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Circuit Breaker for Model Railroad

Discussion in 'General Electronics Discussion' started by LKO Railroad, Aug 9, 2013.

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  1. LKO Railroad

    LKO Railroad

    43
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    Aug 9, 2013
    Hi,

    Can you guys please help a model railroader with limited electronics knowledge?

    My railroad has continuous filtered 14v DC on the rails. The locomotive is controlled with a radio decoder in the locomotive. The railroad is divided into multiple circuits much like a house is wired to a load center. I want to use this circuit as my circuit breakers.

    Are any modifications needed to change it from 12v to 14v?

    The diode across the motor is not possible inside the locomotive because of the decoder. The circuit will be located at some distance from the load. Is the diode absolutely needed?

    Are there other components needed to make a functional circuit that are not shown on the schematic?

    Are there other concerns that should be considered?

    Thanks in advance for your help.

    [​IMG]
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    11,513
    2,651
    Nov 17, 2011
    Could you please elaborate a bit more detailed? I don't think I get all of it.

    What do you intend to do with the circuit breaker? Is this kind of a resettable fuse?

    The circuit you show is not particularly well suited to placement far from the motor as it needs thermal coupling between the motor and the PTC. How do you think to do that if you don't even have access to place the diode across the motor?

    Have a lok at these resettable fuses. These will open if the current exceeds the threshold and will stay open if the fault (overload, short circuit) persists. After removing the fault, allow for some coling of the fuse and it will conduct again. Easy and inexpensive. You can put a fuse into every circuit, thus ensuring continued operation of other circuits even if one circuit has a fault.
     
  3. LKO Railroad

    LKO Railroad

    43
    6
    Aug 9, 2013
    The loco decoder takes power from the rails and then supplies it to the loco motor in PWM to control speed. Motor lead polarity is reversed by the decoder to operate the loco in opposite direction. That is why the diode is problematic. If the diode could be placed on the track side of the decoder then not an issue.

    Is it possible for the circuit to operate correctly without the thermal protection? Heat in the loco motor is no concern. Derailed cars and locos causing short across rails is the purpose.

    The railroad is powered by a single 14v dc switching power supply with 12 AWG bus wires running the length of the railroad (approx 120'). The tracks are electrically isolated into blocks (circuits). A circuit breaker protects each block. If a short occurs in a block the breaker for that block prevents the whole railroad from shutting down. Only the affected block shuts down. Each circuit breaker is connected to the 12 AWG bus. Radiating from each circuit breaker is a secondary 12 AWG bus running the length of the block. Many, very short 20 AWG "feeders" every 3 feet of track connect to the secondary bus.

    The power supply has a fast acting electronic auto-reset circuit breaker within it. I do not the exact trip time but it is super fast. Faster than a conventional fuse will blow. It goes without saying the circuit breaker has to be faster to trip than the power supply. Looking at the resettable fuses they don't appear to be fast enough.

    Thanks for your help. Please let me know what other information I can provide.
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    11,513
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    Nov 17, 2011
    Why do you need an additional circuit breaker, then? And what is fast ? Microseconds, Milliseconds?
    I agree that the resettable fuses are most probably too slow. Do you need the autoreset feature or could you live if you have to press a button to reset the breaker?
    I still do not think the circuit you show is suitable. At the least it is overkill. You could use a current sense resistor plus a comparator to achieve the same effect.
     
  5. duke37

    duke37

    5,364
    771
    Jan 9, 2011
    If the motor runs in both directions, then you cannot use a diode across it. You could however use a pair of zener diodes connected back to back to limit voltage spikes.
     
  6. LKO Railroad

    LKO Railroad

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    6
    Aug 9, 2013
    The tracks are electrically isolated into blocks (circuits). A circuit breaker protects each block. If a short occurs in a block the breaker for that block prevents the whole railroad from shutting down. Only the affected block shuts down.

    The power supply manufacturer does not supply this information however it is believed to be a few milliseconds.

    Push to reset ONLY if no auto reset function can be found. It certainly isn't desirable since it would force location of the breaker in an accessible place, namely on the fascia control panel which is already quite dense with controls.

    If it performs the function as desired then overkill is OK. I need only build 12 of them - 1 for each district.

    Can you elaborate please?
     
  7. LKO Railroad

    LKO Railroad

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    6
    Aug 9, 2013
    Would back-to-back zeners at the circuit breaker board work well enough considering the board may be 15-20' away from the traveling locomotive where the DC motor is located?

    Also, the motor leads are connected to the decoder which is then connected to the rails via the wheels. The circuit breaker won't actually see the motor. It will see the decoder.
     
  8. Harald Kapp

    Harald Kapp Moderator Moderator

    11,513
    2,651
    Nov 17, 2011
    Look at this circuit.
    It is not a breaker, but a current limiter. I think in your application that would be fine.
     
  9. LKO Railroad

    LKO Railroad

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    6
    Aug 9, 2013
    Do I understand correctly this circuit limits the current to a certain level but does not completely shut off is an over-current situation is detected. If so, then it will not work for the application.

    When a train derails (usually at a turnout) the derailed wheelset often bridges two opposite polarity points on the rails. 4A of current flowing through the wheelset is enough to melt the small plastic parts of the car's wheel truck.
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,496
    2,837
    Jan 21, 2010
    A circuit I used in the long distant past worked like this...

    The power to the regulator passes through the NC contacts of a relay.

    When the voltage across a sense resistor rose to above 0.7V it turned on a transistor which triggered an SCR to pull in the relay.

    The reset switch was a momentary contact switch across the SCR.

    The advantage of this was that pressing the reset switch actually cuts power to the circuit, so holding it down keeps the circuit engaged rather than reset.

    Note that this circuit does nothing to limit current during the period of the short circuit, however that is possibly already handled by the circuit. If it is, the "fuse" current needs to be lower. This may be achieved by using the existing current sense resistor and another small resistor (perhaps 10 to 20% of it's value) in series with it. The sense for the fuse is both resistors, and the sense for the current limit is just one of them. Note that not all current sense resistors need to drop 0.7V at the current limit...
     
  11. LKO Railroad

    LKO Railroad

    43
    6
    Aug 9, 2013
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