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Temperature circuit help

Discussion in 'Electronic Design' started by kmillar, Feb 9, 2005.

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  1. kmillar

    kmillar Guest

    I'm looking for some help designing a crcuit to measure temperature in
    a central heating system.

    Sepcifically, I have a ZWorld OP7200 programmable display which has
    several Analog inputs.

    I have no trouble writing the sofware to read the analog inputs, but
    would like some help deisgning the external circuitry. What type of
    temperature sensors should I use? Temperature Dependant Resistors?
    Something else? There will be 2 of them in total, one recording
    temperature of flow from the boiler, and one reading temperature of
    water returning to the boiler, by being clamped onto the copper pipes,
    near the boiler.

    Any help would be gratefully accepted, schematics (JPG or Eagle etc)
    would be fantastic. It doesn't need to be specific to the OP7200.
  2. j.b. miller

    j.b. miller Guest

    When I designed remote energy management systems, I used the National
    LM34DZ temperature sensor and an LM358 dual opamp. Used one stage as a
    buffer the other a simple x4 amplifier.
    My systems also monitored boiler emps as well. Simply use a gearclamp and
    lightly hold the LM34 onto the pipe.
    It's not real 'rocket science' and you will have to experiment to get the
    gain right for you application.Nice thing about the LM34, 70*F = .700 volts
    Good luck , it is easy though

  3. Tim Wescott

    Tim Wescott Guest

    Thermistors work well if you can stand accuracies to a few degrees.
    With calibration you can do better yet.

    I've used the circuit below in the past, and it's worked well. In spite
    of appearances it does a good job killing common-mode noise from the
    thermistor. It takes the exponential resistance from the thermistor and
    shunts it so you have an S-shaped voltage vs. temperature characteristic
    that's easy to solve for temperature -- I've done it either by
    calculating the temperature from first principals using floating point
    arithmetic or by building up a look-up table in a spreadsheet and doing
    linear interpolation on integers.

    Figuring out the Vout vs. thermistor resistance and Vref is left as an
    exercise to the reader. Once you've figured out the thermistor
    resistance you can get thermistor temperature using equations from the
    manufacturer. Note that if the board's Vref is available to you this
    circuit is entirely ratiometric.

    .-. Rf
    | | Rbias ___
    | | .---|___|----.
    '-' Ri | |
    | ___ | |
    <-----------o--------|___|------o |\ |
    To '---|-\ | Vout
    Thermistor | >-----o------
    ___ .---|+/ To ADC
    <-----------o--------|___|------o |/
    | |
    .-. Ri |
    | | .-.
    | | Rbias | |
    '-' | | Rf
    | '-'
    | |
    === |
    GND ===
    created by Andy´s ASCII-Circuit v1.24.140803 Beta
  4. Guest

    Guest Guest

    I would side-step all the analog stuff and go directly to digital.
    Maxim/Dallas has a chip that speaks 3-wire SPI and would be ideal for your
    It's their DS1626/DS1726.

  5. Michael

    Michael Guest

    Also DS1620 and DS1820. I use gobs of DS1820's.

    No calibration with these digitals. I like that. Also, attachment is
    easy; I use wirewrap wire even for long runs. 3 conductors twisted
    together with the aid of a hand drill. I have yet to see how long such
    a run can be and still have a DS1820 work, but I've made many 30 foot
    runs without encountering troubles.
  6. Robert Baer

    Robert Baer Guest

    I have found the LM35DT to be linear and accurate to at least 185C.
  7. tlbs

    tlbs Guest

    There will be 2 of them in total, one recording
    What is the max. exit temperature. If this is a pressurized boiler,
    then temperatures could get up to 1000F and a semiconductor temp.
    sensor would "fry".

    Is this design part of an industrial control system, or just a
    lab/school experiment? If it is for an industrial control, you will
    need to use devices qualified to the local building codes and national
    electric codes.

    If it is just a school lab experiment, then pretty much anything will
    probably be OK (within reason). The suggestions, above, are all valid
    so long as the temperatures are within the range of the devices. If
    the expected temperatures are higher, go to -- they have a
    very wide range of temperature sensors. They will have something to
    meet your needs.
  8. Ted Edwards

    Ted Edwards Guest

    Is this a hot water system for your home?
    That's the one I was thinking of. Linear, accurate and no hassles
    assuming the answer to the above question is yes. If much above 100C,
    then thermocouple and more complicated circuitry.

  9. kmillar

    kmillar Guest

    The max output temp is 85 degrees C
    It is a domestic central heating system, split into 2 zones., the CH
    and the Hot water.

    I wish to measure the temp of the 'flow' and 'return' close to the
    It's all part of a bigger project to autmate my home central heating so
    I can monitor and control it via the web.

    Thanks for all the replies. I think I have all I need now.
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