Non-invasive AC power adapter for exercise equipment

We often look at battery-operated hardware and shake our heads at the wastefulness of throwing away disposable batteries. There are some devices that minimize the waste, like those TV remotes that seem to never need new cells. But the C cells that [Quinn Dunki] kept replacing in her elliptical trainer were only lasting about three months at a time. The manufacturer hadn’t cared enough to build a power jack into the machine, so she built her own AC adapter without modifying the stock hardware.

The first thing she did was to patch in a couple of wires between two of the batteries. This let her measure the current consumption, which topped out at around 200mA. This is good news because that’s easily sourced with a cheap linear regulator. Out of the junk box came a 12V/1A wall wart transformer, which just leaves the need for a fuse and some capacitors to finish out a voltage regulator circuit.

Since [Quinn] didn’t want to permanently alter the exerciser, she came up with a way that it could take the same physical space as the batteries. Two long stand-offs are used as prongs to interface the spring terminals in the battery compartment. They attach to a piece of protoboard which hosts the rest of the circuitry. Now she just needs to remember to unplug this from the wall after each session and she’ll be in business.

27 thoughts on “Non-invasive AC power adapter for exercise equipment

    1. Hi B- I actually used rechargeable batteries for a while, but the machine didn’t run as well on them, and they went dead so often (seemingly just from sitting) that they needed constant charging and weren’t ready when I wanted them. This has been my experience with rechargeables in most of my devices, unforuntately. Of course, YMMV.

    2. @B – @Quinn already responded, but I could chime in also: some electronics do not run well (or at all) with the lower voltage supplied by rechargables.

      The lack of a boost circuit also means @Quinn was most likely throwing away GOOD disposable batteries. What I mean is, the trainer probably acted like the disposable battery was “dead” as soon as it approached 1.2v… yet there’s still a lot of life left in those weak batteries, for devices that aren’t quite as sensitive for voltage drops.

      It’s already been commented that transformers make lousy power and need regulation. I would add – avoid transformers altogether, and just use a switching power supply. A universal PS should have a setting for 6V, and it would not leech nearly as much power when not in use. (although you can always unplug the transformer to combat power vampirism).

      1. Also, batteries for stationary equipment like this is just ridiculous. Even rechargeables will wear out and have to be thrown away, which is still wasteful. The adapter will likely not wear out for decades unless it is overloaded. Even if she gets rid of the machine, the adapter can be reused for something else.

    1. The higher quality stuff usually does (such as you’ll find in a gym). This is a fairly low-end machine, and the computer is an optional attachment (and clearly an engineering afterthought), so it has its own power.

      1. Next step would be to tap the power you generated like the above stated. A couple magnets, some magnet wire, a capacitor, and a diode or four should do the trick.

      2. I like this idea- to do it cleanly would require tearing into the machine quite a bit, but that’s never stopped me before. The existing regulator circuit would still be useful…

      3. Actually you could adapt your existing board to use machine-generated power. You could use a bike-light charging kit as the basis, and run it in to your board.

      4. @Jaakko – No regulator means that 12V transformer she had, would quite possibly put out 15V or even 18V. With transformers, voltage means “at LEAST this much”. That’s why switching power supplies are so much better..

  1. 5V wall wart + 3V zenier would have made it even easier. to build. Although I do like the idea of the big cap for filtering just in case.

  2. I like this nondestructive build. Wonder if one could have gotten away without regulating the supply since the battery pack is going to fluctuate maybe 1.2 volts over its lifetime so most likely there is some kind of regulation done already internally or the device can take it.

    Of course if suitable transformer is not available then I see this more than justified.

  3. Personally, I would have hunted down a 6V wall wart, and installed a power connecter on the computer housing. Could look stock enough, as not to hurt any resale value. Interesting how minds work when it come to household current. While it’s true adding a fuse is never a bad idea, but the fuse here is fusing the “mains”. Not Quinn’s solution isn’t great as is.

  4. She’s spent more time hacking this than most owners of these machines use them.

    Still, three months between battery changes and this is a problem? I wouldnt even bother.

    Wait, yes I would. Its in our hacker blood.

    Good job!

    1. Well, the switch would have to be on the 120V side of the transformer to prevent the vampire effect. On the low-voltage side, it doesn’t need a switch because the computer has an auto-off. I haven’t been actually unplugging it, though I probably should.

      I’d be interested to see a breakdown of the resource costs of making, shipping, using, and disposing of batteries compared to the power I’m wasting leaving the wart plugged in. Am I still coming out ahead, or do I need to be unplugging it for this hack to be an improvement?

  5. @ScottinNH yes, as i was saying “Of course if suitable transformer is not available then I see this more than justified.” and by that I meant if there had been a transformer that had a output near that so you wouldn’t need to regulate it.

    Assume the transformer was rated for 4,5V so if the transformer was full-wave rectified so Vrms=Vpeak/sqrt(2)
    Vpeak=Vrms*sqrt(2)
    =6.364V
    and if you take 4*1.5Volt batteries with full charge I’d quess it would be somewhere around 6.4V. So I’m still saying it might be possible without discrete regulation _IF_ you have proper transformer for it.

  6. It seems overengineered in my opinion. Why not just use an old 5V cell phone charger? Everyone seems to have at least three laying around…

      1. lol well of course i could! in fact, i can do many things!

        sorry but i guess i’m still chapped about that one post where someone used his “totally ruined heheloljokes” smart phone to open a garage door. fking terrible!

        but this simple and clean project gives me hope!

  7. yes… why do they need batteries or a power cord at all? in those things you have a big resistor or a belt to “destroy” the energy to heat which you produce in training… why not harvest a fraction of it to power the machine itself?

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