volts-20-amps

Build 20A High current adjustable power supply

  • All the devices in the circuits. Devices can be soldered onto the PCB as shown in Figure 5. Unless you change the input capacitor-C2 has increased these. I will have to install it on outside of PCB.
  • Bridge diode must be attached heat sink neatly. To help extend the life and durability.
  • For IC-LM338 that you need to install on a big size heatsink as well. Be careful, the body of the IC to the heat sink Short decisively.
  • When all is finished soldering equipment, test input AC power to this project.
  • Then adjust VR1 until the output voltage as needed, and test the Load and adjust VR1 until the output voltage is the voltage should be unchanged.

Related Posts


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  1. iftikhar ahmed

    its very usfulfor ples tell can i add mor LM 338 for more current in this circuit


  2. Michael Mbamobi

    Yes. any additional LM338 is additional 5A.

  3. Michael Mbamobi

    Ahmed. Remember that if the current increases that you should consider the size transformer also.

  4. Randy Boulter


    Will a 110vac transformer work with this project as long as it provides at least 18vac and 30A?


  5. Dean

    “To optimize the circuits for the capacitor-C2 should better use of 20000MF”

    Where can I get a 20000MF? Does MF stand for Mega-Farad?

  6. Alexandre

    Hi, for works a transformer 15vac – 12Anp.
    3 -LM338 it is good to work? the circuit and pcb,
    work with less resistor and less LM338? thank for help.

  7. Sefa

    Is it 5 Watts enough for 0.3 ohm resistors? If 5 Amps(peak 7A) current flow, what is the voltage should be? 1V? I don’t understand how this circuit works? I don’t want to copy – paste. I must know how its works. Because it will be voltage regulator of the my school project. I have to write a report. So, can you explain the circuit?
    Please help me.

  8. Bonjour, je cherche le schéma d’une alimentation régulé et stabilisé, réglable de 24 v a 30 v et de 40 ampères ; auriez vous cela ? merci

  9. j jos selvin

    Lm338 k circuits R1 , R2 5v 20ampes R1, R2 value please

  10. geraldus bagus

    I built this and worked flawlessly, I use a 10.000 uF capacitor on the Vin, thx

  11. Jaydeep Rathore

    Can i attach a 42 volt 25amp transformer, will lm338k can handle upto 50 volts??
    and what modification i’ll have to do with capacitor….

  12. no Jaydeep, it will now work properly with 50v, I built a power supply recently and the transformer was 24 + 24v which rectified and filteres was giving around 52v , the regulation worked but the ability to provide current was zero. when I modified the input to be 24v then the lm338k magically started delivering current. properly.

  13. Jaydeep Rathore

    Ty Mario…it did’t worked at 50v, i had to reduce to 28v….but it worked and was delivering upto 10amps as my requirement.

  14. Dorin

    Hello, I’ve seen your site and it is quite interesting, and I have a question:
    – can you design me (not for free, of course) an circuit to stepdown voltage for car headlamps (dimmer), from 12-14 v to 8 v, by using an LMC555CM and an IRF1405?

    Best regards,


  15. Pratap Nath

    How I can do CV & CC Mode power supply above this LM338K circuits?

  16. Jonathan Calleja

    Hello. I just want to confirm with you if this circuit is capable to have deliver 0 to 30 V ? And last thing, can i buy this PCB – if yes from where pls?



    Hai, can u please give me a solution for my question it is that how to produce 7amps current as an output by giving dc voltage as a input one important thing is without using transformer please help me guys

  18. niharika

    details of diode bridge please. its urgent

  19. Ericson

    Can I use this as battery charger? tnx.

  20. Romano

    can i confirm if this circuit is a single layer pcb board?

  21. Hakan Kandemir

    Can I use a TIP3055 or other power transistor instead of the LM338? If the transformer primary voltage is 24 volts, how is the effect of the circuit (such as resistor and capacitor values)?

    • Hi, Kris.
      I am not sure. The PCB size. I suggest you print it with a laser printer in scale 300dpi per inches.
      It may be real size.
      I am sorry. I never build this circuit. it is a very old circuit. It may useful or a good idea for you.
      Have a good day!

