Introduction: 300+ Watt Linear Power Supply
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
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.
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
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
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.