# THE EXTREMELY SIMPLE DIGITAL DC-DC POWER SUPPLY!

74,025

169

29

Published

## Introduction: THE EXTREMELY SIMPLE DIGITAL DC-DC POWER SUPPLY!

Hello Instructables Community!
LET'S BUILD A DIGITAL DC-DC POWER SUPPLY TOGETHER!

Before we get started, let me tell you a little about this kit.  I have a digital DC-DC power suppy that I use daily.  However, it cost a bunch of money that I didn't want to spend and it is limited to an output current of 500mA. This supply has a current limit of 1.5A, as that is what the LM317 IC is designed to output.  I decided that while I'll stick with my current power supply, there may be people out there who are looking for a simple and practical solution for their Arduino projects and such.   I hope you all enjoy this instructable!  If you have any questions, I'll do my very best to help!  I'm also going to offer hardware kits for this project, so read on to find out where they can be found!

THE BREAKDOWN:
This circuit is made to work with a wall transformer or a battery.  You can use either.  There is no on-board AC-DC rectification, so don't apply 120VAC to the input by mistake!  The power supply for the digital voltage display should be kept under 18v.  If you're going to use a wall wart AC-Dc transformer, I suggest a 12v-14v @ 1A transformer.   You can use a 9v battery if you'd like =)  We will talk more about that in a while.

When you feed DC voltage to the input terminal block, it acts to supply power to the digital voltage display, as well as the variable DC voltage LM317 regulator circuit.  The LM317 is the key behind this device.  The LM317 is a 3-pin chip that acts to offer a variable DC voltage based on the resistance of the on-board variable resistor.  The voltage at the output terminal block is variable and is displayed for you on the 4-digit 0-20VDC digital display.   Please note that this design has limitations.  Mainly that the output is not protected from shorts.  If you short your output pins together for a short time, the circuit will likely be okay.  However, this can cause permanent damage to the LM317, in which case you'd need to replace it.  Be very careful not to short the output pins.  If you can get your hands on a fuse, you can easily protect your supply.  Place the fuse in series with the positive lead on the input DC line of the device.  Otherwise, be careful.  As well, this supply does not read out load current.  On top of that, the kit does not come with banana wires.  The neat thing about this project is that you can customize it to your liking.  We do sell parts and a kit for this project in our hobby electronic stores that can be found here: http://www.electroniclessons.com  and http://www.engineeringshock.com

Here is a video that gives you a demonstration:

This video is for our new kit that is based on this instructable:

## Step 1: What You'll Need!

HERE IS A LIST OF WHAT YOU'RE GOING TO NEED:

COMPONENTS:
1x 0-20VDC Digital Voltage Display Module
1x LM317 Voltage Regulator IC
1x On/Off Switch (Included in http://www.engineeringshock.com kit, but not seen in image)
2x Terminal Blocks
1x Heat Sink for LM317
1x 5k Ohm Variable Resistor
2x 200 Ohm Resistor
2x 100uf Electrolytic Capacitors (Rated for 20v or higher)
2x 0.1uf  Ceramic Capacitors (Rated for 20v or higher)
1x 7cm-9cm PCB
1x DC Power Source (Battery of Wall Transformer) - Not included in http://www.engineeringshock.com kit.

TOOLS REQUIRED:

1) Soldering Iron
3) A Hand Drill
4) A Drill Bit (I used 1/16)
5) About two hours of time, give or take.  This is based on your electronic background, and soldering experience.
6) A Multimeter to troubleshoot, in case you have any problems along the way =)

## Step 2: The Schematic Diagram

As you can see, there really isn't too much to this schematic.  We will be begin from the left-hand side.

