Introduction: Pocket-Size Power Supply

About: My name is Jason Poel Smith. In my free time, I am an Inventor, Maker, Hacker, Tinker, and all around Mad Genius

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.

Step 1: 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

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

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

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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