Add an AC Adapter to a Battery-powered Device

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Introduction: Add an AC Adapter to a Battery-powered Device

About: I've been an experimental high-energy physicist for 20 years (since I started graduate school in 1988). I got my BS in physics from UCLA, my Ph.D. at Caltech, and did a post-doc at UBC before moving to SLAC...

With a new baby, we are acquiring an astonishing number of battery-powered devices -- bouncy seats, swings, activity jumpers, mobiles, ... -- and burning through an even more astonishing number of batteries. Now I know why Costco sells those immense packages of AAs.

My wife asked me whether I could attach an AC adapter to our baby's mobile. I'd seen an article in MAKE about modifying noisy toys, so I knew it was possible. It turns out to be surprisingly easy, provided you (okay, I) don't make some dumb choices along the way.

NOTE The project I describe will void the warranty for whatever toy you modify. The manufacturer (rightly) will not support you or provide you with assistance in doing this. If it works, be happy: If You Can't Open It, You Don't Own It. If it doesn't work, take it as a lesson not to meddle in the affairs of Corporations, for they are Subtle and Quick to Anger.

Step 1: This Instructable Is Incomplete

You'll notice that two of the Steps don't have useful images. I started writing this up after completing the project on one of our daughter's mobiles, and before starting the second. I definitely don't want to take the first one apart again just for some photos, and we've since decided not to do the AC adapter on the second mobile (the cord wouldn't be adequately secured).

Nevertheless, the descriptions in the Steps are accurate and complete, based on lessons learned the first time. I think this can still be useful to folks.

Step 2: What to Modify? a Mobile

First, choose the toy you want to power from the wall. I'm going to show you a very nice infant's mobile from Tiny Love. The steps will be similar for whatever device you choose, although the internal details (and maybe the screwdrivers) will be different.

Step 3: What Do You Need? Parts and Tools

The parts you need to add an AC adapter are fairly simple, but harder to find than I expected. I ended up going to Radio Shack -- they carry specific-voltage adapters, individual tips, and panel-mount sockets to match the tips. At most places you have to by a "universal" adapter, and can't get the sockets at all.

For the TinyLove mobile, which uses 3 AA batteries, I bought

4.5V adapter (700 mA) 273-1765
Size "M" (2.1mm ID, 5.5mm OD) plug 273-1716
Size "M" panel-mount socket with switch 274-1582

For your application, choose an adapter with an output voltage that matches the type and number of battery your device uses.

You will also need lengths of thin (24 gauge or smaller) wire, one with black insulation and one with red. I cannibalized mine from some left-over four-conductor signal cable. Start with 6" lengths and trim them back when doing the soldering (Step 5). A wire stripper will be quite handy, but someone with some skill (not me) could use some small wire-cutters or scissors.

You'll need some tools: The TinyLove mobile is held together with weird triangular-bit screws. I found the bits at McMaster-Carr Industrial Supply (items 5941A11 through 5941A14).

A 21/64" bit will make the exact hole needed for the panel-mount socket. You could also use a 1/8" Dremel bit, followed by the Dremel ball grinder to enlarge the hole.

A soldering iron, solder and flux are needed to connect the wires. A solder-sucker might be helpful, too, when disconnecting the leads from the battery terminals.

Step 4: Open the Box

Remove the batteries first; soldering the terminals with the batteries in place isn't the best idea. Open the mobile using the triangular bit on the four back-panel screws. Be careful when opening up the mobile, as the wires inside do not have a lot of slack.

Identify the red (+) and black (-) wires at the terminals of the battery compartment. Choose a spot on the side of either the back or front halves to mount the AC adapter socket, close to the battery terminals, so the red and black wires will reach it. You'll need enough space inside to fit the whole socket so it can be pushed through its mounting hole. Mark the outside of the panel with a Sharpie where you're going to put the socket.

Step 5: Prepare and Mount the Socket

Drill out the 21/64" (5/16" plus a little) hole at your mark for the AC adapter socket. The threaded end of the socket needs to stick out far enough to tighten the nut. You may need to clear some additional plastic from the inside in order for the socket to fit snugly up against the curved surface.

Mount the socket from the inside, with the washer and nut on the outside. Make sure that all three contacts are accessible (the non-contact second ought to be downward). Tighten the nut as much as you can with pliers, so it won't twist around or come loose (you may want to use thread-lock).

Step 6: Move the Existing Wires

Disconnect the red and black leads from the battery terminals using the soldering iron, and the solder-sucker if necessary. Make sure you keep track of which terminal is which!

Solder the red lead onto the center pin connector (at the 9 o'clock position) of the AC socket. This allows the AC adapter to feed current into the circuit.

Solder the black lead onto the shell connector (at the 12 o'clock position) of the AC socket. This is the return path for current from the AC adapter back to ground.

At this point, the project is "complete," in that the AC adapter will provide power. However, you've disconnected the battery system entirely, so the mobile is not portable (nor will it work if the power goes out).

