# 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:
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hsosna6 months ago

where can i get the display at?

hsosna6 months ago

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

6 months ago

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

bsatbolt7 months ago

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.

ignasg7 months ago

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.

EngineeringShock (author)  ignasg7 months ago

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.

7 months ago
Thank You for info about capacitors!
chandubandara1 year ago
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!
1 year ago
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 ...
charliebande1 year ago
very intresting project.thanx.
sjroth1 year ago
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?
Kira_gelo1 year ago
Can i use any voltage display(7-segments) for this one?
bbmuck2 years ago
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?
jpeg2 years ago
I wish there was a bigger version of that schematic!
2 years ago

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!!!
louph3 years ago
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
EngineeringShock (author)  louph3 years ago
Hi
Yes, very easily. You can place it directly on the output.

Thanks
Pat
tim1273 years ago
where could i get a digital voltage display module?
Steel Chameleon3 years ago
Is it possible to change the LM 317 for a LM 150 to supply a higher current output? Just in case my project is pushing 1.5A+
mattman23 years ago
awesome thanks for replying, nice instructable by the way
mattman23 years ago
just wondering could you use capacitor with more than 100 UF or less cause that's all I have available
EngineeringShock (author)  mattman23 years ago
Yes, that is fine =)
mattman23 years ago
I was also thinking that if the display dosn`t have a separate source of power or the power consumption is not subtracted out of it the reading would be inaccurate.
gormly3 years ago

Is this the same as the kit on your website?
I want to biuld one for a project using two 3000mAH batteries at 3.7v each, connected in series for 7.4v and be able to vary output from 3 to 5v.

I have most of these parts already.
greyhawk833 years ago
Patrick, All your instructables are very educational, enjoyable-great work.
My question about super capacitor is how large do they get and do you carry them?
MROHM3 years ago
I Would HIGHLY RECOMMEND Patrick's E/bay store as I have purchased 4 Kits from him and all 4 kits worked when completed,,,,and some of the Instructions were even in Chinese!!! Do you have any ideal how much a Store Bought Variable Power Supply Costs????Like a Horror Movie,,,Very Very Scary!!!
Electronics Man3 years ago
Very nice!!!!!!!
jensenr303 years ago
nice instructable!