A Reddit Refugee. Zero ragrets.

Engineer, permanent pirate, lover of all things mechanical and on wheels

moved here from lemmy.one because there are no active admins on that instance.

  • 9 Posts
  • 256 Comments
Joined 9 months ago
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Cake day: December 22nd, 2023

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  • Relays operate by flowing current through a coil. The coil generates a magnetic field, which attracts a ferromagnetic plate that then closes contacts. Without current flow, no relay can operate.
    This coil has a standard resistance/impedance based off the length of wire coiled, ans is engineered depending on the size of relay, designed operational voltage, etc. This cannot be modified.

    With a series of solid state op amps, one could turn a “no current/infinite resistance” signal into a relay switching effect, but it would require some other outside power source to run the relays or mosfet switching. Seems complicated. Maybe look for a “solid state relay” or “digital relay”, I’ve not ever used one though.
    I know you weren’t operating a car horn directly through a relay coil either, so there must definitely be some other relay or more likely a BCM computer with a MOSFET on board.

    What year/model of car? Is there any chance you can locate a oem wiring diagram to understand better how the horn system works? In older cars, the horn is a ground-switched circuit, where the horn unit is always hot +12v and then it is directly connected to ground through the steering column when the contacts on the horn pad are pressed together. No intermediate relay. On a system like this I would connect the relay effectively “in parallel” with the horn, with one side of the coil on +12v hot, and the other on the non-grounded, switched side of the horn pad. This way the horn and coil sit at the same potential and no current will flow anywhere except the horn pad ground circuit when pressed.

    I cannot guarantee any newer car uses a system remotely like this though, especially through a computer so YMMV.

    And realistically you could in a pinch pull coil power from the horn itself. Just connect a wire to either side of the horn so your relay coil is in parallel, one side switched and the other side always hot/ground. This may/may not be safe though as the horn circuit will likely be in a relatively large fuse and it’ll be impractical to relocate the relay or run an additional 14 feet of small gauge sense wires from the cabin to the front of the car.


  • Hahaha, now I’m picturing an IC with an angry face just before it farts out Magic Smoke.

    Well, that’s basically how they behave too lol. Solid state power components are generally not very tolerant and require careful surge suppression and filtering to not have them blow up frequently.

    I bet if you took that 25 year old driver apart, sanded off the commutator rotor, and put new brushes in it you’d suddenly find it’d have more power and use less battery. (And thats something you can do with older tools!)
    When brushed motors get old and oxidation/dirt builds up the resistance across the brushes to the rotor coils grows and you’ll lose motor efficiency.


  • I mean, yes and no. A lot of that is modern tools are going to be more carefully engineered to operate as close to failure as possible, as to advertise more power with a cheaper device. Thry have small wires and encoder sensors that can be prone to failure.
    Yes, the driving electronics are also sensitive, however magnetic transients are less of a deal on the scale of a cordless drill. When dealing with huge motors, those can be significant multi-kilovolt spikes that make solid state components Very Very Mad.
    But the brushless motors in a drill also do not have brushes that wear down rapidly in a very dirty/dusty contaminated environment like older power tools would. So it’s a bit of a 50/50.


  • Well, most all DC generators these days are actually AC alternators with the output rectified, because alternators can be run a lot more efficiently. So you’re already losing on efficiency there.

    You need to consider the consumer side as well. Dinky residential loads like your computer would be fine on DC. But most of the world, especially heavy industry, runs on synchronous or induction AC motors, big ones. Big huge tens-of-megawatts motors that often run upwards of 97% line efficiency, which is insane for any industrial process.
    The best you could replace those with would be modern brushless DC motors, which require really expensive inverter controls that die frequently due to the magnetic transients and still top out at an efficiency of only 90% if you’re lucky. And that would incur huge costs that just aren’t worth it.