Pocket-Size Power Supply

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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.
davehartles692 months ago
Im currently working on a breadboard version would you like a pic when finished?
Sure. If you want to post it in the comments, it might be helpful to other people.
davehartles693 months ago
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.
agis683 months 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)  agis683 months ago
You can use a 12V battery without making any changes to the circuit. You would just want to use a different battery connector.
Wo0kiE6 months 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)  Wo0kiE6 months 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.
DragonDon6 months ago
This is a great idea!
wkoepp7 months 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.
atharva128 months ago
nathan6221678 months ago
Please send me the schematic diagram... Thank you.
The circuit diagram is on step 2
Dr.Bill8 months 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.
Orngrimm9 months 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. 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.
PetrichorXFi9 months ago
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.
thegrendel9 months 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.
the values in the circuit are fine, remember, batteries drop in output rapidly. AND standard resistors still 5% +/- tolerance. so in the end you would be literally measuring resistors to find the "perfect" values.
halamka9 months ago
I have a question. I bet that the x is a / in the equation. What are the output voltages? Something is wrong with the equation.
DIY Hacks and How Tos (author)  halamka9 months ago
The output are approximately 1.25, 2.80, 4.33, 5.86, 7.39, 8.92, 10.45, 11.98 with a max output of 1.5 below the supply voltage. I believe that the equation is correct. What are you uncertain about?
Excellent instructable. How about adding a small heat sink to the regulator? As it is I guess the max current you draw into the load is limited by temperature rise of the regulator before it shuts itself down is about 2W/(9-5)V = 1/2 A. The 9-5 is the 4 V difference between input and output. But with a small heat sink at least it might not burn your finger. ;)
adding a heat sink might be a good idea but you will never get the 2 watts you think . the LM series ( in this instance the LM317) are self limiting on current levels, the T series are all 1 amp max... this has nothing to do with temperature rise coefficents, it is simply a design parameter of the three terminal regulator. I've been utilizing the LM series regulators since 1975. They are wonderful devices, but if you want more current you need to add pass transistors, and these are where you will start to deal with the temperature coefficents limiting your current. The Regulator itself is actually more of a simple IC than a transistor as they can not be used as an amplifier.
PhilKE3FL9 months ago
Very nice idea, but as has been said, unless you're only powering very low power devices some heat sinking will be needed. Another thing that others may not know is that a 9V battery was designed for low power devices and can only handle something around 30 mA, that's 0.030Amp it most definitely will NOT be able to power something requiring 6V and 1000 - 2000mA (1 - 2A) the battery would die almost instantaneously even if the regulator could handle the power requirement of 6 - 12W. You'll stand a better chance testing out those baby swings with a nice 12 V battery, and some good heat sinking & probably a 3 - 5 Amp regulator if needed.

Today's electronics is using less and less power so perhaps a 9V "transistor" battery will work like this. (How many even remember that the original common name for that 9V battery was the "transistor" battery?) But some baby stuff requires, as you noted, C or D cells. If they require that size cell and multiple cells, 4 = 6V, 6 = 9V, & 8 = 12V then they require something on the order of 1 or more amps of current and a 9V Transistor battery maxes out around 30 mA that's 0.030 Amps, it might just work for some very small fraction of a second to power such a device. Take a 12V battery or power pack unless you know you're only going to be testing very low current devices.
this regulator, even with a substantial sink will only do up to 1 amp so the current sourcing of a 9volt is moot. This whole setup is geared around low current devices.
Ericc8159 months ago
This is a wonderful 'ible for some gadgets.
Now I ask - Would you consider a more substantial 'ible using a mini 12V sealed battery - a potentiometer instead of a switch & resistors - a 3 digit Vout readout???
This is what I would consider a true all purpose portable power supply which would be adjustable with heavy load capacity like some 1-2 amp hardware requires.
this regulator is only good for 1 amp with a decent heat sink
usbg3rd9 months ago
y not use a variable resistor instead of many resistors and switches ?
this gives you fixed voltages, a variable you would need a meter to read the voltage setting
DIY Hacks and How Tos (author)  usbg3rd9 months ago
A variable resistor would also work. I just wanted a setup where you could quickly dial in the output for various battery combinations.
rhooie9 months ago
Firstly I would like to admit I am an electronics illiterate. I have 2 solar panels at 12v and would like to set them up to be able to charge a cell phone or AA and AAA batteries. Could I use this to do so?
sschoemann rhooie8 months ago
You Betcha! set your regulator to 5 volts (if this is what your cell phone calls for) and use the correct connector... also, add a blocking diode at the solar panels output, and feed it to either a rechargeable 12 volt battery or the largest capacitance that you can supply via ELECTROLYTIC capacitor(s) at the input of the regulator. You want about 1000uF minimum, wired like batteries on the input of the regulator (parallel or positive to positive negative to negative) so that you have what is called a constant current source.
gmh5760 rhooie9 months ago
Try using the search function at or google. :) Seriously, quite a few how-to pages out there for solar chargers.
Omar Amir9 months ago
I prototyped in on a breadboard and it didn't work. Should I put the components on the bread board in the same way you put them on the perf board? If yes, I did it on my bread board, but I don't get why it doesn't work. I used a 9V battery and I tested the alligator clips' ends, but it doesn't show any voltage!
breadboards have nasty habits of losing contact after a certain amount of use. inside the plastic are copper fingered strips. these lose tension after a number of lead insertions and eventually don't make contact anymore. You can take the board apart and close the fingers and put it back together. If you do this however I recommend picking up some tarnex and cleaning and rinsing the contacts, drying them with a hairdryer and handling them with cotton or surgical gloves when reassembling to prevent your fingers from contaminating the copper after you clean them. There IS a liquid treatment that will tin copper, but I have never used it and am not aware if it will add to the live or shorten the contacts life. It would seem to me that it would lengthen the life but if it has a caustic carrier it would risk damage to the copper, thereby shortening the life of the thin copper.
DIY Hacks and How Tos (author)  Omar Amir9 months ago
The location doesn't really matter. All that matters is which components are connected to which as indicated on the circuit schematic.
mazzabazza9 months ago
I'm not really chip savvy. Could someone please direct me to the most appropriate lm317 to use in this situation. So far, I have seen K, M and T variants. Please and thank you. Mat
DIY Hacks and How Tos (author)  mazzabazza9 months ago
The one that I used was purchased from Radio Shack. It was the "T" version. Other styles could work too. But they would need to be mounted differently.

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