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.