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temp controller

Discussion in 'Electronic Design' started by Krash, Dec 17, 2003.

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  1. Krash

    Krash Guest

    I need schematic of a controller that will turn a light bulb on [regular
    120v] at a certain temp, and when the temp rises to a set level, turns the
    light back off, any ideas?
     
  2. Jerry Girard

    Jerry Girard Guest

    Use a thermistor into comparator and the comparator switching a TRIAC to
    control the light. Use the comparator hysteresis to control the on and off
    cycle rate.
     
  3. Jerry Girard

    Jerry Girard Guest

    See attached schematic link:

    http://www.lillyct.com/temp.gif

    Add your own +5V power supply. The values suggested for the hysteresis
    feedback should get you started. Enjoy.
     
  4. Roger Gt

    Roger Gt Guest

    Nice, a really simple circuit that is also elegant. Kudos'

    Roger Gt.
     
  5. Mac

    Mac Guest

    I doubt anyone will draw a complete schematic for you. It's a lot of work.
    But what you need is a temperature sensor, say a temperature to voltage
    converter. This could be connected to a comparator with a reference
    voltage corresponding to your temperature set point. Be sure to put in
    some hysteresis so the bulb doesn't just flicker on and off.

    Then the comparator output would have to actuate the light bulb. This
    means your low-voltage DC circuit would have to somehow open and close the
    120 V circuit containing your lighbulb. A relay might serve this purpose.
    There are probably solid state devices, for this, too.

    The sensor/comparator part seems pretty straighforward to me. However, I
    don't have any experience switching house voltage from a DC circuit. I'm
    sure others here have done this, though.

    Mac
     
  6. Ban

    Ban Guest

    Jerry Girard wrote:
    || See attached schematic link:
    ||
    || http://www.lillyct.com/temp.gif
    ||
    || Add your own +5V power supply. The values suggested for the
    || hysteresis feedback should get you started. Enjoy.

    It should be mentioned that the shown circuit is very difficult to
    calibrate, because the two pots are interdependent, much better would be to
    use the second comparator for the lower threshold and the other to switch
    off. No additional components needed. :)
    VCC ___2k2
    +-----------------|___|-+
    | ___ |
    .-. +-|___|-+
    | | | 2M2 |
    | | | |\ |
    '-' +-|+\ |
    | | |A >--+ LM393
    +---------+---+-)-|-/ | |\
    | | | | |/ | +--|-\
    .-/> | | | LM393 | | |B >-to MOC3010
    |/| --- +-)-------)---|--|+/
    / |Therm. --- | | | |/
    </'-' |0u1 | | |
    | | | | |
    === === | | |
    GND GND | | |
    | | |
    | | |
    VCC | | |
    | | | |
    .-. | | |
    | | 10k | | |
    | | | | |
    '-' | | |
    | | | |
    .-. low threshold| .-. |
    | |<-------------+ | |<-+
    | | 10k 10k | |high
    '-' '-'threshold
    | |
    === ===
    GND GND
    created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

    Another difficulty is that the specialized optocoupler MOC3010 is not easily
    available.
     
  7. It depends on how precisely you're trying to control, and what you're trying
    to control. You could use a thermostat; they're very cheap in hardware
    stores. If you need precise control, or the system you're trying to heat
    has a lot of thermal lag, you may need a device called a "PID controller" -
    google for that to learn more.

    I hacked my espresso machine to use a PID controller rather than a
    thermostat :)
     
  8. hamilton

    hamilton Guest

    I like espresso.

    Did the better temp control of the water help the brewing process ??
     
  9. Yes, quite a bit. For two reasons: first, the temp is more reliable from
    shot to shot; second, I can adjust it for different kinds of beans. (Darker
    roasted beans like the water a little less hot.)

    The machine is a Gaggia Classic, which is already a good machine with a
    powerful heater and a lot of thermal mass. But the PID has really made a
    surprising amount of difference. It used to be that one in three shots was
    really good; now I can consistently get the same quality.

    There's often discussion of this on alt.coffee, if you want to learn more.
     
  10. hamilton

    hamilton Guest

    Walter,

    Would you please post more information about the PID controller.

    Did you build / program the controller yourself, or
    did you buy an off-the-shelf controller ?

    Any information about the controller itself would be fun.

    Thanks

    hamilton AT dimensional DOT com
     
  11. Well, as I said, the best place to learn more is on alt.coffee (or on
    http://www.coffeegeek.com). Quite a few of the folks there have done this.

    But in brief: I used an off-the-shelf PID controller and a solid-state
    relay. The controller cost about $90, second-hand but unused; it's a little
    thing, about 1"x2.5". These live in a steel chassis that sits underneath
    the espresso machine. Within the espresso machine, the thermostat is
    disconnected and the wires instead run to a connector on the back of the
    espresso machine (and thence to the controller chassis), and a K-type
    thermocouple is also run from the controller into the espresso machine and
    attached to the boiler.

    The PID controller has an auto-tune mode, that takes about half an hour to
    run. It heats up and cools down the boiler a few times and watches the time
    constants.

    I'll post a picture and a schematic to alt.binaries.schematics.electronic
    for your entertainment.
     
  12. hamilton

    hamilton Guest

    Thanks for that.

    I am looking at this as overkill.

    A simple temp controller was posted here ( or comp.embedded ).
    The thermal mass of the water means that the boiling will take a while,
    keeping the temp at a set point temp ( within 1 degree F) should be easy
    even with an analog temp controller (as posted).

    I am looking at getting an espresso machine after xmas. (for myself)

    I will put together some low cost temp control designs myself.

    I found a web site with PID controller info. This looks great.

    <http://www.quiknet.com/~frcn/Coffee/Coffee63.html>

    But adding $200 to an already $350 unit seems a little crazy.

    Have to admit, the mods look great.

    We all have our hobbies. :)

    hamilton
     
  13. Arguably, owning an espresso machine at all is overkill... anything beyond
    that, such as modifying it for better coffee, is surely overkill. As you
    say, we all have our hobbies :)

    There's a substantial time lag between when you start dumping energy into
    the boiler, and when the temperature sensor detects a change. That time lag
    is your enemy, in terms of process control. I think you'll find that a
    naive system has considerable overshoot, and takes a long time to stabilize
    after pulling a shot, if it ever stabilizes at all. More likely, it'll
    constantly oscillate (at 0.05Hz or so) over a range of several degrees.

    If the problem of accurately controlling water temperature in a boiler was
    really all that easy, PID controllers wouldn't exist... it's one of the main
    things they're used for, I think.

    That's a good page, as are the other ones it references.

    The question is not whether adding $200 to a $350 unit is crazy, the
    question is whether having a $550 espresso machine is crazy. Arguably it
    is. Me, I keep thinking about moving up from the Gaggia + PID combo to an
    $1100 Isomac. So yes, I'm crazy :)

    -w
     
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