Frankenstein’s Lawnmower

[phantompinecone] has an electric mower that worked great for about 4 seasons, and then the battery started to die. A replacement was installed but it started being a pain after the first season. Since the battery was brand new (and probably costly too) there must be something else.

Checking the brushes, which were fine, the next logical place was the switch.These mowers are just a battery, motor, and switch. Yanking it apart there was indeed a problem, they were chewed up and corroded, not allowing full electrical contact. So [phantompinecone] replaced the simple mechanical switch with a MOSFET.

Electrically there is an IRF1405 MOSFET, some resistors to pull signals around and a couple diodes to A) keep the back emf from the motor in check, and B) drop the voltage going into the fet from 24volts to 12. Problem solved, and the motor should not have anymore trouble caused by a junked up switch.

22 thoughts on “Frankenstein’s Lawnmower

  1. Welcome to “Hack A Day” dude. LOL

    I was gonna say that block of styrofoam may be an ESD hazard for that PCB.

    Aside from that it’s awesome and looks fun.

    “Buy a switch”, hehehe these kids today…

  2. @ xeracy, Because taking that course of action is very risky. We are not sure if a new switch would work, so instead backwards engineer the hardware and build something that will!

    Although I do find the lack of 555 and Arduino’s to be disturbing, how can this lawn mower function as a coffee bean grinder without a brain?

  3. @Mr_Bishop: LOL

    But you have to pay compliment to the guy who spent the time on this. Seriously, HP loves over-engineering (look at the silly dual-sided neodymium+and+spring+loaded magnet latch thing for the convertible tablets.. I mean, really; and you can bump the tablet open with almost no force so this expensive over-engineered latch doesn’t even work). Plenty of cause to hack this thing out, plus the looks from passers-by while mowing the front lawn is priceless.

    “OMG that guy supercharged his lawnmower!”

  4. @dan fruzzetti

    Buying a corded version is irrelevant… both models use the same kind of cheap switch. I’ve had two of these mowers now which were corded and needed switch replacements.

    The switches don’t last long in these mowers because the inrush current of these motors at stall is very high and also there is no protection on the switches to reduce inductive kick-back.

    All in all, it’s just a poor design…

  5. I had a mower where the switch shorted the motor in the off position, so the motor acts as a blade brake. I didn’t keep it long, but I assume the switch wouldn’t last very long as there was a brilliant glow (visible through the housing, in daylight) every time I would turn it off!

  6. Looks like it should go on the “I fixed it” website, right along side the bottle opener car door handle and duct tape patched leaky roof.

    He should simply have spec’d out the motor’s amperage at startup and bought a proper switch. It wouldn’t look as stupid, be as likely to turn on unexpectedly to take somebody’s hand off or start a fire, and would have a chance of lasting through the lifespan of the motor.

  7. this also opens the option of adding all kinds of new features — blade braking (maybe with energy recovery?) reverse, a huge capacitor or second battery for a temporary “overboost”, playing music by altering the motor speed….

  8. Hmm not sure what all the hate is about. He more or less did replace the switch, with a solid state one.
    And unlike just replacing the switch, he actually took the back EMF problem into account.

    And hell now if he wanted he could add another FET to short the motor to provide dynamic braking if he wanted.

  9. ive always wanted to see someone hack one of these mowers – albeit to something cooler with them. i had one and after the first year it ran for 15 minutes. completely outlasted my gas mower 2 fold the year before. it was a 6 month ordeal to get black and decker to fix it. they ended up shipping it back to the wrong address without securing the battery which flopper around smashing the mower to bits. after months of no response and then threating a small claims suit they shipped me a new mower as replacement. i used it twice and bought a gas mower again. now i just have parts laying around waiting for an idea.

  10. Nice hackin job there, Lou. You’ll make sergeant for that :)
    It would be a nice addition to have a “power boost” button (alternate switch with less resistance/ more juice-regulated of course) to give a little more power for tree stumps and kids toys and gophers in the yard. He he.
    Sorry about that switch man.

  11. Buy a switch? What?, and have to replace it when it fails?
    Remaking something differently and usually better than it was sold. Not necessarily the easiest wya just a different one.

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