Picture of 300+ Watt Linear Power Supply
In the theme of the site's motto, "share what you make" I would like to present this power supply that I made. I would like to point out first though that this is not my original design. I found this schematic along with many others on the Internet. I prototyped a few and liked this one far more than other ones similar to it. So I made a permanent version of it for myself to have, and use.

Features of the circuit:
  • Adjustable Voltage 1.2- close to maximum input
  • Adjustable Current once a set threshold is hit voltage drops to maintain a constant current
  • Low Parts Count
  • Precision Regulation
  • High Current Output
This is one bad boy of a bench power supply! It is the one you want to build. Take it from a guy who has built dozens of power supplies over the years. I am proud to have this gracing my electronics workbench today. It is my primary go to power supply when I am noodling around in my shop today.

In the image I am load testing the supply over 100 watts and it isn't even breaking a sweat. The heatsink didn't even warm up. The analog meter is on the 10 amps scale and is reading 8 the digital meter is on voltage and reading 13.7. 8 X 13.7 = 109.6 Watts!

I have no doubt my supply can deliver beyond 300 watts with the components I have used to build it. More on that later.
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Step 1: The Schematic

Picture of The Schematic

I found this on a Tripod web page.


There is a lot of schematics on pages there and I built a number of the circuits and liked this one far more than any others I made.

Whoever it belongs to the credit is theirs. I am putting this file up here in case someone else wants to make their own. This is what I used, schematics are helpful if I want to replicate a circuit. I assume this will be helpful to someone else as well. I could have redrawn it and claimed it was mine I guess, but I didn't. I did convert it from the original gif it was though. The original file name of this was ps2-30v.gif Make one and enjoy!

Step 2: Parts

Picture of Parts

I built my supply out of all junk box parts I happened to have around. I found this circuit to be very forgiving with parts substitution. I did not use any of the transistors the schematic calls for but instead I choose functional equivalents for them all. Parts substitution is a skill that cannot so much be taught as came upon through experience.

I can offer some tips for those that have never substituted parts in an electronic circuit before though. One very powerful tool that can be used for parts substitution is the Internet itself. Today almost every component can be searched, and its data sheet located on the Internet. This is a beautiful thing! For much of my time with electronics this wasn't the case and I collected big thick heavy books chock full of datasheets which I poured over constantly in vain hopes of locating the information I desired. You no longer need to do this so avail yourself of the power of the Internet and find the information you need online.

As an example lets look for the main component that can be substituted in this circuit the bypass transistors. In the schematic the circuit calls for 2N3055 transistors. A fast Internet search of the term "2N3055 datasheet" leads me to:


This is exactly what we want. It shows us the case style of the device, or what it physically looks like. It gives us every possible specification so we can thoughtfully determine a suitable replacement for it. Specifications such as maximum voltage, or current capacity, even dissipated Watts are all figures we can take into account in selecting a suitable replacement from our sources.

Me, I just grabbed some big honking TO-3 NPN cans out of a drawer and called it a day to be honest. I didn't even have to look the 2N3055 up because I knew exactly what it was when I first looked at the schematic. they're ultra common and often substituted, or even used as substitutes. Still if I hadn't known the device the process of unknown parts substitution is much the same time after time.

But let me discuss another component I had not the slightest clue what it was when I first examined the schematic then say where I got my replacement from. This information could be useful to others in their endeavor to construct this circuit. The LT10A04 reverse biased diode in the output. The note next to it is some clue 10 A This is a large diode in case the circuit becomes reverse biased I guess.

Yeah well I have some big diodes in my collection of parts but which big diode would be best to choose? Well I searched the part numbers for a few and hit upon using one side of a back to back diode I'd scavenged out of a computer power supply. Turns out those diodes are massive! Something like 50 amps and they're fast acting too. So if you need that diode and have a dead PSU lying around look for the back to back diode pack by the buck coil. It is a big ugly thing that looks like a power transistor but has a graphic of two diodes facing one and another on it usually. It worked for me.

The LM723 you pretty much have to use an LM723 or one of its many variants UA723 etc. or you won't be building this circuit. They're pretty common. I have managed to find loads of them in my scavenging.

Step 3: Construction

Picture of Construction

How electronics hobbyists build their projects is a subjective personal decision often dictated by the resources they have on hand, or their end goals they wish to achieve. Initially I prototyped this circuit on a breadboard along with several others to test out its functionality and suitability to meet my needs. Doing this also ensures that I have an adequate understanding of the circuit sufficient to build it in a more permanent fashion. Read I lashed up a real sketchy version and gave it a few judicious pokes here and there, kicked the tires so to speak.

I don't always do that but in this case I did. Sometimes the prototype is the final version. Sometimes I toss the whole mess back into a scrap bin too. But in this case I was looking for something a bit more so I made the extra effort of prototyping this circuit. Then once I gained sufficient understanding of it I modeled the components physically.

