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24 Volt Contactor/Relay Wiring

Discussion in 'Electronic Repair' started by William R. Walsh, May 29, 2009.

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  1. Hello all...

    I recently dreamed up a way to add thermal control to a window air
    conditioner that I'd picked up off the curb some time back. It worked,
    outside of the fact that whenver the thermostat was connected, the fan motor
    would eventually grind down to a halt. It made no sense to me, seeing as the
    compressor is the only thing switched by the thermostat. So I did the quick
    and dirty thing, and I bypassed it so the compressor is running whenever the
    unit is on.

    My first means of control was a heavy duty timer that could turn the unit on
    for a while and then turn it off. This was cumbersome at best, especially as
    the timer had no way to "know" when it was too cold for A/C.

    So I got a programmable thermostat, a 24 volt AC transformer and a heavy
    duty contactor with a 24 volt coil. This setup works great to cycle power to
    an outlet placed especially for the air conditioner, and the parts were all
    freebies. But I've noticed that the coil on the contactor gets hot, and I'm
    wondering if this is normal. I've looked around for wiring diagrams and even
    looked inside some furnaces like the one where the the parts came from to
    see if I'd need a current limiting resistor or something. But I don't see
    anything like that in use. It looks just like the wiring comes from the
    transformer, with one end going to the contactor and the other going through
    the thermostat.

    Am I going to ruin the coil, or do they just run hot when energized?

  2. Hi!
    The relay is a 24 volt AC relay. Both items were salvaged from the same
    defunct cooling system. It is quiet, and seems to operate normally. Output
    from the transformer is reasonably stable when loaded, and ranges from 23-26

  3. PeterD

    PeterD Guest

    Also, define 'hot'... Have you measured the temp, and what was it?
  4. Bob Larter

    Bob Larter Guest

    Then don't worry about it, if it's only getting warm, rather than hot.
  5. (posting from a "real" news server this time, in hopes of attracting
    more eyes!)

    Not being able to find my noncontact thermometer, I can only guess.

    After the coil has been running, and with the power *off*, I noticed
    heat emanating from it. So I touched the coil body and found it was
    nearly hot enough to make continued contact undesirable.

  6. Hi!
    That's the thing. The outer surface of the coil does get hot after it's
    been on for a while. It's too hot to stand touching it for more than a
    few seconds. (It isn't so hot that you immediately pull back--there is
    more than enough time to think "gee, this is somewhat hot".)

    On the other hand, it has been running trouble free for some time now,
    and I can't find any evidence from the original unit it was in, that the
    power was in any way limited to the relay, such as by a resistor.

  7. PeterD

    PeterD Guest

    My suspicion is that they contactor has a short in the coil perhaps.
    If it is too hot to maintain continued contact, then the rule of thumb
    (sorry, bad pun...) is that it is too hot.
  8. alchazz

    alchazz Guest

    If it's that hot on the outside, think of how hot it is at the core. My
    advice, replace it.

  9. Baron

    Baron Guest

    Its quite possibly normal for the coil to get quite hot ! AC contactors
    have a shorting loop on them to stop the armature chattering due to the
    alternating flux. If it bothers you that much reduce the voltage
    across the coil a little. Just make sure that it pulls in solidly if
    you do.
  10. Jamie

    Jamie Guest

    Or, just put a single diode in series with the coil to remove one half of
    the cycle so that a AC coil can operate normal at %50 duty with DC.
  11. Not sure how helpful this is, but this reminds me a lot of a repair I
    did to a house I lived in about 10 years ago. House had an old old gas
    furnace whose gas control finally gave up the ghost. The control was a
    standard one, it turned out, operated through the thermostat with a
    24-volt transformer.

    Seemed simple enough: I installed the new control, wired it up. It
    worked, but as soon as the thermostat called for heat, it made a really
    horrible buzzing noise (60-cycle) that could be heard through the entire

    I carefully checked the installation instructions and the existing
    transformer, which was emitting as close to 24 volts a.c. as no never
    mind. The control was clearly rated for 24 v.a.c. Called the place I
    bought the control, who were as puzzled as I was. They finally gave me
    another unit as a replacement. But after installing it, got the same

    I even contacted the manufacturer of the control, who tried but couldn't
    offer me any fixes.

    I finally did what any reasonable tinkerer would do: inserted a resistor
    in series with the control, which reduced the buzzing to a soft,
    tolerable level. I had to determine the value by trial and error, high
    enough to reduce the sound but low enough so that the control's solenoid
    would engage reliably. After that, the furnace worked fine, for several
  12. Baron

    Baron Guest

    I've seen AC relays where the shading ring has not been fastened
    properly buzz like that ! Often a tap with a hammer and punch in the
    right place solves the noise problem.
  13. Hi!
    I'm beginning to think that it must be. It is definitely a coil rated for
    use with AC power.

    I talked to an HVAC guy and asked what he thought the other day in the
    grocery store. He wasn't sure how hot the contactors ran, but he did raise a
    good point. In this thing's normal application, it would have received some
    "relief" (if you will) from the circulating fan in the equipment. I hadn't
    thought of that, but it makes sense.

    In any event, I put an inline meter on the power wiring going into the
    transformer. With the contactor closed, the transformer is pulling about 7.2
    watts at 125 volts AC.

    What I've finally decided to do is to add a fuse to the transformer. It's a
    3 amp slow-blow fuse (hey, it was handy!) from my parts box, but it should
    stop any gross meltdowns before they happen.

    The coil runs silently when powered, with no humming or chattering at all.

  14. Baron

    Baron Guest

    Now you mention it, it hadn't occurred to me either. There is usually
    one or more extractor fans in the top of the system cabinets to remove
    heat generated by the multitude of contactors and inverters !
    That seems about right for a small single phase contactor.
    You could always use a salvaged PC PSU fan to provide an airflow !
  15. Hi!
    This was set up in such a way that the contactor was mounted inside the part
    where the interior air circulation fan goes. It would benefit from the air
    flowing by to meet the fan and get pushed through the registers in the

    It was easy to do, so I also went out and had a look at the contactor used
    on my central air conditioner. Couple of screws and one small metal panel
    later, I saw pretty much the same unit as the one I'm using for this other
    project. And it can't get much, if any cooling air where it is situated. The
    makers chose to put it in a relatively weather-but-also-air tight box. It's
    run for 14 air conditioning seasons the designers must not have
    done too badly.
    Small, but with two sets of switched contacts. It could be used to switch
    both the hot and neutral portions of the line, but I suspect that it
    actually switched two 120 volt hot lines in the original application. (The
    original unit is no longer around now for me to check.)
    I thought about doing that, but I don't want to precipitate a fire if one
    should ever occur. Nor do I want the dust buildup in the box that all this
    stuff is going to be put inside. I have noticed that when bolted to a metal
    plate, that the contactor sheds heat through it. It would be easy to work
    that into what I'm doing.

    Thank you for all your help and information so far.

  16. Baron

    Baron Guest

    William R. Walsh Inscribed thus:
    You're welcome.

    I agree you don't want to encourage a build up of dust around it ! It
    wouldn't do the contacts any good. Using the metal box as a heatsink
    is probably how any excess heat is got rid of, particularly if there is
    air flow around that.

    I reckon that you have got it well taped ! ;-)
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