Introduction: 5A Variable Linear Power Supply

This power supply is a simple project that I have built on a weekend and I thought it would be a nice idea if I shared it here. This is the first instructable I write so please, sorry if any mistake.

It has a simple circuit and it's really easy to understand how it works. The original schematic is from the LM317 datasheet, but I did some modifications like adding a LM7812 to power up the fans, a lower resistor on the IC input and other things. This bench power supply is really stable and very useful in many applications.

Step 1: The Circuit

The way the circuit works is very easy to understand. The LM317 adjusts the voltage. As current starts to flow through the 8,2ohm resistor, a voltage drop will appear across it. When this voltage drop is higher than 0,6 volts, the TIP42C starts to conduct and it drives the two 2N3055. The 2N3055 is on a emitter follower configuration and it will supply current to the output of the power supply. This starts happening when the IC LM317 is outputting 73mA, as ohm's law show us: 0,6/8,2 = 0,073.

A easier way to understand it is that the two transistors will start to "help" the LM317 with current, while the LM317 adjusts the voltage.

So, this is the basic function of the circuit. The LM317 controls voltage and the 2N3055 supplies current.

The two 0,22ohm resistors on 2N3055 emitters is for ensure that both of them work together preventing one outputting more current than the other leading them to burn. (I used a 0,33ohm because I already had them at home.)

The 500ohm potentiometer I added on LM317 feedback is to adjust the maximum output. The 500ohm resistor between the 2N3055 emitter and base is for it don't lose its base-emitter referential, the power supply would work without it, but less stable.

For filtering a 10000uf/50V capacitor was used because it was laying around at my home, but you can use a 4700uf/50V capacitor, just remember to modify the PCB.

You can use a transformer from 12 to 24 volts/5 amp. It will only change your maximum output voltage. I used a 24V/5A transformer. You can calculate the maximum output with the formula: ((Vout transformer * 1.41) - 3,4).

A 25A diode bridge, that I have bought on a local hardware store, was used. It won't be too difficult for you to find one like mine, just search for "diode bridge 25A" on eBay. A 15A or 10A diode bridge will be fine too.

The inductor is from one old PC power supply I had, you can salvage one from a old PC power supply. It does not need to be exactly like mine, anything with more than 25 turns and a wire diameter higher than 1mm will be just fine.

The bill of material is:

1 - Transformer (see the text above)

1 - Diode bridge (see the text above)

1 - Inductor (see the text above)

1 - LM7812

1 - LM317

2 - 2N3055

1 - TIP42C

1 - 10000uf/50V Electrolytic Capacitor

2 - 100nf/50V ceramic capacitor (polyester is fine too)

1 - 47uf/50V Electrolytic Capacitor

1 - 8,2ohm/5W resistor

2 - 0,22ohm/5W resistor

1 - 4k7ohm 1/4W resistor

1 - 500ohm trimpot

1 - 120ohm 1/4W resistor

1 - 1N4007 diode

1 - 5K potentiometer

Step 2: Designing the PCB

I used expressPCB to design the PCB and the file I uploaded here is free for you to edit as you like. Edit it as you wish to fit the components you have. The inductor I used on this circuit is from a old PC power supply and you might want to edit it as you don't have the same inductor as I have. If you don't want want to edit it you can use a wire to connect one point to the other bypassing the inductor or you can use another inductor that you have, it's fine too.

A PDF version of the board has been uploaded too if you don't even want to edit it or generate the PDFs yourself.

Update: I've update the mounting places of the components on the PCB picture.

Step 3: Tone Transfer and Etching

I used the toner transfer method to transfer the printed circuit to my PCB. Just print the circuit using a good laser jet printer and you will get good results.

A good toner transfer tip is: use an iron that can get to 200ºC (392ºF). This is the temperature that toner starts to melt. For etching the board, I used iron perchlorate. If you know any other method to etch a board, just do it as you like.

Step 4: Soldering the Components

I did not took any picture while drilling the holes for the components pad, sorry about that. A drill with 1mm will do just fine

All the lines that would pass high current was reinforced and you should do it too as a 3mm line won't stand 5Amp. See the top view picture for more details on the connectors and bottom view picture for more details on the lines you should reinforce.