  22. how can i make it to auto cut off full charge indicator using a relay?heres my email add [email protected]. anyways thank you so much for the effort,really much appreciated

  23. czesław

    Hello, if you can ask for data on the tile pattern paths, arrangement of components. Please, please

  24. Gilou

    Je suis à la recherche un schéma alimentation 30V/30A
    Votre présentation est intéressante, il faudrait ajouter deux LM338 suplémentaires.
    je voudrai ajouter un réglage pour l’intensité, je ne voit pas comment réalisé ce réglage.
    Je pense qu’avec un LM741 suplémentaire et quelques composants et un potentiomètre de réglage.
    Ma question est la suivante auriez vous une proposition ou un montage d’adaptation à ce montage.

  25. claude

    Bonjour Gilou
    Avez vous trouvé votre solution ?

    • Gilou

      Je n’ai pas trouvé la solution, je suis en essai de réalisé un montage avec un 723 est le schéma courant élevé ci-dessus et le réglage intensité de la revue Elektor avec un 741.

      Je ne sait pas si cela va fonctionner. Cordialement.


  26. Claude

    I have power supply unit (PSU) ATX so I think it possible to connect this to the schem Figure 4….
    But I have two questions :
    Q1 : Are the two capacitors C2 and C3 are needeed ?
    Q2 : The positive of PSU is connected to the IN line of the LM338 so OK….But where is connected the negative (ground)
    Is it possible to complete figure 4 (sorry my english is poor)

  27. What is the uses of 0.3Ohm 5w resistance, if it is necessary than please suggest any alternative too.

    • Hello Atul Kumar,

      I would like to express my opinion.

      When we bring the devices together in parallel. We often add a small resistor in series. To compensate for the current And protect those devices. You may be able to use 0.33 ohms 5W.

      Although this circuit is difficult (for me), I believe you can do it, hopefully you will benefit from it.

      Don’t forget to share me too.


  28. Palgunadi

    I need to have adjustable dc power supply from 5 to 60 Vdc, and provide current up to 20A. Could I used the 4-20 vdc 20A adjustable power supply circuit above and replaced the transformer 20A? please help. Thx

    • Hello, Palgunadi,

      Thanks for your visit. How are you?

      In truth, I haven’t built this circuit yet.

      But I like this circuit and I think it will be helpful to others.

      I think it cannot be used with 60V. The LM338 can get 37V max input only.

      However, I believe you can make it. And having fun with electronics


  29. Livio Ortega Morales

    En los circuitos anteriores,puedo reemplazar el LM317 por LM317HVT,para obtener en la salida voltajes hasta 57 Vdc,siempre y cuando el transformador tenga en el secundario 40 Vrms.
    Todos los reguladores deben ir montados en disipadores de calor,cuales serian los cálculos para obtener las dimensiones de las características de los disipadores.

  30. Theodore Alexopoulos

    Hi I have in place 2 toroidal identical transformers 12V/300VA. Can I use them in “parallel” with two bridges, so I can have 24V/300VA output and feed this circuit?

    • Hello Theodore Alexopoulos,

      First Thanks for your visiting my site.

      How are you?

      When you are interested in this circuit, you are a good person. It is a big size circuit.

      I’m not quite sure. I understand your question well enough? But I would like to express my opinion.

      Normally when we parallel the transformers. The current will increase. But the voltage remains the same.

      So it is 12V 600VA output.

      You may connect it in series. They will get 24V 300VA. https://www.eleccircuit.com/multiples-ways-to-increase-the-power-supply-current/

      Hope it can help you

      I like your question Please come back again.


    • Anton K

      As the author said, you can do that, but you need to connect transformers in series

  31. where can i buy a pcb for the High power DC regulator 4-20 volts 20 amps by LM338

    • Hello oekkedakke,
      Thanks for your visit. I am so happy that you are invested in this circuit.
      It is a big circuit for me. I have never built it.
      And, I am sorry. I do not where to buy a PCB for this circuit.

      However, I think you can build it.


  32. chris breedon

    Is there a way to use a small transformer and get the same output Volts and Amps? Looking to use the smallest transformer possible as space is limited.

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