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A latching relay question

Discussion in 'Electronic Design' started by garyr, Apr 21, 2013.

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  1. garyr

    garyr Guest

    I'm using a latching relay to switch between two gain-setting resistors for
    an instrumentation amplifier (INA128). There are two such amplifiers on my
    board, hence two relays. I would like to connect the set and reset coils
    of both relays in parallel and switch both at the same time. Latching
    relay guidelines I found on the Panasonic site states that if the coils in
    two latching relays are connected in parallel, there should be a diode in
    series with each. It doesn't say why this is necessary and the diagram that
    illustrates this doesn't show any snubbers in parallel with the coils. Would
    the series diodes be necessary if snubber diodes were connected in parallel
    with each coil?
     
  2. Mark

    Mark Guest

    maybe there can be unwanted coupling between the set and reset coils
    such that when you pulse say the set coil of RL1 it may feed some
    energy though the interconnected reset coils and create an unwanted
    reset to RL2. In your specific case since BOTH set and reset are
    connected in parallel, it probably won't be a problem. In other cases
    where they are only partially interconnected, there might be unwanted
    state changes.

    From the construction of the relay, does it look like there could be
    significant coupling between the set and reset coils?

    Mark
     
  3. OldeGuye

    OldeGuye Guest

    After serious thinking garyr wrote :
    Don't the diodes act a balancing ballasts too?
     
  4. Syd Rumpo

    Syd Rumpo Guest

    Just in case you didn't know, latching relays (at least the ones I've
    used) have a minimum physical separation - if they're too close, the
    magnetics can interfere.

    Cheers
     
  5. garyr

    garyr Guest

    I like your scheme; much simpler that what I had in mind.

    What is the purpose of Q1 and Q2 in your schematic?

    Many thanks.
     
  6. garyr

    garyr Guest

    maybe there can be unwanted coupling between the set and reset coils
    such that when you pulse say the set coil of RL1 it may feed some
    energy though the interconnected reset coils and create an unwanted
    reset to RL2. In your specific case since BOTH set and reset are
    connected in parallel, it probably won't be a problem. In other cases
    where they are only partially interconnected, there might be unwanted
    state changes.

    From the construction of the relay, does it look like there could be
    significant coupling between the set and reset coils?

    Mark

    The relays are sealed so there no way to determine how they are constructed.
     
  7. garyr

    garyr Guest

    In my case they will be about one inch apart. Is that enough?
     
  8. Jasen Betts

    Jasen Betts Guest

    no. the relays resistance and inductance do a better job of that.
     
  9. Syd Rumpo

    Syd Rumpo Guest

    Well, it'll be in the data sheet, but yes, one inch will be plenty.

    Cheers
     
  10. garyr

    garyr Guest

    I started changing my schematic to control the relays using your method and
    realized that I could do the same thing with two inverter gates. I must be
    missing something; what is the reason for using two 2-input nor gates?
     
  11. Guest

    On Sunday, April 21, 2013 4:31:51 PM UTC-4, garyr wrote:

    Diodes in series with each coil will prevent a sneak circuit from the COM though the coil of the first relay then back into the coil of the second relay through it to its COM ( if that is the configuration you're dealing with). I can see where there could possibly be this interaction via magnetic coupling within the relay so that the relays end up interfering with each other if the turn-off dynamics are not exactly identical. So, it is not a bad idea, diodes are cheap, use them.
     
  12. Guest

  13. Guest

    The diodes block current in the mesh of the two coils in series through the relay coil COM node. Obviously Panasonic ran into the problem before. If he doesn't have the headroom for diodes then he has two go with four separate drivers.
     
  14. garyr

    garyr Guest

    I switched to a one-coil relay.

    Thanks again to all who replied to my query.
     
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