Introduction: 300+ Watt Linear Power Supply

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.

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.


SagirF (author)2017-03-17

Could you give us some hard numbers for ripple at different amperages and voltages? Oscillocope waveform output screenshots would be nice.

p_e_r_r_y_ (author)2017-02-04

Would more than 60v be possible?

Hi, I'd like to give this a go, with a transformer I happen to have. The only problem is, after rectifying it puts out 62v. Would it be possible to use this same schematic, but replace the LM723 by a L146CB?

pfred2 (author)p_e_r_r_y_2017-02-07

The method here is using a pass transistor to increase current capacity. That is universally applicable. I am not familiar with a L146CB, but if it can handle the input voltage, and the transistors can too, then it should work. Then again it could all go up in smoke with a loud bang too. But that's what makes power electronics so much fun. When experimenting with this kind of stuff it is good to have plenty of spare parts to fry. Because that's how we learn.

crisdarville (author)2016-12-14

Hey guys, very interesting project. I was wondering if any of you guys could point me in the right direction here. I'd like to build a LPSU for an audio computer I'm putting together. All comments are much appreciated!

My ultimate goal would be to have something like this (LPSU-wise)

Bverysharp made it! (author)2015-12-10

Here is my version of this one. I have never seen a 10 turn potentiometer...... I used a "fine" adjustment pot, 1/10 of the value of the main pot, in series with the main pot, to allow for fine adjustments. I also added a digital volt/ammeter.

Thanks again for a great design!

darkmns (author)Bverysharp2016-07-28

Hhello. How did you wired the pot's?

I didn't find any reference for fine/course pot wiring (or didn't tried hard enough/wasn't lucky).


Bverysharp (author)darkmns2016-07-29

I just wired them in series, so that the "fine" adjustment pot is part of the total resistance of the adjustment pot. The reason that I did this is so that I didn't have to purchase a multi-turn pot, which is far more expensive than buying 2 standard pots.:
I put the "fine" pot in one leg of the "main pot" and tied the centre terminal of the "fine" pot to one of its outer legs.

darkmns (author)Bverysharp2016-09-02

Hello. I was asking myself the same questions over again, then I found this:

Best of luck!

pfred2 (author)Bverysharp2015-12-12

I found the schematic online myself. But it was on Tripod, and I don't even know if that domain exists anymore. The supply you built looks like it came out nice. How many VA is the transformer you used? You need to use a big transformer like the size of one in a 10 amp battery charger to get the full power of this circuit. Although that would be too low voltage. Well, it'd work, but it would limit the highest voltage you could output. If you can find an old stereo that put out a lot of Watts it might have a suitable transformer in it. Then again it might be too high voltage too. You could also rewind an old microwave oven transformer. Those are good for a few hundred Watts. I use a transformer I pulled out of an old minicomputer. It is huge.

It powered a computer 6 feet tall.

Basho317 (author)2016-04-04


I'm brand new to Instructables. I've recently retired and wish to build a bench top power supply. Initially I was going to use a PSU from an old computer until I read your comments Re: 300W Linear Power Supply, I'd very much like to build this unit.

My problem is that I'm not familiar with any of the terminology, or where to purchase/scavenge the required parts. You appear to be well respected and knowledgeable in this field. Any tips?


pfred2 (author)Basho3172016-04-05

My tip is if this is your first PSU then build a different circuit. The fact is if you're going to pursue electronics you're going to be building lots of power supplies. So there's plenty of time to tackle this one down the road. I started out building LM317 based power supplies. I'm still building those today too. Because they're cheap, easy, affordable, effective, and just about every other positive thing I can think of.

So buy a lot of 10 LM317s and go wild with those. You can add pass transistors to them to increase current output, and even wire them up as current regulators too.

Then down the road if you salvage a nice set of power transistors you can tackle this circuit. You can even build this circuit with just one pass transistor. That's how I made it when I prototyped it.

What you need to do is get the datasheets for the parts in the schematic then match the component values to parts you have on hand. The candidates for substitution are the power components. Like the LT10A04 diode (I got the one I used out of a PC PSU), or the 2N3055 transistors. Any NPN transistor with the voltage, and current capacity works. The 1,000uF 40V capacitor is just a smoothing capacitor too. So lots of values would work there. A 2,400uF cap would work just as good. You just have to make sure it can handle the voltage.

The kicker with this project is finding the right step down transformer. That is what makes linear supplies unattractive. Which is why everything is SMPS today. Off the top of my head I can't even think of a readily available source of nice high current step down transformers. Other than rewinding your own Microwave Oven Transformer (MOT).

