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Circuit breaker with parallel resistor

  1. Jul 22, 2011 #1

    Redbelly98

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    In my battery-powered lawnmower there is a circuit breaker. I have replaced the breaker, and when I cut apart the old one I found a 68 Ω resistor in parallel with the breaker switch. My question is, what is the purpose of this resistor?

    Here is the basic mower circuit:

    MowerCMM1000_circuit_2009-08-15.gif
    (Imagine that the Main Switch is in the "On" position when the circuit breaker opens.

    I'm thinking the Charger Interface does not really interact with the circuit during normal operation, only when I'm charging the batteries.

    The Battery Gauge is reading the motor voltage, and is a series combination of 680Ω, 16V Zener, and solenoid (to deflect a magnetic indicator needle):

    Code (Text):

           \
    (+) ---|<|-------(coil)-----/\/\/\/----- (-)
           \ 16V                 680Ω
     
    The motor runs on 24 V DC, is the permanent magnet type, runs at 12A during normal operation or as high as 40 A to trip the breaker.

    Mainly I want to know about the 68Ω because I just purchased some cheap breakers from an alternate supplier, to be ready for the next time this happens. I opened one up and these do not have the resistor, so I'm wondering if I should add a resistor in parallel if I use one of these in the future.

    Oh, just taking an educated guess here: without the 68Ω, if the breaker opens then the motor, being an inductive load, would try to drive it's current through the Battery Gauge and fry one or more of its components. That's my guess but I'm interested in other opinions, especially if you have come across this yourself.

    Thanks in advance!

    p.s. photo of the breaker:

    MowerBreaker40A-1.jpg
     
  2. jcsd
  3. Jul 22, 2011 #2

    dlgoff

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    I did a little googling and found that the original circuit breaker is no longer available from the manufacturer. According to this site anyway: http://www.ereplacementparts.com/circuit-breaker-p-100616.html?osCsid=aksc0nc3sk91tlnkp2bl99su55"

    [STRIKE]I'm guessing here, but I'm thinking the old breaker used the 68 Ω resistor as a heat source for breaking the breaker on overload. I'm also guessing the new breaker uses a bi-metal type switch and doesn't require the resistor.[/STRIKE]

    WRONG
     
    Last edited by a moderator: Apr 26, 2017
  4. Jul 22, 2011 #3
    Probably the breaker was a bi-metal type. The 68 ohm resistor kept the breaker open as long as the main switch was on. This was to keep the mower motor from starting when the breaker cooled and reset.
    If you use a breaker without the resistor or without a mechanical reset YOU ARE DISABLING AN IMPORTANT SAFETY FEATURE.
     
  5. Jul 22, 2011 #4

    dlgoff

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    Good point. That makes perfect sense.
     
  6. Jul 22, 2011 #5

    Redbelly98

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    Yes, exactly. I bought a new one of those 2 years ago, and that's what's in the mower now. I recently realized they were discontinued, that's why I bought the ones from an alternative manufacturer.

    Makes sense, thanks. That would also mean just adding a 68 ohm somewhere physically outside the breaker would not work.
     
    Last edited by a moderator: Apr 26, 2017
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