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Dual Coil Latching Relays

Discussion in 'Electronic Components' started by Joseph Sroka-10.2.8, Apr 16, 2005.

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  1. In another posting I asked for ideas on how to add red and green LEDs to
    the switch tracks of an N Gauge model electric train. See below for my
    earlier post.

    It seems like the way to go would be with DPDT Dual Coil Latching Relays.

    I would like to obtain a few (< 10) DPDT Dual Coil Latching Relays that will
    work with both AC and DC impulses to the coils.

    Digikey, current catalog page 1229, has

    24VDC 3,200 7.5 2 2 16.8/16.8/48 255-1068-ND

    a DPDT dual coil latching relay.

    A couple of questions:

    1. The 16.8/16.8/48 is in the Pickup/Dropout/Max. Voltage column of the
    catalog page. Why isn't the middle entry a "-" as it is for some of the
    other dual coil latching relays? Does this mean that the coil must have
    a continuously applied voltage of at least 16.8 volts? This doesn't seem
    to make it a *latching* relay.

    2. All of the relays on page 1229 state that the coil voltage is DC.
    Could some of these also be used with AC, if the AC is just a momentary
    pulse, say for a relay with a 16.8/‹/48 entry in the Pickup/Dropout/Max.
    Voltage column?

    3. Does anyone know where I might obtain an AC/DC DPDT relay that holds
    the setting with no voltage other than a pulse to the coil?

    A self-latching relay seems like the simplest solution, except for the
    questions that I have (above).

    My previous post:

    I have an N-gauge train with some remote-controlled switch tracks,
    "snap-switch" type.

    This is an older style train, not the DCC type.

    I would like to add red and green LEDs to the switch tracks. The switch
    tracks are operated from the "accessories" terminals of a small power
    supply which are nominally 17 V AC, (I measured it open-circuit at 17.7 V

    The switch tracks are operated by pressing momentarily on a momentary SPDT

    I'd like a simple way to hook up the LEDs either at the remote location or
    at the SPDT switch. Since power to the switch track is applied only
    momentarily, I suppose that some sort of latching mechanism is needed.

    The 17 V AC is part of the complication, it seems.

    For reference, the switch tracks are Atlas #'s 2580 and 2581; the power
    supply is Model Rectifier Corp. Railine 370N.

    The switch tracks are apparently solenoid powered.

    The power for the tracks only is 15 V DC. I wouldn't want to power the
    LEDs from that terminal pair because it turns off when the train is
    stopped, and includes the speed control.

    Anybody here ever do this stuff with this type of remote turnout?

    My guesses are some sort of relay or solid state relay, but I have no idea
    what the specific part number or circuit would be.



    --- Joe
  2. Ken Taylor

    Ken Taylor Guest

    I seriously doubt you'll find an AC/DC relay. Use a DC one and feed it from
    the AC supply via a bridge rectifier and a capacitor.


  3. John Fields

    John Fields Guest

    In a dual-coil latching relay, 'pickup' and 'dropout' translate to
    'set' and 'reset', and since the set and resert voltages are the
    same, I think the dashes imply that. In any case, a pulse will be
    sufficient to set anmd reset them.
    I wouldn't think so. The relays show polarized coils, the implication
    being that there are bias magnets in the relay to assist in switching.
    If that's true, then placing AC on the coil would have the effect of
    making and breaking the contacts at the line frequency, so where the
    contacts wound up would be unpredictable.
    I have a few SPST AC latching relays, so I think DPDT are available,
    but they would be huge and expensive. Besides, if all you want to do
    is switch between two LEDs you don't need DPDT, you need SPDT. Or,
    maybe, just a couple of transistors.
    Well, there _are_ a few more things that need to be worked out, like
    you need DC for the LEDs, you need some way to sync up the LEDs to
    correspond to the track switch position, and you need some way to
    convert the single signal from the momentary pushbutton switch into
    the two signals needed to run the relay coils or whatever's switching
    the lamps.

    I'll be happy to help you out, but I'm just wondering, aren't there a
    set of electrical contacts in the track switches which could be used
    to do what you want?
  4. Just a WAG (I'm not clairvoyant[1]), but it seems he wants to use one
    set of contacts on the relay to do the higher current AC switching for
    the rails, and the other set of contacts for the LEDs. But all that's
    needed is a diode and I limit resistor in series with each LED and it
    can then be put in parallel with the rail.
    [1] If palm readers are clairvoyant, why do they have to ask you for
    your credit card number? :)
  5. Dave

    Dave Guest

    Hi Joe,
    I used some latching relay's on my layout and found that they required
    more current than the Model RR power source was capable of supplying.
    To overcome this problem I found it necessary to use a capacitor
    discharge circuit.
    This circuit is also used on some RR's to drive the switch solenoid,
    run AC into bridge rectifier through a resistor to a suitable
    capacitor. The voltage across the capacitor is then used to pulse the
    latching relay and the resistor will limit the current from lowering
    the RR power supply voltage.
  6. John Nagle

    John Nagle Guest

    Yes. The prototype solution is a Compton and Greaves
    QL1 DC Magnetically Latched Relay. These Q-series relays
    are much smaller than the classic vital relay designs
    from General Railway Signal and Union Switch and Signal.
    They're widely used in India today and are still in use
    in Great Britain.

    John Nagle
  7. Guest

    Put your ac thru a diode and you have chopped dc. This should work, but
    you'll need to apply a higher v than rated, maybe 2x.

    Larger relays are as happy on ac as dc, though they need 2x the v at
    ac. Small relays however are too fast responding to ignore the 50Hz,
    but can be run on chopped ac if latching.

    bistable relay

    I doubt it, logic is usually much easier implemented electronically
    than electromechanically.

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