The Input Terminal Block:
This is where you're going to connect your input DC source.  In the video, I mention that 18VDC is fine, but I suggest keeping it to 14v or under.  if you are looking for a higher voltage, you can use a 18VDC source at the input.  In all likelihood, most of you are going to want to use this for digital applications, or perhaps for powering amplifier circuits.  Anyhow, through the terminal block, there are two capacitors in parallel with the line.  The 100uf capacitor is a smoothing capacitor.  While the DC source is likely coming from an already smooth DC source, you never know!  This capacitor isn't 100% necessary, but I do recommend that you use it.  The 0.1uf capacitor is a decoupling capacitor.  It is used to filter out unwanted high frequency signals that might find their way to your circuit..   It is good practice to have both of these capacitors in parallel with the DC input line.  The positive DC input line feeds voltage to the input of the LM317 chip (Pin#3), and to the digital voltage readout.

The digital voltage readout has four pins on its rear.  Pin#1 is connected to the voltage reference (Our output voltage), pin#2 and pin#4 are connected to the ground line, and pin#3 is connected to the DC input supply.  This device requires a power supply at pin#3 of anywhere between 6VDC-18VDC.  Nice range, eh?  This supply powers the circuitry on the device.  As long as their is power at this pin, you will see the voltage display light up.  If thre is no votlage connected to the reference pin (Pin#1), then the display will read 00.00v.    Again, our reference pin is reading the voltage at the ouput terminal block.  The voltage at the output terminal block will be displayed numerically on the voltage readout.

The LM317 Varaible DC-DC Regulator:
As mentioned before, the input supply pin for the LM317 is pin#3.  The postiive DC supply voltage is connected here.  Pin#2 is the output pin.  There is a 200 Ohm resistor tied in parallel with pin#2 and pin#1, and a 5k Ohm variable resistor between pin#1 and ground.  This variable resistor adjusts the voltage at the output.  When you adjust the variable resistor, the output votlage goes up or down depending on which direction you're turning the variable resistor.  The lowest output voltage that the LM317 can support at the output is 1.2v and the highest output voltage is 37v.  However, we're never going to go that high, as our input voltage will be less than 18v to protect the digital readout..  The LM317 can support up to 1.5A of current, but it needs to be heat sinked.  In this case, a heat sink DOES come with the kit =)

The Output Terminal Block:
Like the input terminal block, there are two capacitors of the same values in parallel with the output line.  This is used for the same purpose.  I do suggest you implement them.  You will also notice that the reference pin from the readout is connected to the positive line of the outputer terminal block.  We talked about this before.  This terminal block is where you will connect your load.  It may be a digital circuit.....  It may be an analog circuit...  The possibilities are endless!

## Step 3: Putting It All Together

When it comes to putting this all together, there really is no right way or wrong way.  I soldered each of the big components on first, such as the heat sink, the terminal blocks, the variable resistor, and the digital voltage display.

IMPORTANT NOTE!!!
When you're about to place your heat sink and terminal block into your PCB, you may notice that the leads are too big for the board.  In this case you're going to need a hand drill with a small drill bit.  I Used a 1/16 bit.

Once you place all of the larger components into the board, drop the LM317 into an area of slots in front of the heat sink.  DO NOT SOLDER THE LM317 YET!  First, use the screw to screw the LM317 into the heat sink.  For those if you who don't know anything about heat sinks, they are used to dissipate heat created by current running through the IC.  It helps to keep the IC cool, and acts as protection.

Once you have the LM317 screwed tightly against the heat sink, solder the leads to the board.  Now, solder the switch and all of the passive components to the board.  From here, all you have to do is make all of the connections on the bottom of the board, and you're set.

Please note that I am very experienced in soldering.  Don't feel discouraged if you're having trouble soldering paths like I did.  If you are having problems, use jumpers and wires.  It doesn't have to look neat to be cool!

## Step 4: Applications - What Can I Do With My Power Supply???

Well.... It is a DC power supply, so the world is your oyster!    I use my power supply every day.  Mainly for design purposes, but I play with my hobby kits from time to time.

WHAT CAN I DO WITH THIS???

Tune the output to 5v to work with TTL ICs!

Charge super capacitors with your new variable power supply!  Please keep in mind to limit that charge to the super capacitors before you do so!

Use it to power small motors!

Use it to power your own circuit designs!

Build the Eye of Thundera and Destroy Mumm-Ra!

THE POSSIBILITIES ARE ENDLESS!

