Regulated Power Supply

I decided that it would be useful to have a regulated DC power supply to power my electronics projects. I went online and found a lot of websites which offered plans and instructions. I chose this design because it is relatively simple. However I got carried away and made my own pc board, because I had been wanting to try that for a while. You certainly don't have to go to that trouble if you just want to make the power supply - you could just solder the small number of components onto a perfboard.

Step 1: The Schematic

Here is the schematic for the pcb part of the power supply. The input is coming from a transformer (not shown in the schematic), which steps 120 volts AC down to 24 volts AC. I bought it at Radio Shack. There's a bridge rectifier, a large smoothing capacitor, two linear voltage regulators, and a few small filtering capacitors on the output. The power supply takes 120 volts AC from the wall, and produces a smooth steady 5 volts DC, and 12 volts DC.

I produced this schematic using Eagle, a widely used and freely available CAD program for designing circuit boards. I wanted to learn how to use it. Of course you wouldn't have to go to that trouble if you just wanted to make the power supply. This circuit is pretty simple.

I wanted this power supply to be able to handle 2 amps, so I used the L78S05CV and L78S12CV voltage regulator chips, which are rated for 2A. The popular LM7805 and LM7812 are only rated for 1.0 amps.

Step 2: The Box

I bought a simple aluminum project box and planned how to lay out the components. The paper represents where the pcb will go.

Step 3: Drilling the Box

The aluminum is thin and easy to drill through. You could probably use any drill and drill bit you have. I made it even easier by getting a step drill bit, which is ideal for drilling holes of various sizes in thin material.

Step 4: Painting

I decided to spray paint the box. Even though the color I used is similar to the original aluminum it still looked nicer with the paint. This was for aesthetic purposes only.

Step 5: Mounting Parts

The transformer is mounted by bolting it to the bottom of the box. There are
binding posts for output in the front, an on/off switch, and a power indicator light. I used an AC power entry module in the back (like the kind you find on computers), and I also installed a 2 amp fuse for safety. I have included a complete parts list at the end of this instructable. The pcb is mounted on bolts and standoffs. I made my own standoffs by cutting segments of a BIC pen tube, but that's just because I forgot to put them on the list when I was ordering parts.

Step 6: A Note About Grounding

Because this project is powered by line voltage from the house, I wanted to make it safe. The AC socket in the back requires a 3 prong grounded cord. The ground terminal is solidly connected to the metal box, as it should be for safety. However this ground is not connected to the 'ground', or common, on the output side of the transformer, so the output of the power supply is isolated from earth ground, which is typical. In that regard, the output is like a battery.

Step 7: Making My Own Pcb (this Step Is Optional)

Here's the part where I made a DIY pc board. If you're not interested in doing this at this time, and you just want to make the power supply, you can skip all of this and just solder the components to a perfboard. I wanted to try making a pcb, so I went for it. I won't go into detail here - there are tons of websites and videos on YouTube showing you how to make a pcb. In a nutshell, I used the 'positive photo resist' method. Using Eagle to design the circuit board, I printed the image on a transparency, covered a pretreated circuit board, exposed it light, and developed it with photo developer.

Step 8: Etching

I etched the board with ferric chloride.

Step 9: Drilling the Board

It's recommended that you drill the board before cleaning off the remaining photoresist. You need small, hard, drill bits, with a lot of rpms. I bought a pack of carbide steel micro bits, ranging in size from .5 mm to 1.5 mm, and I used my Dremel. The smallest bit I attempted to use was the .8mm. The issue here is that I don't have a drill press, and all the websites and YouTube videos I looked at said that it is nearly impossible to do this without one. The carbide steel is very brittle and the bits are tiny, and the slightest wobble will break the bit. I concentrated and steadied my hand and pulled it off, but if I were to do this more in the future I think I would want to invest in a small precision drill press.

Also, you can just take your output files from Eagle and send them to a pcb manufacturer, and in a few days, for a modest investment, you will get a beautiful, professionally made pcb.

Step 10: Soldering the Components Onto the Board

All I can really say about this is that if you don't have a good soldering iron, electronics can be a frustrating hobby. I could have done this project with my fixed power, low wattage soldering iron, but I finally invested in a variable voltage soldering station, and it is such a delight.

Note the heat sinks on the linear voltage regulator ICs. This is recommended.

Step 11: It Works

The output terminals supply 5 volts and 12 volts of smooth, regulated DC power. Note that with a simple modification of this circuit, you could make a variable voltage power supply.

Linear power supplies like this one are not the most efficient. A lot of power is wasted in heat, and the transformers are big and heavy. In the future I may build another power supply and use a more modern and efficient design, like a switched mode power supply.

Step 12: Parts List

Parts List

25.2 V 2.0 A Heavy Duty Chassis Mount Transformer, Radio Shack Part #273-1512

50v Bridge Rectifier Radio Shack Part #2761146

6800 uF 35 V Electrolytic Capacitor

78S05CV Linear Voltage Regulator IC

78S12CV Linear Voltage Regulator IC

Heat Sinks (2)

2.2 uF Electrolytic Capacitors (2)

100 nF Ceramic Capacitors (2)

Single Sided Presensitized PC Board, Photo Developer, Ferric Chloride

7" x 5" x 3" Aluminum Project Box

Binding Posts, (1) black, (2) red

Red Neon 120v lamp

SPST Toggle Switch

AC power Power Entry Module Qualtek .250 Q-C

Fuse holder and 2 amp fuse

nuts, bolts, standoffs, and wire,