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24VAC Relay troubles

Discussion in 'Electronic Basics' started by kingery, Dec 6, 2003.

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  1. kingery

    kingery Guest

    One year ago, I installed a DPDT relay in my furnace to allow it to use an
    antique thermostat that I really like. The old thermostat has two contacts
    that are on the same bimetallic coil that tend to "bounce" and allow the
    furnace to cycle too frequently. I adjusted the thermostat so that as the
    temperature drops, first contact "A" closes, then at a lower temperature,
    contact "B" closes. Conversely, as the house warms, first Contact "B"
    opens, then Contact "A" opens.

    I then installed a Idec RR2P 24VAC DPDT relay in the furnace and wired it so
    that the relay closes when both "A" and "B" are closed, and the relay won't
    open until both "A" and "B" are open. This was accomplished by placing "A"
    and "B" in series with the relay coil and utilizing one of the two relay
    "contact" circuits to bypass "B".

    The system has worked fine for 1-year, but the relay began to intermittently
    fail to close this past year. I have tried replacing the relay, but the new
    relay behaves the same. I have also tried bypassing the thermostat and the
    thermostat wiring when the relay is in the malfunctioning state, but still
    the relay will not close. I next suspected that I had a poor connection, so
    I measured the current flowing into the relay in the malfunctioning state,
    and reset all of the wiring connections. The measured current through the
    relay coil was 110 ma (the relay is rated to use 105 ma, with a 182 ma
    inrush current. This all seemed fine, but the relay is still not closing,
    and is getting noticeably warm. My next suspicion was the power
    transformer.

    The power transformer is a 40 VA, 24-VAC transformer that is currently
    outputting 28.48 VAC when the furnace is off (the furnace circuit board uses
    126 ma in this state). It outputs up to 840 ma at 26.2 VAC when the furnace
    is running and the relay is closed. The transformer is outputing 310 ma
    when the relay is in the malfunctioning state. I am stumped. It seems that
    the transformer has plenty of power and the relay coil has the proper amount
    of current flowing through it. The system cannot be made to start by
    tapping the relay, but it will always start if the system is "reset" by
    turning off the thermostat (or the furnace power), then turning the
    thermostat (or furnace power) back on. It seems unlikely that I have two
    bad relays (they look like high quality units and work most of the time).
    The wiring all seems good, and the transformer seems to be working. Any
    ideas on why my relay isn't closing?
     
  2. Try the relay on a separate power supply that meets its needs. If it fails,
    then you know it is the relay. If it does not, then you have some other more
    subtle problem.
    In some cases, you can have problems with the switching conditions that
    require adding a capacitor to the coil of the relay- this will momentarily
    extend its switching time. Unfortunately, this trick will only work with a DC
    system.
    I suspect the contacts are at fault somehow here. Try cleaning and
    adjusting them.

    Cheers!

    Chip Shults
    My robotics, space and CGI web page - http://home.cfl.rr.com/aichip
     
  3. kingery wrote:
    (snip)
    If your thermostat wiring has gotten some additional resistance in it
    that it didn't have when the relay worked, it might explain the
    failure to pull in. It normally takes the higher current to get the
    relay pulled in, and then, the increase in the coil inductance drops
    the current ot the holding value. If your wiring cannot deliver the
    182 ma with the armature held open, you have some resistance limiting
    the current to deal with.

    AC relays generally run a little warmer then the equivalent DC relay.
     
  4. als

    als Guest

    A 40VA transformer should be good for a couple of definite-purpose type
    relays, or half a dozen control-type relays. Probably not your problem.
    What are you measuring current with? What is the "furnace circuit board"
    and what is it doing?

    Things to check:
    1) voltage at the contactor coil when operated
    2) mounting of relay. I have seen some that absolutely did not like
    being mounted horizontal (i.e., to a vertical panel)
    3) resistance across both switches (mercury?). Remove any connections.
    Should be very low. Check at terminal points (relay/transformer).

    <als> alsAThalDASHpcDOTorg
     
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