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controlling a strip heater?

Discussion in 'Electronic Basics' started by Bill Kraus, Jul 5, 2004.

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  1. Bill Kraus

    Bill Kraus Guest

    Hi -

    I'd like to build a circuit that will control the temperature generated by a
    high density strip heater. The goal is to use a temperature controller (with
    a thermocouple) whose output is wired to the heater - the temperature range
    is from 10 to 100 C.

    The strip heaters I've seen are driven by either a 120 or 240 voltage
    source. The temperature controller I'm looking at has an input voltage of
    either 12 or 24 VAC (what is the advantage of each?) and an SPDT relay
    output at 4 amps (26 VAC resistive). I assume that I can use a transformer
    to go from the household outlet to the controller, but do I need a
    transformer from the controller to the heater strip?

    Any other advice/suggestions would be appreciated.
  2. Controllers made to operate on low voltage are a move to miniaturize
    and make these devices safer and cheaper. They are so small, now,
    that it is getting dangerous to have line voltage wiring on the same
    terminal strip as the thermocouple wiring. Also, the alarm outputs
    are often wired to low voltage systems like 24 volt PLC systems. A
    wall wart (small receptacle mounted transformer) is probably available
    that will provide the power to operate your controller.

    I would use an external relay to boost the contact rating for long
    life (and easy replacement) rather than operate the heater directly
    from the tiny internal relay. A solid state relay may be a good
    choice for long term reliability, especially if your controller has a
    high temperature alarm to handle the failed on state that may follow a
    severe line surge. For redundant protection, wire a mechanical
    normally closed relay operated by the high temperature alarm in series
    with the control contact. That way, if the normal control contact
    fails on, you will still have rough control accompanied by repeated
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