There you have to kind of do what the homemade welder crowd does, but instead of the extreme amerage they're shooting for go for a bit less, but more voltage. So more turns of thinner secondary wire. There's loads of videos on the net of people chopping up MOTs to rewind. You want to chop off the finer windings. Because MOT are actually step up transformers. MOT are very high VA though. 1000+

Basho317 (author)pfred22016-04-06

Thank's for your sound advice, I'll revert to plan A.

Kind regards,


pfred2 (author)Basho3172016-04-07

Yeah this power supply is not for everyone. I posted this article

and it is more my idea of a beginner power supply. Maybe even only the positive half of it? Although with the positive, and negative supplies you can tackle dual rail op-amp circuits.

Arex_x (author)2016-04-07

Ok. This looks neat. I was looking for something like this to go with a large transformer I have.

I wonder, is the 390 ohm resistor such a small value in order to drain the filter caps quickly because of voltage sensing under fast changing loads?

Bverysharp (author)2015-12-16

Its only a little transformer. about 70VA @ 24 volts. I built this supply for a bloke @ my local men's shed, and he only wanted it for light bench top work. I initially did rewind a MOT, but he only wanted something that would fit into a limited space, able to output about 3A. I will save the MOT for the next build...I really love the simplicity of this design.

pfred2 (author)Bverysharp2015-12-21

You probably could have gotten away with using less pass transistors in parallel then. One might have been sufficient? Actually I'm pretty sure one transistor would have been just fine. I'm glad you like the circuit. I tried a bunch out before I settled on this one myself. I was watching a video on Youtube the other day of a guy that bought a bench supply. Actually he was tearing it down because it did not work. He called it a cheap imported model. He still paid 60£ for it though. Sometimes you don't even get what you pay.

Bverysharp (author)2015-11-01

This circuit is a total GEM! Thanks for sharing it. I will be building it ASAP!

pfred2 (author)Bverysharp2015-11-01

When you build it come back and post pictures. I'm sure everyone would enjoy that. Most of the circuit is very straightforward. The only part that tripped me up was around the adjustment pots. I made some dumb mistake there and initially my prototype did not work. But once I paid a bit more attention there things turned out better for me. So if you can make a bit more effort when working on that section of the circuit things should go smoothly for you.

MROHM (author)2011-09-28

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)MROHM2011-09-28

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!

adamdwaldrop (author)pfred22014-10-13

What was the ohms and watts for those resistors? Thanks!

pfred2 (author)adamdwaldrop2014-10-13

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

MROHM (author)pfred22011-09-28

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 Powell (author)2014-10-02

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. :-)

adamdwaldrop (author)2014-08-18

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.M (author)2011-09-28

i have the same volt meter you like yours

pfred2 (author)R.A.T.M2011-09-29


R.A.T.M (author)pfred22011-09-29

hot damm the small cheap red one

pfred2 (author)R.A.T.M2011-09-29

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.

KylerKraus (author)pfred22013-03-22

lol i have about 15 of thoose free ones

pfred2 (author)KylerKraus2013-03-22

They do most of what a multimeter needs to do. The ones that work anyways.

R.A.T.M (author)pfred22011-09-30

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.M2011-09-30

Yes the Red one. I was just saying I picked up the one with the red clip at HF too.

R.A.T.M (author)pfred22011-09-30

oh ok do u like it

pfred2 (author)R.A.T.M2011-09-30

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 (author)pfred22011-10-03

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.M2011-10-03

It happens.

MROHM (author)pfred22012-12-26

Your Multimeter Collection Rocks!!! I'm still in love With The Simpson Meters,,,Thanks for the Picture!!

pfred2 (author)MROHM2013-01-01

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 (author)pfred22011-10-01

Nice Display of Meters!!! I have Some catching Up to Do!!

pfred2 (author)MROHM2011-10-01

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/

Nerdz (author)2011-09-28

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)Nerdz2011-09-28

My CNC project.

Nerdz (author)pfred22011-09-30

Ah thats true. Theres even 1000 Lumen LED's that consume up to 3 amps..Heh.

pfred2 (author)Nerdz2011-09-30

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.

Robot Lover (author)pfred22011-10-16

What kind of motors are you using that draw so much power?!

pfred2 (author)Robot Lover2011-10-16

A Rockwell 1.5 HP induction motor. On my table saw.

Robot Lover (author)pfred22011-10-16

Wow! Sounds powerful. Carefull now!

pfred2 (author)Robot Lover2011-10-16

Fear is the only thing that keeps my fingers attached.

Jason-B (author)2011-09-28

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.

About This Instructable




Bio: I was pfred1 but moved, changed my email address, and lost my password. I suppose worse things could happen.
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