In this case what modeling meant was putting a piece of perfboard over a piece of foam and sticking parts into it in various configurations until I arrived at something I felt confident I could assemble into the final circuit. I find this step to be easier than soldering then unsoldering things as I progress building a circuit then suddenly decide I don't like the parts placement for whatever reason. Maybe your spatial relationships are more highly honed than mine are but I find this step often helps me to do. It is optional but yields me a neater looking finished circuit.

Examine this attached image which is just a wider shot of the picture on the previous step. In it you can see the progression of the project as I did it, from my scribblings on a print out of the schematic, to my breadboarded circuit, then my model, and finally the completed device.

One more thing to note:

In this picture you can see a black piece of zip cord coming off a barrier strip on a tan box. That is stepped down AC out of a transformer that is in that box. That zip cord is going to the AC inputs of a diode bridge rectifier which is going to a filter capacitor. That is how I am supplying DC to my circuit. I just figured I should mention that for full disclosure of how this whole circuit works. Those items are not on the schematic but are assumed needed for its proper operation. I'm sure you can feed this circuit DC from another source as well, but this is the simplest method I am aware of.

Once I publish this I'll anxiously await somebody posting a picture of theirs in the comments. Or maybe you'll build one and think it stinks. Either way I'd love to hear from whoever regarding this circuit. I love mine! I do hope you like yours as well.
MROHM3 years ago
I can't even describe what to say about this circuit but I am going to collect the parts and build one Myself!!! I Believe anyone who dabbles with electronics should have in their collection a Variable 10-20 amp Power Supply. with this much headroom you'll never out grow this!!! 2N3055 are everywhere so they aren't hard to find!!Commercial units of this caliber run around $125.00 and I'm sure you built yours much Cheaper. One of the BEST CIRCUITS for a test bench I have ever seen on Instructables,,,,,,Awesome!! Gotta Go.....I smell my Weller.
pfred2 (author)  MROHM3 years ago
Cheap is one way of putting it, free would be another. Mine is 100%a junk box special! About the only part you really can't substitute in this circuit is the LM723 and I have quite a few of those so it wasn't an issue. To make up the current limiting resistors I twisted two together for each transistor. I subbed everything!
What was the ohms and watts for those resistors? Thanks!
pfred2 (author)  adamdwaldrop9 months ago

Resistors are 1/4 Watt unless otherwise indicated. All Ohms values are in the schematic.

MROHM pfred23 years ago
You're once again Showing us how it pays to have a well stocked junk box. By building your 100% Junk Box Special You have saved (At Least) $125.00 over Store Bought Units and should be the envy of all Instructables electronics buffs!! Another advantage of Building Something like This is That You should Be able to repair this yourself( therefore Saving more $$$$$) Try Repairing a Store Bought Unit,,,,,,,,,,Not Fun ,,,,,No Sir!!! I should know this,,,,I had to repair some store bought Units in the past,,,,Once again I didn't enjoy it!!!! The Biggest piece of Junk was a LAG-126 Audio Generator,,,,,,,Finally worked after I repaired ( and Replaced) Half the Board!!! Original cost of Generator (1982) $ 875.00 Cost of Repair(2009) Parts=$12.00( Still a deal here!!!)
Robert Powell10 months ago

That is a lot of power but I used a transformer from an audio system and modified this same circuit and I have a 30 amp output to run my mini welder! It's not that powerful but it works for spot welding tabs to batteries, so i'm happy! But it is really 30 amps dc so it is real, I would take a picture but my laptop doesn't have a camera. :-)

adamdwaldrop11 months ago

can you post a drawing of your final schematic with a full part list please? I would like to build one without going through trial and error, thanks! btw HUGE filter cap I love it, what size is that?

R.A.T.M3 years ago
i have the same volt meter you like yours
pfred2 (author)  R.A.T.M3 years ago
R.A.T.M pfred23 years ago
hot damm the small cheap red one
pfred2 (author)  R.A.T.M3 years ago
Ah the HF special? I have 3 of those now. I was picking one up every time I went for a while there. I think with 3 I'm good for a bit. The little amp clamp with the red top came from them too. It's OK. I've a bad habit of blowing up more expensive meters. The cheap ones seem to last me forever!

The most important piece of test gear any of us can own we keep between our ears anyways.
lol i have about 15 of thoose free ones
pfred2 (author)  KylerKraus2 years ago
They do most of what a multimeter needs to do. The ones that work anyways.
R.A.T.M pfred23 years ago
not with the amp clip the solid red one its brand is Cen-Tech

i got it for free at Harbor Freight when i bout some files

pfred2 (author)  R.A.T.M3 years ago
Yes the Red one. I was just saying I picked up the one with the red clip at HF too.
R.A.T.M pfred23 years ago
oh ok do u like it
pfred2 (author)  R.A.T.M3 years ago
Th little red meter? If yes, then yes. Enough that I bought 3 of them. I keep them here and there. One lives on top of my 300 watt linear power supply. It is sort of the dedicated output voltage meter for it I guess.
R.A.T.M pfred23 years ago
sorry i need to check my comments more i had 2 diffident volt meters now only the red it works good but the one that i had i spent an arm and a leg for and axadently ran 14k thou it and broke the whole thing
pfred2 (author)  R.A.T.M3 years ago
It happens.
MROHM pfred22 years ago
Your Multimeter Collection Rocks!!! I'm still in love With The Simpson Meters,,,Thanks for the Picture!!
pfred2 (author)  MROHM2 years ago
I have another 260 now that I need to restore. All of the potentiometers in it are shot so I need to get replacements for them all. The meter movement is still good so it is worth it for me to do. It only cost me $3 at a flea market too.

