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temperature controller ?

Discussion in 'Electronic Basics' started by Chan, Dec 28, 2003.

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  1. Chan

    Chan Guest

  2. Baphomet

    Baphomet Guest

    The ratio of R6/R8 defines the amplifier gain. If the circuit gain is too
    low, there will be a large hysteresis around the balance point of the
    temperature bridge input. If the the gain is too high, the relay will tend
    to chatter around balance.
     
  3. John Fields

    John Fields Guest

    ---
    It's 'jitter'.
    It doesn't, and can actually add to it.

    The circuit is shown incorrectly in that the end of R6 going to IC1-2
    should be going to IC1-3.

    In this application the 741 is being used as a voltage comparator and
    for a slowly-changing signal like the one which will appear at the +
    input, when the signal gets close to the switching point (the voltage on
    the - input) the slightest noise on the signal coming from the
    thermistor can cause the output of the 741 to chatter wildly between its
    full-on and full-off states, easily causing the relay to chatter.

    To get rid of this very real problem, the output of the 741 is coupled
    back to the + input through a rather large-valued resistor so that when
    the output goes high it forces a tiny extra current through the
    thermistor, which forces the voltage on the + input to go a little bit
    higher than it would if the resistor wasn't there. This little increase
    in voltage causes the + input to quickly 'step over' any noise that may
    be on the input line, forcing the output to stay high. When the
    temperature rises, the same thing happens in reverse and the output's
    going low forces the + input through the noise threshold, keeping the
    output low and the relay off. This action is called 'hysteresis' and is
    almost universally used in circuits like this.

    Some other things which wouldn't hurt to do would be to put an
    electrolytic and a ceramic cap across the Zener to minimize noise on the
    Zener supply as well as from IC1-2 to ground in order to minimize on
    the - input, which is just as bad as noise on the +input. I'd use about
    a 10µF electrolytic paralleled with a 0.1µF ceramic in both places.
     
  4. Michael

    Michael Guest

    I'm sorry - this is probably a really basic question [but hey we're in SEB
    so it's OK :) ] But what is RL1? I'm not familiar with that term. Thanks,

    Michael
     
  5. John Fields

    John Fields Guest

     
  6. Michael

    Michael Guest

    oooooohhhhhhh... It all makes so much more sense now... thanks :)
     
  7. Chan

    Chan Guest

    Excellent, thank you !!!
     
  8. Baphomet

    Baphomet Guest

    YIKES! I'm going to have to look at schematics more carefully before
    commenting in the future. Do I hear a New Year's resolution in the making?
     
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