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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


I found this on a Tripod web page.

http://hmin.tripod.com/als/andysm/pages/analogs3.html

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!
<p>Here is my version of this one. I have never seen a 10 turn potentiometer...... I used a &quot;fine&quot; 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.</p><p>Thanks again for a great design!</p>
<p>Hhello. How did you wired the pot's?</p><p>I didn't find any reference for fine/course pot wiring (or didn't tried hard enough/wasn't lucky).</p><p>Thanks</p>
I just wired them in series, so that the &quot;fine&quot; 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.: <br>I put the &quot;fine&quot; pot in one leg of the &quot;main pot&quot; and tied the centre terminal of the &quot;fine&quot; pot to one of its outer legs.
<p>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. </p><p><a href="http://i.imgur.com/vcy2INB.jpg">http://i.imgur.com/vcy2INB.jpg</a></p><p>It powered a computer 6 feet tall.</p>
<p>Hi,</p><p>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.</p><p>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?</p><p>Basho</p>
<p>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.</p><p>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.</p><p>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.</p><p>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.</p><p>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).</p><p>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+</p>
<p>Thank's for your sound advice, I'll revert to plan A.</p><p>Kind regards,</p><p>kk</p>
<p>Yeah this power supply is not for everyone. I posted this article </p><p><a href="http://www.instructables.com/id/Dual-POS-NEG-Power-Supply/">http://www.instructables.com/id/Dual-POS-NEG-Power...</a></p><p>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.</p>
<p>Ok. This looks neat. I was looking for something like this to go with a large transformer I have.</p><p>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?</p>
<p>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.</p>
<p>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&pound; for it though. Sometimes you don't even get what you pay.</p>
<p>This circuit is a total GEM! Thanks for sharing it. I will be building it ASAP!</p>
<p>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.</p>
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.
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!
<p>Resistors are 1/4 Watt unless otherwise indicated. All Ohms values are in the schematic.</p>
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!!!)
<p>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. :-)</p>
<p>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?</p>
i have the same volt meter you like yours
Which?
hot damm the small cheap red one
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!<br><br>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
They do most of what a multimeter needs to do. The ones that work anyways.
not with the amp clip the solid red one its brand is Cen-Tech<br><br><br>i got it for free at Harbor Freight when i bout some files <br><br>
Yes the Red one. I was just saying I picked up the one with the red clip at HF too.
oh ok do u like it
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.
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
It happens.
Your Multimeter Collection Rocks!!! I'm still in love With The Simpson Meters,,,Thanks for the Picture!!
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. <br> <br>I love restoring old analog meters. It is very satisfying to me.
Nice Display of Meters!!! I have Some catching Up to Do!!
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/
You know..If you combine this with a rewound MOT, you would have one killer power supply....<br><br>But I do have to wonder what you would need 15 amps of power for.
My CNC project.
Ah thats true. Theres even 1000 Lumen LED's that consume up to 3 amps..Heh.
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 <a href="http://i.imgur.com/RJS8T.jpg">this image</a> 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.<br>
What kind of motors are you using that draw so much power?!
A Rockwell 1.5 HP induction motor. On my table saw.
Wow! Sounds powerful. Carefull now!
Fear is the only thing that keeps my fingers attached.
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.
Thanks!<br><br>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.<br><br>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.
Great work, Pfred!
Thank you!
Is my Math Correct??? If we PUSH this power supply to 15amps,,,each 2n3055 will &quot;DRAW&quot; only 5 amps each???( 15/3= 5amps),,, That's not even close to Ic=15 amps!!! Well as rimar2000 said,,,,Great Work,Pfred
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.<br> <br> 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.<br> <br> It is bigger than it looks in this picture:<br> <br> <a href="http://i.imgur.com/RJS8T.jpg">http://i.imgur.com/RJS8T.jpg</a><br> <br> Someday for laughs I'll have to do an article about the housing I made for it. I went all out!<br>

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