I love restoring old analog meters. It is very satisfying to me.
MROHM pfred23 years ago
Nice Display of Meters!!! I have Some catching Up to Do!!
pfred2 (author)  MROHM3 years ago
I buy mostly sad cases in distress! Typically in the $1 to $3 range. Hot tip: If you buy a used meter and it doesn't work on your namesakes scale change the ground wire in it. It is probably all corroded from having had a battery left in it for too long/
Nerdz3 years ago
You know..If you combine this with a rewound MOT, you would have one killer power supply....

But I do have to wonder what you would need 15 amps of power for.
pfred2 (author)  Nerdz3 years ago
My CNC project.
Nerdz pfred23 years ago
Ah thats true. Theres even 1000 Lumen LED's that consume up to 3 amps..Heh.
pfred2 (author)  Nerdz3 years ago
Or one motor I am controlling now that draws close to 3 times the current my little supply can output at ten times the voltage. Puts it somewhere in the 7,000 Watt range. I forgot to link this image in my last reply. It is the transformer that runs my supply. I got it out of an old minicomputer that was the size of a closet. It is about the size of an MOT. It is wound for high current.
What kind of motors are you using that draw so much power?!
pfred2 (author)  Robot Lover3 years ago
A Rockwell 1.5 HP induction motor. On my table saw.
Wow! Sounds powerful. Carefull now!
pfred2 (author)  Robot Lover3 years ago
Fear is the only thing that keeps my fingers attached.
Jason-B3 years ago
Ive been looking for a good schematic for a variable PS, glad to see this one worked for you. I just may have to try it. Nice job.
pfred2 (author)  Jason-B3 years ago

It works lash one up for yourself. It is variable current limiting and variable voltage output too. It is kind of neat when you set the current threshold the voltage just keeps dropping when it hits it so as to never exceed the preset current limit.

Makes the unit pretty much indestructible I'd imagine. Shorting the output leads together doesn't even phase mine in the least. Might even save some things under test too if you set it low ahead of time.
rimar20003 years ago
Great work, Pfred!
pfred2 (author)  rimar20003 years ago
Thank you!
MROHM pfred23 years ago
Is my Math Correct??? If we PUSH this power supply to 15amps,,,each 2n3055 will "DRAW" only 5 amps each???( 15/3= 5amps),,, That's not even close to Ic=15 amps!!! Well as rimar2000 said,,,,Great Work,Pfred
pfred2 (author)  MROHM3 years ago
This isn't my circuit just my build. It is an awesome circuit. What is worse is you could daisy chain even more transistors if you needed more power but you might have to add another drive stage for all the base current you'd need then. As it is this thing literally welds on my desktop when I touch the leads together. Like when I touch them together they stick a little to each other. But it is short circuit protected so it is OK to do.

Just make sure the supply you feed this regulator with is up for the task. The transformer I'm using looks like it fell out of the back of a welder. I actually got it out of a PDP 11/34 minicomputer and it drove the whole thing, all 7 feet tall of it. So it is a big block of a transformer. I'm using the 2 13.5 volt coils in it that ran the 5 volt rails together. By the looks of the wire they're about 12 gauge windings.

It is bigger than it looks in this picture:


Someday for laughs I'll have to do an article about the housing I made for it. I went all out!
MROHM3 years ago
Nice Design here!!! I always like Circuits related to Test Gear and this is such one!!! I'm happy you chose the 2n3055...Why you ask???? The ratings are Awesome,,,,,,Ic= 15amps,,,Pd=115 watts and Vceo=60volts. I had a old P.S. that I replaced the old transistor with a 2n3055 and it ran for years!!! The 2n3055 has been around for years and still proves it's worthiness..............Another GREAT INSTRUCTABLE..
pfred2 (author)  MROHM3 years ago
I didn't choose the 2N3055 it was what was called for in the original schematic I substituted it in my build with 3 bigger NEC TO-3 cans I had. If I had a matched set of 2N3055s kicking around I'd have probably used them though.

A 2N3055 is what I used when I prototyped it, it is on the flip side of that black heatsink in the breadboard picture. I just didn't have 3 of them for the final version. Thing is any big NPN transistor could fill in there just depends what someone has on hand. This design is very flexible.

Thanks for the compliment I appreciate it. Build one. This thing is like the He-Man master of bench power supplies. It sings, "I got the powah!" No, it doesn't really sing, but if it could that is what it would sing.