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Hot Tub arching

Discussion in 'General Electronics' started by Nirodac, Apr 15, 2004.

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  1. Nirodac

    Nirodac Guest

    I have a hot tub control that switches the heater element on and off to set
    the water temperature.
    The thermostat has 20 amp contacts, that tend to burn out. I replaced the
    thermostat , then installed a slave mercury relay (contact rating 25A at 240
    volts AC),(controlled by the thermostat) to switch the actual load of the
    heater. The relay started to over heat and latched up. Replaced the merc
    relay with a "BDR" contactor (big darn relay) with contacts rated at 30A 240
    volt AC. Everything works fine, except the relay contacts arc when the
    relay operates and releases. Question is, how do I stop the arcing. The
    current measured with an amp meter at the heater is 22 amps . I was
    thinking of also using a 40 amp 600 volt Triac, any opinions.
  2. Nirodac,

    Are you sure the control input of the mercury relay was rated for the
    voltage you used to drive it? It is strange that a mercury relay is
    overheated in a application that it's made for. So maybe the voltage too
    high or ac/dc problem?

    To prevent the arching I advise to use a solid state relay. Best you look
    for a type that has a zero cross switch and that can be controlled by the
    mains voltage. A 25A type should be good enough. They become hot so so will
    need a heatsink.

  3. jtech

    jtech Guest

    Perhaps you can parralel connect a .01 capacitor with the relay switch. It
    will squelch most if not all of the arcing. fwiw.
  4. Robert Baer

    Robert Baer Guest

    Arcing would seem to indicate an inductive load.
    But heaters are supposed to be resistive.
    Get the eXplicit, raw facts from the hot tub makers as to *EXACTLY*
    what they use and have inyour *particular* make and model.
    Also get fromthem, exactly what *they* recommend for a controller.
    Be sure all info comes fromthe factory and not a salesperson.
    In short, get the raw oats (info from the factory) before the horse
    eats them; you do not want re-processed oats from the back side of the
    horse (salesperson).
  5. Nirodac

    Nirodac Guest

    The heater is resistive, I've had it replaced myself. The thermostat is a
    bulb and capillary tube type that operates a "microswitch" rated at 20 Amps.
    What I'm trying to do here is save the microswitch from constantly burning
    out. The tub manufacturer is local, so up to now I was buying the parts
    from them, (the complete thermostat) directly. The thermo part of the
    thermo switch never failed, it was always the switch. It is possible that
    they sub'd a higher wattage heater this last time, as this one physically
    looks different.
    Maybe I shouldn't have called it an arc, as much as just a bright spark.
    The plate on the electrical box rates the heater at 4KW.

  6. Robert Baer

    Robert Baer Guest

    4KW (RMS) at 120VAC (RMS) .. P=I*E or I=P/E or I=(4000W)/(120V) or
    I=33.3 Amps (RMS).
    No wonder a 20A contactor does not last!
    ...Unless someone got wise and used 240VAC, which would make the
    current 16.7 Amps (RMS), which is too close to the contact rating.
    At minimum, the contact rating should be at least twice the peak
    current being handled.
    Perhaps a mercury plunger type of a contactor would be best; rated at
    50 Amps (!!) for a 240V supply voltage.

    BTW, a .01 capacitor is orders of magnitude too small to have any
  7. Nirodac

    Nirodac Guest

    It is 240VAC, The current relay I installed is rated at 30 amps. The
    measured current (clamp on meter) is 22 Amp AC, for the heater only.

    So, just how much heat would I need to dissipate, if I used a 40 amp triac.
  8. Robert,

    The OP told already in is his first post he's using 240Vac .

    What makes me curious is your statement that "At minimum, the contact
    rating should be at least twice the peak current being handled." Why should
    a manufacturer specify a current rating that we have to double for practical

  9. How about a 480V 50A solid state relay?
  10. Nirodac

    Nirodac Guest

    Just ordered a 40 amp 800 volt triac and a zero crossing detector, will see
    if this works


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