Variable Voltage Regulator - Filtered Linear Power Supply

This Instructable will show how to Make a 1.6v-6v Filtered Linear Power Supply using a LM317 for the variable voltage regulator and a 1kohm Potentiometer (trimmer). It can source power from anywhere between 7v to 14v for this version of the power supply. The power supply in the above image is at the top and you can see a 7.4v battery plugged into it, with a 3.3v cable harness attached to the output on the right of the power supply.

Filtered means that it is good for sensitive IC's as it will provide the most accurate selected voltage possible. This is accomplished by using filter/bypass/decoupling capacitors on the power rails to reduce noise. This power supply can source power from a range of power supplies within 7-14 volts @ 500ma. The lm317 can sink more than 500ma of current but at the sacrifice of having to use a lower voltage, this is because the LM317 has a max rating of 3watts total. Sinking 1.5 watts of power uses a fair size heat sink so if you want to have more power running though it you may want to move to a larger Voltage regulator.

This power supply is not suited (or needed) to power standard MCU's like the arduino uno or mega. It works great with a few tweaks to power a stand alone or breadboarded ATMEGA328 though, because the power is filtered and you have a nice easy way to adapt it to the same sources the standard arduino's use. Just beware that the ATMEGA328 is capable of drawing up to 560ma with just it's digital pins and this will stress the heck out of the power supply.

Step 1: Parts

So there are two ways of going about this. The totally home-brew way, or the lame way where you just go out an buy a 5v/3.3v power supply . There are lots of different kinds of these available but the one from SparkFun is great because you support a great company and it's breadboard compatible (awesome!).

For those of you who want to build it yourself AND save yourself a few bucks then that's what you'll learn to do. Lets start with the parts list:

1x protoboard - doesn't really matter what size as long as you can make it fit. 2"x2" should be fine but the heat-sink will likely hang off the edge unless you're creative
1x LM317 (type-t) Variable voltage regulator
1x heat-sink capable of dissipating up to 3 watts.**
1x 1N4001 1A Diode
2x 3.5mm pitch 2-pin screw terminals*
2x Break Away Headers* - (Right Angle or Straight depending on your design)
1x 100uF electrolytic radial capacitor
1x 10uF electrolytic radial capacitor
1x 0.10uF ceramic disc capacitor
2x 330ohm 1/4W 5% Carbon Film Resistors (standard resistors)
1x 1k ohm adjustable Potentiometer/Trimmer.
1x LED 5mm (your choice of color [may need to adjust resistor value])
Some wire - 22 gauge.

note - you'll also want to have some thermal compound handy for the heat-sink.

*The headers and screw terminals are how I decided to attach my power source. My supply is set up to accept power from either the screw terminals (from a 9v battery lead) or directly from a 7.4v 1600mah battery pack. These can be adapted however you want and I would suggest adding a 2.0mm DC Power Male PC Mount (2-pin) if you want it to work with a 9-12v wall transformer. That way it has a nice clean interface and you cant mess up the polarity. 

** The heat-sink is important and there are several different ways to go about using it. The heat-sink is for the LM317 voltage regulator, it needs to be sinked for our purposes and while a 1.5 watt heat-sink is what i'm using and does a fine job for what i use it for, if your dropping a lot of power and using a lot of current you'll want to consider using a larger one. Just remember that this specific voltage regulator only supports up to 3Watts total even WITH the heat-sink.

It seems like a lot, but the total cost for just about everything here is less than $5 when ordered online. You can still get most of the parts from local hobby shops (or radio shack) for less than $10 and i'm sure if you've been working with electronics for a bit now you'll have most of these parts on hand anyways.

Step 2: Schematic

Below is the schematic designed from the power supply I use. There are a lot of these online you can get your hands on, but this one should give you everything you need to know on how to make this specific one, as well as the ability to adapt it to your projects needs. Some of you may notice the trimmer seems oriented wrong or that pin 3 also connects to ground. pin 3 is optional and not used for this design, and the trimmer is nothing more than a resistor, making it bidirectional (in short it doesn't matter. Also note that the LED is on the adjusted power rail, This can be moved wherever you want so long as you calculate the correct type of resistor to use.

Step 3: Protoboarding

This part will be mostly up to you to follow the schematic and adjust as you need to. I personally used a two-way three-throw switch, that way i can toggle the power to be controlled by a micro-controller if i wanted, as well as just flip it off or leave it on permanently. There are lots and lots of mistakes on this board that had to be corrected, but hey, thats why their called protoboards. The next step has a pcb etch ready to go for the same power supply using the same parts (with a couple locational changes) in-case you want to skip the mess. 

I'm sure some of you are wondering by now how you can measure what the output voltage is while setting the potentiometer. I simply use a multimeter and touch the top of the screw terminals. This can be a pain sometimes but you can get a small voltmeter for <$10. and if you don't already have one, you need one. As mentioned previously, if you're not interested in trimming values or using a multimeter to check the voltage frequently, you can simply use a switch and several calculated resistors rather than a potentiometer to give you exact values.

It's also important for me to note that if you hook up two power sources at once, something is going to cook (probably a couple things). One thing I would add to this design would be a power source selector. That way if you have a need to plug in both a wall jack AND a battery (or some other auxiliary source) you would know for sure that there is no way to accidentally melt your new fancy power supply.

Take some time to review the photos included. I tried to get good angles on everything so that if you're trying to follow it you should be able too. Also don't hesitate to ask if you need clarification on something, or if you see a correction that needs to be made.

Step 4: Next Steps and Modifications

If you reviewed some other linear power supplies at all you'll notice a couple of things that are different. First of all, they usually have selectable source switches on them (like I mentioned in the previous step). Also, rather than having the fine-tune adjustable 1k potentiometer that I used in this power supply, they have a switch or jumpers that allow you to select which exact voltage you want to use. Adding one of these switches to easily select between voltages is simple enough and are they simply change the resistance on the LM317's adjustment pin. If you are interested in doing this there are a bunch of calculators you can find online that will help get you the correct resistor combinations for the voltages you want. I specifically wanted to make sure I could tweak the output voltage a tad so I could ensure that all of the components I'm attaching get the correct rated voltages. For example I have a 5.8ghz transmitter that is rated to run at 3.3v. When hooked up with it's other components and with the power supply set to 3.3v, the actual amount of power reaching the transmitter sits below 3.1v. While this is a negligible difference in most cases it makes sense to ensure that everything is getting as much juice as it wants (it also helps curb my OCD). 

The other thing you may have seen are the much smaller versions of this power supply here (once again usually with at least one of the above mentioned alterations). The key to making those is getting the PCB designed and printed out. For the fun of it, I made a fairly simple single sided PCB that should do the trick just fine. I used express PCB and the print cost for 10 of these boards is roughly $90 through the express pcb supplier. Printing your own is also fairly easy and there are plenty of other instructables that will help you with that if you're interested.

Step 5: Help!

If you run across errors in my design or have a better way to do something let me know!  I'm not very versed in how to best select filter/bypass capacitors and the PCB design is only an amateur attempt. So once again, don't hold your tongue if something doesn't seem right. I'd love to learn the better way.

Also don't hesitate to ask if you have any questions.

Enjoy!