Picture of Pocket-Size Power Supply

I am a big fan of garage sales, flea markets, and thrift stores. They are great places to find used parts and materials for your next project. But one problem that I often run into is not being able to test battery powered electronics to see if they work. Because there are so many different combinations of batteries that are used in portable electronics, it isn't really practical to carry around batteries for testing. One device may need 6 AA's and another may require 4 D's. So I came up with this simple pocket-sized variable power supply. It can plug into either a 9V battery or a 12V battery pack. You can then adjust the output voltage to match the device that you want to test and attach the output wires to the end terminals on the device's battery connectors. This lets you power the device long enough to see if it works.
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Step 1: Materials

Picture of Materials
LM317 Adjustable Voltage Regulator
0.1 µF Capacitor
1 µF Capacitor
220 ohm Resistor
7 x 270 ohm Resistor (preferably 1/8 watt)
8-Position DIP Switch
Perf Board
9V Battery Connector
2 x Alligator Clip Wires

Note: All these parts are available at Radio Shack. I highly recommend using 1/8 watt resistor because they take up less space on the board which makes it easier to fit everything into a smaller space. Unfortunately I only had five 1/8 watt resistor so, I had to use two 1/4 watt resistors.

Step 2: The Circuit

Picture of The Circuit
The standard LM317 regulator circuit uses two resistors to set the output voltage according to this formula:

Vout = 1.25V x (1 + (R2/R1)) + (Iadj x R2).

Since Iadj is small (about 0.1 mA), the formula can be simplified to Vout = 1.25V x (1 + (R2/R1)) as long as R1 is also relatively small. Because of this, R1 is generally kept to about 240 ohms (you can substitute a 220 ohm resistor). R2 is then selected to get the desired output voltage. Often a a variable resistor is used for R2 to make the circuit adjustable. 

The circuit for this project has one major modification to it. The variable resistor R2 is replaced by an array of resistors and switches. This allows the output to be adjusted in discrete increments. I did this to more easily simulate individual batteries. Each switch effectively represents a battery being connected or disconnected.

Turing on switch 1 turns on the circuit and brings the output up to 1.25V. Then with switches 2 through 8, turning the switches off in order will each increase the output voltage by about 1.53 volts. 

Example: Initially switch 1 is off and switches 2 through 8 are on. Turning on switch 1 gives an output of 1.25V. Then turning off switch 2 gives an output of 2.80V. Then turning off switch 3 gives an output of 4.33 and so on.

The circuit can use either a 9V battery or a 12V battery pack as a supply voltage. The output will max out at about 1.5V below the supply voltage (7.5V for a 9V battery or 10.5 for a 12V battery pack.) But this isn't a problem because if you need the full supply voltage of the battery, then you can just hook the battery up to the circuit directly. 

Step 3: Solder the Circuit Together

Picture of Solder the Circuit Together

After testing the circuit on a breadboard, I soldered the circuit together on a small perf board. You can either follow my layout or make your own.

Step 4: Finished Circuit

Picture of Finished Circuit
Now you have a miniature power supply. Wrap the wires and the battery connector around the circuit board and it will easily fit in your pocket. To use it, connect the battery, dial in the desired voltage and attach the alligator clips to the battery terminals on the device. This should let you power it long enough to test it to see if it works properly.
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manny1230071 month ago
hi i am a student and i need power supply for my tons of projects,
sir i want to ask u can i use a 12 v power supply, then what specifications do i need so that this project run smoothly,
do i have to increase number of 270 ohm resistor or increase my capacitance,i hope you would guide me....

You can use any voltage supply up to the voltage regulators max rated limit. So yes you can use a 12 volt battery or a 12 volt supply without changing anything. However you need to keep in mind that the maximum output will be slightly lower than the supply voltage as the regulator chip drops the voltage a little.
ArtTech86 made it!4 months ago

I appreciate you creating this instructable! I will benefit from having made one of these. I also know quite a few people who would also benefit from having a pocket power supply so I plan on making some more to give as gifts. Thanks, I will be checking out more of your work.

Pocket Power Supply.jpg
Im only able to source stripboard, does this make the build more complicated or would it be the same please?
It might be a little more difficult to get all the connections right. You may need to use a knife to cut some of the etched lines to make it easier.

Hi there, I came across this project and thought that it would be perfect for a complete noob. After purchasing some parts and discussing it in my local electronic parts supplier, I came upon a problem where I couldn't Identify what types of capacitors you used, please could you list the types too for total novices?

The type doesn't matter. As long as you have the right values, it doesn't make a difference.

makeosaurus5 months ago

Could you do this with a 4 switch DIP because this is what I have???