Step 5: Assembling the Power Supply

I assembled my power supply on a metal box I bought on a local hardware store. There is not a right one, just buy one you like and can fit the circuit board, transformer and other components.

The voltmeter I bought here, but you won't have any trouble to find it on eBay too.

Make sure to mount the LM317 and the two 2N3055 on a big heat sink, because they will get very hot. I even added the fan to force some air through the heat sink.

And that's it. Any thing that I can help just ask on the comments and if you based on my project to make a new one please comment too, I'm very curious to know if you did a really nice upgrade to it.


Erosion139 (author)2016-11-10

Could you tell me the minimum voltage you can get out of this? Im only getting a min of around 9v max 22v. To be clear I am using a TIP32C rather than the TIP42C since it is all I had. Is this effecting my results? Maybe I need to redo the circuitry on my bread board. I am also substituting some of the resistors for multiple smaller value ones. Ex: Instead of 500ohms Im using 200-200-100 to create one. Also on one of my tests my 500ohm potentiometer make alot of magic smoke. So I replaced it with a 200ohm resistor that for some reason didn't even get hot? To be fair some of my components are a bit off the required items but they aren't off by what I think is a significant margin. Any advice is greatly appreciated. Another thing that I should note is that I had no 8.2ohm resistor so instead I had a 10ohm.

Erosion139 (author)Erosion1392016-11-10

Also if you need pictures I will post some as soon as I can.

panzerbuern (author)2016-07-27


just got myself to finally try myself in electronics , but cant seem to make this cicuit work(

can you please help?

this is literaly my first attempt of making a pcb of any kind so i am bound to make stupid rookie mistakes.

the input voltage is the same as output voltage(12V) no matter what i do..

WhatsApp-Image-20160727 (1).jpegWhatsApp-Image-20160727.jpegРисунок (5).jpg

Be careful with these two 2N3055. They must be mounted on a heatsink or they will burn and the worst case scenario is fire.

panzerbuern (author)panzerbuern2016-08-14

nope evrything actually works fine) just learned that powering something meant for ac after the diode bridge doesnt always work)

JulieG42 (author)2016-06-24

Hello, what's up with this components cause I don't understand:

1 - 500ohm trimpot

1 - 120ohm 1/4W resistor

First you say trimpot but in the pictures I see two small resistors and I can't seem to find the 500ohm resistors to buy them, what did you use instead?

HeinojC1 (author)2016-06-20


Two question, in the description you said 4K7 ohm 1/4 W but on the picture below it says 500 ohm 1/4 W did you change it? And also I saw and an another inductor with a red cable, can you tell me what it is for? Thanks


Are you referring the 5K resistor on the base of the tip42c? If yes, I changed it to 4k7 because (If Im not mistaken) 5K is not a comertial value. You can use a 5k1 if you want. It wont affect the psu behaviour. The inductor with the red wire doesnt made it to the final design, I removed it.



No, on the Step 2:Designing the PCB if you check the 2 resistors of 1/4 W show 120 ohms and 500 ohms, thanks.

HeinojC (author)2016-06-06


I saw in the picture that you used (2) 5K potentiometer, but in the description you said just one, the other one is for the current?. Thanks

TrichogasterAzul (author)HeinojC2016-06-06

I didnt notice that. I forgot to put it on the instructable. It is actually a 1K pot for fine adjusting. A single 5K pot for adjust 0-24V is really annoying so you will need a 1K or less pot for fine adjust.If you dont find a pot with less than 1K you can solder a 1K resistor in paralel with the pot (pins 1 and 3).

Tech Works (author)2016-02-05

you said " I used the toner transfer method to transfer the printed circuit to my PCB. Just print the circuit using a good laser jet printer and you will get good results"

But i want to know which type of paper you used here to take print out????


Just normal A4 paper. I think bond paper is the proper term in english.


Bond paper... good that means there will be no change in printing the circuit from paper to PCB board

Lux_Gamer (author)2016-01-24

Hello TrichogasterAzul In your build you used a transformer is that one from a microwave? I hope you answer soon.