## Step 5: Conclusions

This kit is great for people with little electronics background.  It doesn't take a weekend to build, and it can be used as a effective lab tool!
All in all, I'm very happy with this device.  I hope you enjoyed reading the instructable.  If you try to solder the paths like I have, take your time and be careful.  It would be a good to have a solder sucker handy =)

Thanks again everyone!  Have an educational day!

## Recommendations

• ### Large Motors Class

8,637 Enrolled

• ### Oil Contest

We have a be nice policy.

## Questions

Funny the things that appear when you type in LM317 on Google. Thanks a million for the how to, I have several home built power supply's, but I am getting old and forgetful. Here this afternoon I found a package of 10 LM317's and sort of forgot why I ordered them awhile ago. This gave me several ideas. I had planned on building a power supply out of an old AT computer power supply laying in the back room, and using one of these to make a variable output on the supply in addition to the standard 12v and 5V as well as the wonderful 3.3V. However since that time, I have been tearing down old laptop batteries and reviving the 18650's to use in portable power supplies (we do a lot of camping now in our old age!) This will make a nice build for using 3 18650's in a small box for an adjustable to use. I already have some built with some of the cheap DC-DC power converters available on Ebay, some boost, some buck and they work OK but it would be nice to have one that I built the whole thing by hand instead of relying on the junk you get from over there. (well some of it is junk, I have found some very nice items from over there as well, from Banggood and a few other good sellers on ebay, just have to be careful who you order from.)

where can i get the display at?

my only question is where did you get the display from?

also how many pins does it have and how aare they hooked up?

I built it!! Keeps blowing the LM317 after using it a few times. Going to try the circuit with a couple of diodes to protect the IC.

Thats very strange schematics file...

First - You have to use LM317T in such connection, but in picture we have LM7805. Thats wrong 7805 - doesn't have any adjustments (with resistors like here). 7805 produces 5V fixed regulated voltage, thats what it is designed for. For novices - LM317 and 7805 looks almost the same, but they are completely different devices.

Second - why 100uF and 0.1uF capacitors are connected in parallel ? In the result they will sum as 100.1uF capacitor... Of course it will work, but what's the point ? In reference designs from official LM317 datasheet 0.1 uF is on the input and 1uF or 10uF on the output.

2 replies

It is an LM317, but I accidentally labelled it 7805. It is the correct schematic otherwise. The 100uf capacitor is a smoothing cap that acts to compensate for any low frequency spikes on the supply line. The 0.1uf capacitor is a decoupling capacitor that acts to eliminate higher frequency noise. It is common practice. If the output of the LM317 was supplying power to a few different chips, it would be in good practice to have 0,1uf decoupling caps near the power supply to each chip. From a schematic perspective, all of said caps are in parallel with one another.

Common practice.

Thank You for info about capacitors!

It is mentioned LM7805 in the schematic, does LM317 and LM7805 are same, or the author has made a mistake?

Anyway tnx very helpful and awesome project!

The LM7805 and LM317 are not the same. Although they do share some similar features. They come in the same style package, and have the same number of leads. Oh, both of their names start with LM (stands for Linear Monolithic I think) too. But after that all similarities end. Plug either in where the other belongs and boom! 3 terminal voltage regulators blow up sweet. Don't ask me how I know that ...

very intresting project.thanx.

Awesome project! I was planning on making something very similar to this, but without the display. Do you know where I could order a voltage display module?

Can i use any voltage display(7-segments) for this one?

Could I add a fuse for short circuit protection?

if so, what type of fuse would I need and where would I need to install it?

I wish there was a bigger version of that schematic!

Full Page Schematic !!!

{P.S - To enlarge ANY picture anyone has uploaded here on instructables, just click on the 'i' square button on the Top-Left of every single picture, then simply click on what size picture you want displayed from the "Available Sizes" box on the left hand side of the page that opens-up as soon as you have clicked the square "i" button on the picture!!!}

hope that helps!!!

Hi there. Is there a way to put a digital ammeter also? I'm totally new with electronics and trying to build a bench supply. thanks