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Triac or relay to switch AC power?

Discussion in 'Electronic Design' started by Jean, Feb 1, 2005.

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  1. Jean

    Jean Guest

    Triac or relay to switch AC power?

    I want to use a controller to switch for an hour the AC power of a
    light bulb.
    Should I use a triac or a relay?
    Besides the circuit needed to drive the triac or the relay, what are
    the parameters I should be aware of to choose between the triac and
    relay?
    In both cases, the controller has to keep sending the signal for an
    hour; The circuit around the controller will use current for an hour
    and I am concerned about this consumption and want to keep this
    consumption as low as possible.
     
  2. Gary J. Tait

    Gary J. Tait Guest

    Use triac then. The current will be no more than an LED.
     
  3. Michael

    Michael Guest


    Consider a solid state relay. Easy hookup. I use a 25 amp SSR and
    PIC-based timer to cycle a 1500 watt space heater that used to be inop.
    because its thermostat had died. Ten years old last fall and still
    running fine.
     
  4. Joop

    Joop Guest

    The lowest power usage I can think of is by using a bi-stable relay.
    It only uses a bit of current when switching on or when switching off.

    Joop
     
  5. mike

    mike Guest

    Get a wind-up timer and be done with it. Fits right into the box in the
    wall and does everything you asked for.
    mike

    --
    Return address is VALID.
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    Bunch of stuff For Sale and Wanted at the link below.
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  6. John Fields

    John Fields Guest

     
  7. Fred Bloggs

    Fred Bloggs Guest

    The simplest way to achieve success without knowing much about
    electronics is to use an AC relay actuated by a your controller through
    a TRIAC. The sizing of the relay driving an incandescent load has to be
    adjusted downward from the maximum current rating of the relay to
    something like 30-40% because of the cold filament surge and possible
    filament failure scenarios. So if your bulb is say 300W @ 240VAC, then
    I=W/V=300/240=1.25A -use a relay with contacts rated at 5A @240VAC and
    240VAC coil. Then use a 600V TRIAC with 1A or more current rating to
    drive the coil like so:

    View in a fixed-width font such as Courier.
     
  8. Fred Bloggs

    Fred Bloggs Guest

    Maybe the simplest in your case would be to use a DPDT AC relay
    configured as self-latching, and then use timer to pulse two logic level
    mini-relays ( one with normally close,nc, the other with normally open
    ,no, contacts) with AC rated contacts to initiate ON/OFF operation. The
    quiescent overhead from the controller is now zero.
    View in a fixed-width font such as Courier.

    ..
    . DPDT AC PWR RELAY
    . +--------------+
    . | / |
    . LINE -----------|--+-----o o---|----------------->
    . | | | TO LAMP
    . | K|| |
    . | K|| | +---->
    . | K|| | |
    . | K|| | |
    . | K|| | |
    . | | | |
    . | +--------+ | |
    . | | | | |
    . | o | | |
    . | / | | |
    . | o | | |
    . | | | | |
    . +--------------+ |
    . | | |
    . | | |
    . o o |
    . nc | <-. no / <--. |
    . o | o | |
    . | | | | |
    . NEUTRAL-----------+----|---+-----|---------+
    . | |
    . OFF ON
    . | |
    . from two separate
    . mini-relays on
    . controller board
    . 250ms pulse operation
     
  9. Jean

    Jean Guest

    Maybe the simplest in your case would be to use a DPDT AC relay
    Relay DPDT
    Self-Latching connection
    I want to use VCC=5V to drive a 120AC line.
    How would you connect the DPDT relay as self-latching?







    VCC----+
    |
    | | | | | | |
    +-----------------------------------------+
    | | NC COM NO NC COM NO |
    | K|| |
    | K|| |
    | K|| |
    | K|| |
    | K|| |
    | K|| |
    | K|| |
    | | |
    +-----------------------------------------+
    |
    |
    Ground----+
     
  10. Rich Grise

    Rich Grise Guest

    Cheers!
    Rich
     
  11. Fred Bloggs

    Fred Bloggs Guest

    A relay that "self-latches" is not a latching relay. Latching relays
    maintain their last state without power applied to the coils. Because
    you are concerned about power consumption in the timer, you either power
    the relay off the AC line or use a true latching relay. It will be
    cheaper for you to use surplus standard 5VDC coil relays that you
    *pulse*- like these
    http://www.allelectronics.com/cgi-bin/category.cgi?category=500&item=RLY-625&type=store
    to drive a standard AC-line relay like this:
    http://www.allelectronics.com/cgi-bin/category.cgi?category=500&item=RLY-2120&type=store
     
  12. Jean

    Jean Guest

    From Fred's point of view, I think VCC should be replaced by the LINE
    with a connection like this:

    +------------------------------+----------+
    | N.C. = 'off' | |
    LINE----+---o/ o-------+---o__o----+ | |
    N.O. = 'on' | | | | +--- load
    | | | | | | |
    +-----------------------------------------+
    | | NC COM NO NC COM NO |
    | K|| |
    | K|| |
    | K|| |
    | K|| |
    | K|| |
    | K|| |
    | K|| |
    | | |
    +-----------------------------------------+
    |
    |
    Ground----+

    This way, the consumption is not from the controller/timer "board"
    because the board drives the two small relays with pulses, but there
    is still consumption for this "AC" relay from the LINE while the load
    is powered.
    To take the overall consumption at a minimum, a latching relay should
    be used?
    Or how much current, voltage or power this AC relay takes when there
    is current in its coil?

    *********************************************
    120 VAC DPDT 10 AMP RELAY
    Kest # KRLY-2120. 120 Vac, 5000 ohm coil. D.P.D.T. 10 Amp contacts. KH
    "Ice cube" style,clear polycarbonate case. 1.1" x 0.83" x 1.37" high.
    Solder or 0.187" qc or solder terminals.
    CAT# RLY-2120
    **********************************************
     
  13. Rich Grise

    Rich Grise Guest

    I know it's poopy of me, but the only difference between that and the
    circuit I suggested is the source of relay power.

    When you use other people's material like that, it's polite to say
    something like "as Rich suggested, but..."

    Thanks,
    Rich
     
  14. I read in sci.electronics.design that Rich Grise <>
    Jean may not have read your post before posting his/hers.
     
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