WARNING Don't do what I did the first time. If you cut the existing leads and try to attach the cut ends to the AC socket connectors, the halves will probably be too short to reach sensibly when the device is opened, and you'll have to splice in lengths of wire anyway to make up the difference. Just move the whole leads as above, and use a couple of whole new wires for the new connections (Step 6).

Step 7: Reconnect the Batteries

This AC socket includes a normally-closed switch on the shell (ground), so that you can power your device as built with the batteries, but when you plug in the adapter the batteries are disconnected (and not shorted to ground or reverse biased).

Strip the ends of the black wire (and cut it to a nicer length if desired) to expose about 1/8". Solder one end to the negative terminal of the battery compartment, and solder the other end to the NC switch connector (3 o'clock position) on the AC socket. When the AC adapter is not plugged in this provides the return path from the circuit board to the batteries' negative terminal.

Strip the ends of the red wire the same way. Solder one end to the positive terminal of the battery compartment, and solder the other end to the center pin connector (9 o'clock position) on the AC socket. You already have the built-in red lead on that center pin, so you may not need additional solder for this connection.

Now, the batteries are connected into the circuit almost as before. With the AC adapter unplugged, the batteries provide current through the two red leads, and the return path goes up the black lead to the AC socket, through the closed switch, and back to the batteries.

When you plug in the AC adapter, the batteries are disconnected from the circuit and current comes from and returns to the adapter through the original red and black leads.

Step 8: Validation: Does Everything Work?

You're now ready to verify that you haven't fried the device. First, set up the AC adapter by plugging the Size "M" Adaptaplug into the end of the adapter cable. Make sure that the "(+)" mark on the plug is aligned with the "TIP" mark on the cable. This specifies that the center pin is positive, and the outer shell is grounded.

Plug the AC adapter into your device and into the wall. Turn it on. For the mobile (since the armature is disconnected) you should hear the annoying digital music start to play. If not, unplug the adapter and check all of your solder connections.

Disconnect the adapter and put the batteries back into the compartment, without closing it up. Turn the mobile back on, and it should work as before. If not, see above.

Finally, with the batteries in place, reconnect the AC adapter and turn the device on again. It should work as before. You should also confirm (by touch) that the batteries are not getting warm, nor that there's any unexpected smells or smoke. The NC switch on the AC socket ensures that the batteries are cut out of the circuit entires when the adapter is in use.

Step 9: Close It Up

With everything working, you can close up the unit. Carefully fold all of the leads into the space between the case halves (use bits of painters tape to hold them down). Insert and tighten the case screws. Put the battery compartment case back on.

Now you can put the unit back into action, powered either from the wall or from batteries as you wish!

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    62 Discussions

    Hi! I came across this instructable and I have the same question as LearnEveryday71, with the difference of my tag does not state .25W, I see a 20 ohm resistor soldered to the switch for one set of string leds. I am thinking if I solder two sets together, I would have to add another 20 ohm resistor in series to it or replace it with a 40 ohm. Also, I would have to buy a 9V dc power supply. My question is what amperage power supply should I get? Thanks for any advice you can give.

    Here is the post from Learneveryday71:

    Kelseymh, please educate... I have 2 battery powered LED light strings. 20 lights each string. The tag on each string reads: 4.5v DC (3, 1.5 AA's) .25W. I want to wire both together into one string so I can solder a transformer plug. Then buy the proper AC/DC transformer at radio shack or online to plug it in to house current. Now the questions ... If each string is 4.5v, and I want to connect them together, do I need a 9v, 100ma transformer? Do I worry about Ohms or wattage when crunching the math for the proper transformer? Any advice, tips or education you can provide would be appreciated!

    Kelseymh, please educate... I have 2 battery powered LED light strings. 20 lights each string. The tag on each string reads: 4.5v DC (3, 1.5 AA's) .25W. I want to wire both together into one string so I can solder a transformer plug. Then buy the proper AC/DC transformer at radio shack or online to plug it in to house current. Now the questions ... If each string is 4.5v, and I want to connect them together, do I need a 9v, 100ma transformer? Do I worry about Ohms or wattage when crunching the math for the proper transformer? Any advice, tips or education you can provide would be appreciated!

    hey, ok I'm in a lil bit of a Doosey.. Soo I have a toy that uses a 9.6 volt battery at 1600mah and I want to know what kind of power supply I would need to operate it ? Can you help me?

    Hi there. I'm about to modifiy my baby's music mobile to use wall AC adapter to power it. It uses 4xAA alkaline batteries. The batteries are connected in series. My question, what is the best Voltage and mA/current adapter should i use? i believe the voltage should be 6V. But how am i going to calculate mA/current since there is no information on the housing? Any help much appreciated. Thank you.

    1 reply

    That sounds exactly like my own (no longer a) baby's mobile and Fisher-Price aquarium. Typically, battery-powered devices don't draw more than about 1A (above that, and the battery usually discharges very quickly, and does so at a lower than nominal voltage). I used an 800 mA adapter for my project.

