Introduction: How to Build a Bench-Top Power Supply
A key component of any electronics project is the electricity. You could use endless amounts of batteries, or use a simple, compact power supply to power all of your electronic projects. This is a great beginners electronics project for those just getting into electronics, or a fun project for those who haven't wanted to drop the cash for a commercial model. This circuit is capable of supplying a variable voltage output from 1.5 volts to 12 volts.
Step 1: The Parts
All of the following components can be found at your local RadioShack. You can also scavenge most all of these parts fairly easily from old electronics.
1 LM317T Adjustable Voltage Regulator - 276-1778
This is the adjustable voltage regulator. It takes input from two resistors (R1 and R2) and then ratchets the voltage down accordingly. I recommend you take a look at the datasheet if you want to learn more about this part.
1 0-5K Linear Potentiometer - 271-1714
This is R2, and will allow us to control the voltage output.
1 560 Ohm Resistor - 271-1116
This is R1.
2 1N4001 Diodes - 276-1101
There are two diodes to protect against short circuits. D1 will protect the regulator from the capacitors discharging if the input power is short circuited. D2 will protect the regulator from the capacitors discharging if the output power is short circuited.
1 .1 uf Capacitor - 272-135
This capacitor (C1) acts as a smoothing capacitor. It should be only a ceramic disk capacitor.
1 10 uf Capacitor - 272-1013'
This capacitor (C2) improves the transient response of the regulator. It should be electrolytic.
1 PCB mount SPST switch - 275-645
Allows you to turn the power on and off without unplugging the wall-wart.
1 PCB mount terminal strip - 276-1388
This is mounted directly to the PC board and is an easy way to connect your power supply to many different circuits and components.
1 12v Wall-Wart
Provides the power to the circuit. RadioShack has a nice selection, but I recommend salvaging your own as I did. Anything will work as long as the output current is no more than an amp. I choose one that has an 800mA output, but anything over 500mA should cover most basic electronics projects.
1 Small Perfboard - 276-148
This particular board is the perfect size for this circuit, and my layout is based on it. This is a perfboard, but if you wanted to make your own PCB, feel free to use the attached EagleCAD schematic to generate your own layout.
1 Heat Sink - 276-1368
A good precaution. The regulator has built in protection to prevent it from burning itself up, but it does that by limiting current. If you didn't have a heat sink, you might find that you have less current output than you expected. Any piece of metal will work as long as you can attach it metal-to-metal on the tab. Even a large alligator clip will provide decent heat dissipation.
Step 2: The Tools
These are the standard tools for assembling almost all electronics projects. Not all of these are absolutely necessary, but they make the job a lot easier.
Not a soldering gun. Soldering guns make semiconductors cry tears of melted plastic.
Hot glue doesn't count. I've seen it. No joke.
Not required if you don't think you're going to make mistakes.
Small flat-headed screwdriver
To tighten the screws on the terminal strip.
These are those funky things that hold stuff while you're soldering. These are useful for way more than electronics.
Solid core wire
You need this to create the traces. It must be solid core!
Step 3: Breadboard
Breadboarding isn't mandatory if your confident things are going to work out, but it's probably a good idea to do it anyway. If a problem arises now, you can fix it before it's set in lead. Make sure to test the output voltage to insure nothing is out of line.
Step 4: Dry Fit the Components
Your going to want to lay everything out on the perfboard before you solder it. Your welcome to use my layout. One is normal and one is reversed so you can see where the components are when you are looking at the soldered side. The black dots show where the pins go through the board. The black lines are copper traces. The red lines are solder bridges. I got the templates from runoffgroove.com.
At this point, your also going to want to bend the copper traces. Use the wire strippers to strip all of the insulation off of a length of wire, and bend them to the correct length. Bend all copper traces except for the ones that lead to the terminal strip. These are more easily added after all the other components have been soldered.
Step 5: Solder Stuff
Now your ready to solder the components on. Start with the copper traces. Then, the resistor, switch, potentiometer, capacitors, diodes, and finally the regulator (in that order.) It's easier to solder the regulator with the heat sink already installed. Plus, it will protect the component from the heat of the soldering iron. Don't solder the wall wart or the terminal strip yet. Remember that polarity needs to be noted for components like the electrolytic capacitor (C2) and the voltage regulator. Don't solder them in backwards!
Step 6: Solder More Stuff
Now, solder the terminal strip into place. Then, solder in the traces to the strip. Note that the positive trace is routed over the negative trace next to it, so you will need to leave a little insulation on the positive trace to avoiding creating a short circuit. Then, solder the wall wart wires on, noting polarity. Clip all the long leads close to the solder joint with the side cutters. Finally, make all the solder bridges to connect the components to the traces. Take care with sensitive components like the diodes and the regulator. These are sensitive to heat.
Step 7: Quality Control
Go over each and every trace and make sure that you have good tight joints, and that no accidental solder bridges are present. Using a multimeter on the resistance setting is an easy way to see if joints are touching.
Step 8: Moment of Truth
Plug the wall-wart in and flip the switch. If smoke pours out of the voltage regulator, you got something wrong. If nothing happens, things are probably in your favor. Hook up your multimeter and test the voltage. Make sure the potentiometer can adjust the voltage. If everything checks out, congratulations! You're ready to start a long life of powering circuits.
Step 9: Possible Improvements and Modifications
VOUT = 1.25 * ( 1 + ( R2/R1 ) )
The value of R1 should be between 10 ohms and 1000 ohms. Anything higher and the voltage regulator won't behave. If you decide to make any changes to the circuit, you should refer to the datasheet for the finer details. This site is another good reference for using the LM317T.
Ideas from the comments
-You could enclose the entire circuit in a plastic project box. That would prevent the back side of the board from shorting out if it came in contact with a metal tool.
-You could buy a multimeter that is to be used only for tuning the output. Cut off the probes and solder the wires directly to the outputs for a permanent solution. If the multimeter had interchangeable probes, you could buy an extra set for use with other projects.