Thursday, April 8, 2021

Adjustable Power Supply 1.2 to 37V High Current 20A using LM317 and TIP35C + PCB

Fig. 1 - PCB Adjustable Power Supply 1.2V to 37V - 20A

For Portuguese version, click here!

This is a High Current Variable Power Supply circuit, which works smoothly *With 20 Amps. And we will use the old, wonderful, and well-known LM317, which is an Integrated Circuit regulating positive variable voltage with a range of 1.25 to 37V, and 1.5 Amps.  

We obtained this circuit from our partner FVM Learning, which made it available to us, we made few changes, such as increasing the transistors that they speak of in the Original Post, and we made the PCB that was not available until now, the rest is exactly the same.

To make an appropriate Power Supply, it is necessary to have a current well greater than 1.5A, so that’s where the power transistors come in, they’re going to increase the current without varying the voltage.

Because this variation function is up to the LM317, and the current factor is up to the transistors that in this circuit we’re going to use TIP35C.

Circuit Operation

The LM317 is a variable positive voltage regulator, which is responsible for varying the output voltage and delivering the base of the power transistors to this variable voltage. 

Which together with the other transistors form a high current booster, and which depending on the amount of transistor and the sum of the currents of each transistor, we form a high current variable power supply.

The voltage that comes from the power supply enters directly into the TIP35C collector, and is controlled by varying the input voltage at the base. 

And in turn the voltage that will come out of the emitter, will be directly related to the control of the output voltage of the LM317, it is necessary to use a heat sink, as this holding energy is converted into heat.

The TIP35C is a High Power Mospec transistor, with a 25A continuous collector current capacity, thus making it the perfect transistor for this project.

* It is worth remembering that this transistor has a total power of 125W, that means that following the Ohms Law, the current was calculated by the manufacturer for this transistor at 5V, let's check:

Ohms Law Formula: P = V * I

P = Power: V = Voltage: I = Current:
I = P / V ==> I = 125/5 ==> I = 25A

In our case where the power supply will go up to 37V, we can consider that:
I = P / V ==> I = 125/37 ==> I = 3.38A

Did you see the difference? We put two transistors for this work, but if you are going to use this source continuously with the maximum voltage and total current, you will need to add two more pairs in this circuit. 

We can use this power supply with these parameters; Voltage, Current, maximum, but for continuous use, it will overheat the transistors and burn.

The TIP35C has 100V of Vce and Vbe, Emitter Collector Voltage, and Base Collector Voltage. Remember this, these settings refer to the TIP35C, there are other variants such as: TIP35 = 40V, TIP35A = 60V, TIP35B = 80V and TIP35C = 100V, so for this project you can use the TIP35C for greater efficiency.

The Adjustable Power Supply schematic diagram is shown in Figure 2, which shows the arrangement of the components and their connections to proceed with the assembly, which in the general context, is quite simple to assemble, and demonstrates a great effectiveness.

Fig. 2 - Schematic Diagram Adjustable Circuit - 1.25V ~ 37V, 20A CI LM317 and TIP35C

Components List

  • U1 ............................ LM317 voltage regulator integrated circuit
  • Q1 to Q6 .................. TIP35C Power Transistor
  • D1, D2, D3, D4 ....... 1N4007 Silicon rectifier diodes 
  • C1 ............................ 4700uF - 63V Electrolytic capacitor 
  • C2, C3 ..................... 0.1uF Ceramic / Polyester Capacitor 
  • R1 ............................ 220 ohms 1 / 4W Resistor - (red, red, brown, gold)
  • R2 ............................ 10K ohms - 1/4 W Resistor - (brown, black, orange, gold)
  • R3 to R8 .................. 0.22 ohms - 5W Resistor - (red, red, silver, gold)
  • P1 ............................ 5k ohms Potentiometer 
  • J1, J2 ....................... PCB Terminal Blocks - EK500V-XXP 20A - or equivalent
  • F1 ............................ PCB Fuse Clip Cartridge 250V 30A With Fuse 20A
  • Others ..................... Wires, Welds, Posts, PCB, Etc.

For those who want to download materials such as the schematic diagram in PDF, PCB Layout,  GERBER and JPG, we are providing a direct link to download all the material.

Direct link to download

Click in the link  to download the Files: PCB Layout, PDF, GERBER, JPG

If you have any questions, suggestions or corrections, please leave them in the comments and we will answer them soon.

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My Best Regards!!!


  1. The PCB for the 20A supply is not correct. Lm317 pins are wrong. The middle pin should be out and the right pin should be input. You have drawn then in reverse. The adjust is correct and also TIP 35c.
    There is no easy fix the Pcb will have to be redesigned,

    1. Hello Valve 3!

      You've got rason, it was the software library that had the pins reversed, and we don't pay attention to that.
      Thank you very much for your observation and cooperation.
      We made a new PCB, we took the opportunity to compress more.

      It was great, and is now available for download.

      Regards from the entire team

  2. Hi I have 37V input voltage
    But output voltage is only 25,2V without loads
    With loads it's only 21,5V
    Any ideas ?

    1. Hi @WA
      Many times the components are not original, it has happened several times that we buy electronic components on the internet and they present low performance.
      Check this case, also carefully analyze the circuit to verify that there was no inversion of components.
      My regards!

  3. I'm trying another lm317 and all diodes, resistor and transistors, solders. Input Voltage 33,4V when I do loads and have 8,3A but output is drop to 20,3V. I need 24V output with load. Any ideas ?