    Hi, I need help with a Air Mattress powered with 4 D Batteries. I'm trying to use wall AC adapter to power it. I tried all kind of 6 volts with NO success. Which AC adapter do you recommend?

    Thank you

    1 reply

    You need to know how much current ("amps") the pump motor draws. If the motor is visible to you, there should be a data plate that shows the voltage and amperage required. Any adapter which is rated for _more_ than the motor amperage should work.

    So if I were to convert a Hot Wheels Criss Cross Crash set that uses 4 D batteries, what adapter and parts should I buy?

    2 replies

    Thanks for the quick reply!

    Actually I have 3 led strips I want to convert, all using 3 AA batteries each.

    The easy way would be to use three separate 4.5v adapters.
    The ideal would be to connect them all together somehow and use only 1 adapter, maybe a 13.5v?

    Still figuring out the latter. :)

    Hi, I'm trying to do something very similar with a 3 aa battery led strip.

    A question about the adapter you used. If your adapter, was rated 4.5v but 2A instead of 700mA. It would still work right?

    Not very efficiently but the device would still only draw as much it needs?

    1 reply

    Oh, yes. So long as you (a) have the right voltage, and (b) the current rating is higher than what the device needs, you'll be fine. If the current rating is too low, the wall wart will overheat. What you describe should be fine.

    Many thanks for the helpful reply! The batteries are in series as they're connected in a daisy-chain. Also, the housing has "12v" printed in large characters. I tried to find any info about calculating wattage from volts and Ah without success. I found an equation for watts=volts*amps, but that's not the same ampere-hours.

    1 reply

    Right. You can't calculate watts from volts and coulombs (== ampere*second). Watts are units of power, which is energy used per unit time (watt = joule/s). (And, since W = V*A, you have V*A = J/s, or J = V*A*s = volts * Ah/3600). To convert energy to power you need to know how fast you're discharging the batteries.

    When you used the 1A and 2A wall warts, did they get hot when you were trying to use the vacuum? If so, the vacuum was trying to draw more current than those adapters were rated, and you'll need a higher amperage adapter.

    I have a (no-longer-very) rechargeable vacuum I'd love to convert to a wall wart but have been failing with various adapters I've tried. The vacuum uses ten 1.2v (1300mAh) batteries. So that equals 12v total. I thought a 12v adapter at 1.5Ah would do it, but the unit ran very low and kind of pulsed off and on every second or so in a very about-to-die way. I tried a 12v, 2Ah adapter with similar results. The batteries run in series so I know I need 12v. Do I have to multiply the mAh rating by the number of batteries too? That would be 13Ah but that doesn't make sense since mAh represents capacity and not amount of power which is voltage. Any help is much appreciated!

    1 reply

    You need to know whether the vacuum was set up with the batteries wired in series (which would be 12V), all in parallel (which would be just 1.2V), or some combination of parallel and series. If you've already opened up the unit, then you "should" (depending on how it was built) be able to see the outside of the battery holder and trace how the terminals are wired together.

    Also, mAh represents the total charge stored (ampere = coulomb/second). If you multiply by voltage you get total energy (joule = ampere*volt). That doesn't provide any information on the way the batteries are wired, just their total output.

    Alright, so I'm converting my Dremel to use AC.  The Dremel currently runs on rechargeable batteries that connect in the configuration shown in the attached picture.

    The first set of 3 AA batteries connect to 1 and 2.  The second set of 3 AA batteries connect to 3 and 4.  I did a continuity test, and 1 and 4 are tied together and 2 and 3 are tied together.

    So my question is, how do I wire this guy?  I tried wiring it with positive from the adapter socket connected to 2 (current feed), then tying 1 and 2 together, tying 3 and 4 together, and then having negative from the adapter socket connected to 3 (current return).

    I then plugged in a 7.2V 1000 mA AC adapter, and the light on the goes off when I plug in the guy.


    Thanks,
    - Darin

    dremel.jpg
    2 replies

    There must be a switch or something in that circuit somewhere -- otherwise you're just running current around and around, draining the batteries. Because of the way the two sets of batteries are chained in series, you need to provide that same chaining by hand, but without leading to a short circuit.

    What other people have done in this case is to build "fake batteries" -- a wooden dowel with a metal contact at each end. Those contacts are wired to the AC adapter. For example, https://www.instructables.com/id/How-to-Make-AC-Powered-Batteries-for-your-DC-Pow/

    Ok... so I jumped back on this project, and I started-out by re-checking the continuity between connections. Come to find out I was initially wrong. The attached diagram is the correct layout (which makes more sense with the batteries in serial).

    So given the new diagram... anyone have any ideas?

    And as I initially stated, the positive terminal from the adapter socket is connected to 2 (current feed), 2 and 1 are tied together, 4 and 3 are tied together, and the negative terminal from the adapter socket is connected to 3 (current return).

    Should I try connecting the negative terminal of the socket adapter to 2 and positive terminal to 3? I just don't want to break anything.


    Thanks,
    - Darin

    dremel.jpg