Instructables

Is it possible to power a device that takes two AA batteries using the power from a USB port?

I have a small fountain that takes two AA batteries.  It looks great sitting on my desk but I'd love to have it powered off of my USB rather than go through AA's all the time.  Is it possible to use the power from a USB port to power the device, and if so, what would need to be built (as far as circuitry to protect the usb ports and get the correct current, etc) to do this? 

cid23234 years ago
if you don't want to go through the trouble of adding resistors or diodes etc....just get recharable batteries and add a connetor so you can just plug in the charger directly to the fountain.....
Re-design4 years ago
There is probably not enough voltage difference to use a regulator and get good regulation.  Running the pump on 5 volts probably would eventually burn it up.  You could use a resistor to drop the voltage.

It would be better to just get a 3 v. wall wart and use that instead of putting the extra drain on the comp. power supply.
I agree, I always save power transformers from old junked devices, but if you don't, a lot of thrift stores have boxes of 'em you can dig through to find the right voltage and amperage. I think this has already been mentioned but depending on whether the batteries are wired in series or in sequence, two batteries can mean the motor is run off the combined power of the batteries or it can simply be run off the power of one battery with the second one there just so they last longer. easiest would be to take your handy dandy multimeter to the leads that come off the battery pack and go to the motor.
knektek4 years ago
use a voltage divider, get 2 resistors in series (500 ohms - 1000 ohms) and apply power and ground to both sides of the module. In the middle get a diode or something to drop the voltage and on the other end of the component is power. The ground of the fountain is the ground of the usb. I would replace the resistors with diodes if you happen to have no resistors. I hope this helps.
thepaul934 years ago
I think you can get diodes that drop the voltage. Each diode drops the voltage about 1 volt.
Taost4 years ago
If your device is simply a motor run by 2 AA batteries that moves the water, you'll be fine with directly plugging it into your usb, if there's circuitry inside, you'd be better off using resistors or a zener diode (google it) to limit the voltage and current. Another issue is that you may want to protect your computer from any noise from the motor inside, I'm sure that computer of yours is worth more than the fountain. Try installing some small disc capacitors on the motor leads to reduce noise from the it spinning.
Yes!! simpy add a voltage regulator weather the device uses: 1.5 v for 1 AA 3.0 v for 2 AA or 4.5 v for 3 AA because USB have 5 volts.
chilll20094 years ago
i did it with no resistors, on an electronic mad libs. it only raises the clock speed very slightly because i put in no resistors. try @ your own risk!
techturtle24 years ago
You will need a HUGE riststor to cut back the power. A divice that runs on 2 AAs uses 3 volts. (one AA is 1.5) and a USB puts out 5 volts. You would need a resister to cut back the power by 2 volts. To find out what resister to use, use Ohm's Law

http://en.wikipedia.org/wiki/Ohm%27s_law
harry884 years ago
if it is just a direct hook up to a motor aplicane like a fan it might go fast but it would work if not break out a resitor
sticking a resistor in to get it to an acceptable speed should be fine if its just a simple motor inside
^^ This. Trial and error will probably suffice, as long as you use some basic precautions (i.e. common sense).
Yeah it should work I Powered my psp when my yellow bit broke and it went perfectly.
KaydenST4 years ago
 Well, USB gives off 5v, and 2 AA batteries give off a total of 3v, so you will be going over the voltage. Then you need to check the amps. The amperage can't vary a lot. You would need a resistor to lower the voltage, and if applicable, the amperage. If you just need to lower the voltage, the resistor would be THIS
techman94 years ago
 You can but 2 AA's give 3V power and a usb plug gives 5V power. if you want me to show you how to do it reply to this message
khaos944 years ago
 you can do it but why bother? why wire it up when you can have it run off batteries? also, I doubt usb ports are designed to give constant significant power. I just wouldn't bother.
Find out what current it takes first. A USB port will not supply more than 100mA.
Regulation  ? Yes, if you get a Low Drop Out (LDO)regulator - they will work on less than 2 volts of difference.

Steve
Some LDO voltage regulators will work on less than half a volt difference. 

I wonder if the motor in the fountain can handle 5V.  It'll certainly run faster.  It might shoot water out like a volcano though!

Oh, and a USB port can supply up to 500mA if it's powered (ie. a powered USB hub)
pissinguoff247 (author)  jeff-o4 years ago
I tried running it off of a 3v power supply, and it made the motor sound like a small starving jet engine.  5v might turn it into a warp drive.
Did it not sound the same as it did on its batteries ? If it didn't it can only be running on 1.5V not 3V, so a conventional regulator would work.

Steve
 Well, it would be amusing while it lasted.

Definitely go for the regulator then.  It doesn't have to be 3V either, you could get a smaller value (2.5V, for example) if you wanted.
Yes, compared to straight reg, where I wouldn't want to put less than 2V across them, the LDOs are a lot better.

We had a discussion about USB last week. The USB spec says 100mA, more if its ASKED for, but limited to 100mA if it isn't.

Steve
exactly. up to 500mA if a request is made by the device to the USB controller.
seandogue4 years ago
I defer to the sound answers Steve and Re-design have already given.

Now, I'll suggest an alternative. If your computer is a desktop (not a laptop), you can use the inboard power taps to provide 5 or 12V at much higher currents.

If you've ever opened the case, you'll notice that there are a multitude of power connefctors. At least some of them are a 4-wire tap that has a red, yellow and two black wires.  The red is 5V, the yellow is 12V, the blacks are common.

If I were to use these for a low power fountain, I'd be strongly inclined to use a fuse so I don't accidentally damage the power supply if something goes wrong (water, crossed wires, stalled motor, etc.)
pissinguoff247 (author)  seandogue4 years ago
It's a desktop, but unfortunately it's my work machine.  My IT guys might take me out back and shoot me if they catch me messing with the guts.  All bets are off at home, but at work, the machine is a shrine.
oh...that's too bad. Never stopped me, but I'm fortunate in that engineering gives you a little more leeway with your office gadgets.

wallwart time.