Yes. You will just have fewer voltages to select from.
davidwhitt467 months ago

hi i am confrese i put in the LM317 Adjustable Voltage Regulator and the 8-Position DIP Switch and the 7 x 270 ohm Resistor now do solder the wire and the red wire and do and do i solder all to pin of the swich togladed and the Resistor bottens


If nothing else, just try to match the pictures.

floowed the pic and got 8v and 1.25v but nuffing betwins what i doing rong


Sorry. But I am not really sure what is going wrong.

ok thing it the 8 resistors as im unsure on thank go to swich but ends unsure wher go the wing goings if can do on the pic and wher go be nice of you

MagicTK9 months ago

Pretty cool, simple design. I like the fixed DIP switches for set voltages. I came across this after posting my adjustable regulator, and thought I would show my appreciation for your design.

EET62310 months ago
is there a more detailed picture of where everything was soldered together on the back?
DIY Hacks and How Tos (author)  EET6239 months ago

I have added a new picture of the soldering to step 3

Im currently working on a breadboard version would you like a pic when finished?
Did you ever happen to complete the breadboard setup?
Sure. If you want to post it in the comments, it might be helpful to other people.
Arduino Tech10 months ago
this project is from MAKE
Yes. I work for Make. My name is Jason Poel Smith. I create projects for them and post them to several sites for a wider exposure.
Do you have a picture of breadboard setup?
Sorry. This was one of the rare occasions when I skips prototyping and went straight to the finished circuit board.
agis681 year ago
awsome!!! since the lm317 can handle up to 40V is suggested to use 12V A 23 mini battery? What changes have to do on above circuit

DIY Hacks and How Tos (author)  agis681 year ago
You can use a 12V battery without making any changes to the circuit. You would just want to use a different battery connector.
Wo0kiE1 year ago
I only have one Q?
How do you know how much current is being supplied across the circuit?
DIY Hacks and How Tos (author)  Wo0kiE1 year ago
The voltage regulator keeps the output voltage constant regardless of how much current is being drawn (up to its operating limits). If you really wanted to know the current, you would probably have to use a current meter at the output.
DragonDon1 year ago
This is a great idea!
wkoepp1 year ago
Even though the idea of this was to test electronics at say a yard sale, this design is excellent for breadboard projects if you don't have a heavy duty power supply or just don't want to pull it out. This is a nice little project I plan on doing soon. thanks.
atharva121 year ago
Please send me the schematic diagram... Thank you.
The circuit diagram is on step 2
Dr.Bill2 years ago
Good part of this is with a heat sink you can attach to a 12v source too. The lower the voltage it is set to the warmer the chip. I use this circuit without the resistors but I do use the potentiometer preset voltages for low power apps like AM/FM radio or a Digital Blood Pressure Monitor. These run my 6v powered speakers and provide voltages for experiments. Good choice for low powered devices. Mine are attached to a 200Ah battery.
Orngrimm2 years ago
simple, but in some cases wastefull due to the linear-regulator.
a more efficient (But more complex!) solution would be a DC/DC-Converter with variable feedback-divider.
http://www.linear.com/product/LT1376 would be such a solution (Buck-Converter) with quite low parts-count. Simply adjust the voltage-divider from Vout to generate 2.42V to the FB-Pin and you have your variable Output up to 1.5 Amps with an efficiency of mostly over 80% and also often above 90%.

But as the efficiency isnt this an issue most of the time, your solution should fit the bill of many DIYers. :)
Actually dc to dc converters in this voltage range would be much less efficient due to the required operating overhead. The only time they are a valid source of power is if you are dealing with devices that require a tightly controlled and specific input. the LM series of voltage regulators are much more efficient. Your 80-90 % figures are not correct, as this output is at an unloaded state and drops tremendously as the operating load increases. The LM series regulator holds a constant 75-80% efficiency regardless of the load. I've had to redesign many supplies where DC-DC converters were grabbed off the shelf for critical use projects and replace the converters with LM series regulators, backed by FET pass transistors to save power for remote system use.
Excellent Instructable. I really like the idea that each dipswitch represents a single 1.53v battery, and also that you explain the voltage loss and facts such as (two 1/4watt resistors == one 1/2watt resistor) Question... I know "standard" voltage regulators follow the "lm70X" naming schema. Yes, I know a little off but it is something like that.... how do I identify variable voltage regulators thus allowing me to scrap them from old electronics, regardless of the fact that they are dirt cheap at Radioshack
Diy Hacks is correct. Solid state regulators are actually ICs so you only get family names IF anything is marked at all, that is an LM on the chip. Then the manufacturers proprietary markings. there are downloadable cross references from various sources, but they go out of date so fast it is unbelievable. your best bet is to go to the parts you are pulling and look for replacement parts through NTE and similar sources to identify them . It's a lot of work. but some times, worth it, mostly for the practice in desoldering more than anything,
I don't know of any universal naming convention. You will probably have to look them up individually.
thegrendel2 years ago
Elegant and useful. A very nice Instructable.

The only change I would suggest is choosing the divider resistors to set
more standard output voltages, such as 3.3 and 5.0v. The key is that these
resistors do not all have to be the same value.
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