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Alternating relay

Discussion in 'Electronic Basics' started by Ryan, Jan 27, 2006.

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  1. Ryan

    Ryan Guest

    Is there a stock item which directs voltage onto one wire the first
    time it is given power, and then to another wire the second time when
    the item is given power?

    I have two air compressor units tied together as one tank. Right now,
    only one of the pumps is hooked up.

    The pressure switch is pneumatically closed/opened. The contacts of
    this switch power the motor on directly.

    I would like to introduce a contactor that, when given this on signal,
    will power on pump #1 for that cycle and then shut it off when the
    pressure switch is disengaged. Next time air is low, I want a
    contactor to engage pump #2 for that cycle and then turn off the motor
    when it is powered off by the pressure switch.

    Back and forth at infinitum so the load is distributed evenly over time.

    The only "alternating" relays or contactors I have found require
    either separate on signals (one for each coil) or else reverse
    polarity in a single coil. I have only one signal: the on / off
    state of the pressure switches contacts.

    My local electical supply houses are either dumbfounded to
    speechlessness by my request, or else don't have anything to fit the
    bill.

    With that having failed, I wonder if there are stock electronic
    circuits that exist, or that I could fashion, and then relay that
    signal into larger switching gear without having to invest in a rocket
    science degree first. (Perhaps I could use a doorbell transformer
    or something easily found to power the coils and maybe the small
    electronics too.)

    The circuit would perform like this:

    Source Off = Motor1 off Motor2 off
    Source On = Motor1 on Motor2 off
    Source Off = Motor1 off Motor2 off
    Source On = Motor1 off Motor2 on
    Source Off = Motor1 off Motor2 off

    I'm stumped.

    -Ryan
     
  2. Lord Garth

    Lord Garth Guest

    Is that pneumatic pressure switch a set of dry contacts?
    I'm thinking you can use a T flip-flop with a relay driver
    on each output to activate the compressors. You will have
    to detect that pressure switch open condition as a sort of
    master off for both relay drivers.
     
  3. Art

    Art Guest

    Simple fix would be a power switch which you would need to manually select
    as to which compressor you care to use. Otherwise you may need a custom made
    switching box produced for your application.
     
  4. Ryan

    Ryan Guest

    I don't think I understand what are "dry contacts."

    I can tell you that the pressure switch does not have an
    electromagnetic coil to actuate the moving parts to which the contacts
    are connected. Therefore the pressure switch does not use a power
    supply, nor do I think it uses the wall voltage in any way on the
    "supply" side.

    The change in air pressure makes the contacts snap shut and then a
    spring makes them click open.

    It's a 220v motor, so it is a double pole singe throw.
     
  5. Lord Garth

    Lord Garth Guest

    A 'dry contact' means that it is not connected to any power source. You are
    free to use it in any way you require.

    All that is needed is a single pole switch so you are in good shape.

    Can you access alt.binaries.schematics.electronic ?

    I can work up a drawing and place it there.
     
  6. Ryan

    Ryan Guest

    A 'dry contact' means that it is not connected to any power source. You are
    Then I think yes. Right now there is wall voltage to one side, and
    the motor connects on the other side.

    I can disconnect both and use it for something else. I can then run
    the wall voltage to some-other-thing, and power the motor using that
    some-other-thing.


    I prefer to automate the system rather than use a manual switch.
    Yup, I see it. I see only 9 posts there, so if that sounds incorrect
    then its expiration may be very short.



    Thank you to everyone who is offering tips.
     
  7. Lord Garth

    Lord Garth Guest

    Your welcome!

    Are you able to obtain digital logic gates? There will be a few required
    in addition to two relays and some discrete components. Of course, you
    will need a power supply to create the 5 volts needed to operate the
    circuit.
    You may need a 12 volt power supply depending upon whether you can
    get 5 volt relays that can switch the compressors.
     
  8. kell

    kell Guest

    I used an alternating relay once that had a single coil. When
    actuated, the solenoid pulled on an alternating mechanism with two
    latching positions, which would reverse every time you actuate the
    coil. Run the motor off the contacts of the alternating relay. This
    way you could switch motors every time the pressure switch turns on.
    Of course the motor would stay on even after the pressure switch turns
    off. You would still need a way to turn the motor off when the
    pressure switch turns off, but it's easy enough to put an ordinary
    relay in series with the alternating relay.
     
  9. Ryan

    Ryan Guest

    I used an alternating relay once that had a single coil. When
    So this altnerating relay would flip and flop based upon a single
    on/off input voltage? If so, then I think I could start with that.

    Do you have any idea what brand or model this was?

    My web searches for alternating relays have yielded products which
    have very sparse descriptions. I think they only flip if the polarity
    is flipped.

    Makes sense I think. I'd power the coils of the contactors with the
    outputs of the alt relay.

    The pressure switch needs to feed the supply side of the contactors so
    that power will shut off even though a coil is still engaged.

    If I do it this way, I need an alt relay whose coil uses 220v,
    correct? (Or at least 120)

    I'll keep thinking about this, but I'm curious how else it might be done.

    -Ryan
     
  10. Chris

    Chris Guest

    Hi, Ryan. ATC makes an inexpensive alternating relay (also called a
    sequencing relay)that will fill the bill for you directly. Look at
    these links:

    http://www.automatictiming.com/pages_misc/app_alternatingrelays/alternatingrelays.html
    http://www.automatictiming.com/pdf_div/ara-arb-duplexor_data.pdf

    Elsewhere in the thread you say you've got a 220VAC compressor setup,
    so you probably want the ATC P/N ARA-240-ABA. Look at the first
    example in the first link for a suggested wiring diagram. By the way,
    make sure to wire it up so pins 4 and 5 are always hooked up to the
    240VAC to ensure proper operation. Also make sure your pressure switch
    has enough hysteresis to prevent switch chatter, which will mess up the
    alternating relay operation.

    If your setup is 3-phase, and/or if the compressor is more than 1/4
    h.p., use only one of the phases for control with your pressure switch,
    then use the relay to drive two 240VAC contactors that can drive the
    two motors.

    For industrial control applications, it's usually far better to
    purchase a one part solution made for the job if you can, than cobbling
    something together with digital logic. Not only does it make
    maintenance of your solution possible, but it also helps avoid problems
    with relay arcing causing EMI/RFI that interferes with the control
    logic.

    You can purchase the part directly on the ATC website for $61.30 plus
    shipping and handling. Even if you cost your time at minimum wage,
    you're going to be hard-pressed to beat that price for an installed
    cobbled-up solution.

    Good luck
    Chris
     
  11. Look for a Potter & Brumfield S89R or S90R series Bistable impulse
    relay. About Can$40 from ElectroSonic in Canada.
     
  12. D'oh! Of course! A racheting relay - there's a solenoid that pulls in
    a pawl that engages a cam on a wheel with a detent. Every time the
    solenoid is energized, the cam advances by one setpoint.

    You can control an arbitrary number of functions this way, but all
    you really need are the two contacts - to use a linear solenoid to
    drag a circular cam around, you'd probably want at least 4 or more
    positions.

    I have no idea if anybody makes anything like that anymore, but if
    you're adventurous, and good with tools, and have parts and materials
    on hand, you could make one in a day. :)

    Good Luck!
    Rich
     
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