By Lux


The transformer is not from a microwave. I bought it brand new on a electronic store.


Thnx for your answers.

But can i use a transformer from a microwave?

And a current adjustment misses is it hard to build one

Lux_Gamer (author)2016-01-24

Forgot somting where is the fan and heatsink at the back for?


Its too cool down the two 2n3055. They get really hot if you draw 5 amps from the power supply for a long time. Sorry for taking so long to answer you. I did not had time to enter on instructables.

adjakfar made it! (author)2015-11-05

Thanks for the project. I have built one and working good. I have re-drawing the schematic using eagle, but still not perfect. Here is what it prototype looks. Thanks.

IMG_20151101_002228[1].jpgNew Layout.png
SadikhaliM (author)2015-11-01

Thank you sir

i need short circuit protection this circuit

adjakfar (author)SadikhaliM2015-11-05

I think you can found it here. I have built it one and working good.

gada888 (author)2015-06-29

I made a replica of this PSU.the only diffirence is some parts i replaced bcz those are in my junk box.for instance,the transformer is a dual output of 10v and which 10v is give to FAN and 14v for power input.Capacitor is 4700uf,inductor was salvaged from a old PSU with 20 rounds of copper thread.

TrichogasterAzul (author)gada8882015-06-29

It is beautiful! Just one question, did you isolate those 2N3055 from the PSU heat sink? If not, you may have some trouble.

gada888 (author)TrichogasterAzul2015-06-30

I wired the 5K potentiometer to volt adjustment,but no avail.I could only adjust 1Volt.

TrichogasterAzul (author)gada8882015-06-30

That's because you connected the lm317 output together with the 2N3055 collector(psu input). The metal part on LM317 is directly connected to its output. If you want to use the same heat sink for the 3 components, you must isolate them. I see you used a old PC PSU case. If you still have the PC PSU circuit, unscrew the transistors from the heat sink. You shall see a rubber and a screw with a plastic part. You can use them to isolate the LM317. And don't worry that rubber you see is thermal conductive. Or instead of doing this, you cans simply buy mica isolator for TO-220 and TO-3 packages. Google this and learn more about mica isolators and how to isolate components from the heat sink.

gada888 (author)TrichogasterAzul2015-07-01

Thanks so much for correcting me,i followed and it did the trick.

gada888 (author)TrichogasterAzul2015-06-29

How to isolate them?

BeachsideHank (author)2015-05-12

Nice handiwork, there is a proper name for the method used in having the '3055's handle current above the lm317's limit, but it escapes me at the moment. Nonetheless, this is a very good build, thanks for the post.


Thank you! I didn't know it had a proper name, I described it as a emitter follower. Now that you mentioned, I will try to find it out.


A "Darlington" arrangement is, I believe, the proper description for the wiring of the power transistors shown. It was driving me nuts, I had to find my own answer. ;-)

Omnivent (author)BeachsideHank2015-05-12

It's not a Darlington pair, which use same polarity transistors i.e. both NPN or both PNP (I use the term "pair" here, as the two 2N3055 is to be considered as one).

The boost transistors are arranged in what is called a Sziklai pair and it is made up of one NPN and one PNP.

For making a "super PNP", you use a PNP to drive an NPN (and v.v.).

It's more lossy than using real PNP's for this application - I personally prefer using the MJ4502 (200W, used to be 250W in the last millenium - although I don't know why they knocked it down 20%) in such applications, although it is as venerable as the 2N3055 :)

BeachsideHank (author)Omnivent2015-05-13

Thanks for the clarification, I was pretty sure someone out there knew what it was:

Fun stuff to learn.

Omnivent (author)Omnivent2015-05-12

Forgot to mention what may or may not be obvious...

These arrangements are used to get a high current gain (Total gain = gain of NPN times gain of PNP) and that holds for both Darlingtons and Sziklais.

And it's necessary with the low gain of the 2N3055.


I hope it is something more professional than "piggybacking", I just can't recall it, maybe someone else can offer up a name. ;-)

About This Instructable




Bio: I love to build and fix things.
More by TrichogasterAzul:Short circuit protection for (almost) any power supply5A Variable Linear Power Supply
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