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Thread: Removable link rule clarification

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  1. #1
    Ocracoke's Avatar
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    Hi everyone,

    With regards to the rules regarding removable links, does the link have to be on the main power circuitry, going from the batteries to the motors or can the link be on the RC equipment which, when removed, causes the robot to lose all power anyway?

  2. #2
    Coolspeedbot's Avatar
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    the link has to be fitted before any electronic hardware in the bot.

  3. #3
    Redirect Left's Avatar
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    Similar to this, excuse my mad paint skills.


  4. #4
    Ocracoke's Avatar
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    Thanks for the clarification - got a date with the soldering iron tonight. :P As The Honey Badger uses SLA batteries rather than LiPo batteries, I presume a inline fuse isn't required?

  5. #5
    Redirect Left's Avatar
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    You should always use a fuse. It'll protect the electronics if the motors start to surge uncontrollably, which can happen when they stall for long periods of time. Some batteries might also surge momentarily if they're suddenly damaged.

    Midi fuses are good for robotics, and very cheap off eBay, including a proper holder for them. Have a quick search on ebay for 'midi fuse' or 'strip fuse', for the price they are, its worth them to protect everything else.

    Also as a note, the 'should always use a fuse' may change to 'must use a fuse' in actual combat competition.
    Last edited by Redirect Left; 26th June 2017 at 22:42.

  6. #6
    RogueTwoRobots's Avatar
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    If you're not using a lipo battery, then a fuse isn't mandatory. However it is highly advised as a protective measure. I never used fuses before I switched to lipo batteries, but I probably would now if I was switching back to an older battery technology.
    Jamie McHarg
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  7. #7
    Sorry to dredge up an old thread, but if there any reason why the interlink can't be on the negative terminal? Cars and heavy machinery are isolated on the negative (which is the opposite to most mains power rules)...

  8. #8
    Redirect Left's Avatar
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    I am not sure if this is why specific to this hobby, however in my experience doing lots of electrics, odd things can happen if you sever a connection on the negative side. Sparking a little more, final pushes of power still powering things for a bit. It's also a bit more likely on the negative side that something is still powered elsewhere in the circuit, however if the link is immediately after the battery on positive side, nothing on the circuit is getting power. It's also probably easier for all involved if its standardized to a specific place for it to be.

    The excess electricity still in the circuit after negative switch pulling may be because from a physics standpoint, electric (or at least electrons) travel negative to positive, rather than what people imagine positive to negative.

  9. #9
    McMullet's Avatar
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    In a car, negative and positive are very different from a practical point of view. The battery negative is grounded to the chassis, so it's not insulated. If you jump-start a car, you just connect the negative side of the live battery to the chassis of the car with the dead battery - the whole car is earthed to the negative side. In theory you could make a car with the positive side earthed to the chassis instead, and everything would work the same as long as you reversed the polarity of the other components (otherwise the starter motor would run in reverse and so on), but components like that are all designed with a negative earth connection.

    The upshot of that is, if you want to make a car electrically safe you unplug the battery negative terminal. Now the battery is ungrounded, there's no return line for any of the positive connections and so no current can flow unless you short the actual battery terminals. If, instead, you unlpugged the positive connection, you could still short between the positive battery terminal and any metal part of the car.

    In a robot the wiring is all insulated, so in theory there should be no difference where you put the link. You have a self-contained circuit with no ground to short against, so no break point is safer than any other. I don't know why you'd see more sparking etc. with a negative break... maybe the way some electronic bits are wired up (electrolytic capacitors have polarity)? But nothing can "flow" to or from either terminal when the other terminal is isolated, the direction that electrons are moving is irrelevant.

    Still, I agree that it's better to just put it on the pos side for the sake of convention! If that's where the tech checker expected to find it, it makes everyone's life easier to put it there unless there's a good reason not to...

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