SENSOR TECHNOLOGY DESIGN 1-Wire Thermocouple standard thermo

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SENSOR TECHNOLOGY DESIGN 1-Wire Thermocouple standard thermo

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SENSOR TECHNOLOGY DESIGN
1-Wire Thermocouple
standard thermocouple combined with lithium monitor chip create smart sensor that communicates with microcontroller over single twisted-pair cable. Awtrey, Dallas Semiconductor Select March 2002
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This article describes method measuring temperatures using
conventional thermocouples that directly digitized cold junction. transducer based recently introduced multifunction chip that communicates with microcontroller) master over single twisted-pair line. significant advantage transducer that each unique 64-bit address that permits positive identification selection master. Because this unique address, multiple sensors share same software automatically recognize process data from given sensor. Although information associated with thermocouple stored within chip itself ("tagging"), unique also allows reference data stored master. design, communication handled single master that executes Touch Memory Executive (TMEX) protocol control 1-Wire thermocouple, transmitting both bidirectional data power over single twisted-pair cable. Data transfers processed half duplex sequential using short long time slots encode binary ones zeros respectively, while power transmitted during communication idle times [1,2].
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1821, Thomas Seebeck discovered operating principle thermocouple: dissimilar metals joined end, voltage (the Seebeck voltage) proportional temperature difference between joined open ends generated. effort maximize performance, numerous combinations metals have been characterized determine their output voltage temperature transfer function. combinations selected industry standards, more popular types Capital letters used indicate composition according American National Standards Institute (ANSI) conventions. example, type thermocouples nickel-chromium conductor constantan copper-nickel alloy) other. While full-scale output voltage thermocouples falls into
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millivolt range, type generates highest Seebeck resulting output almost more than other standard. Obviously, measure this output voltage necessary make connections open ends wires forming thermocouple. These connections turn form second thermocouple, such nickel-chromium/copper series with original "hot" junction when copper conductors used. Historically, Figure Before electronic cold-junction compensation, correct these "cold" thermocouple-to-hookup wire connections were junctions (one each immersed water triple point (0.01°C) reference. thermocouple wire), they were placed bath triple point (see Figure whereas most modern instruments electronically correct reading When electronic correction used, temperature cold junction measured voltage that would generated thermocouple that temperature subtracted from actual reading. voltage temperature transfer function thermocouple were highly linear, nothing further would required correct reading. Unfortunately, since Seebeck varies with temperature, full-scale transfer function usually fairly complex require several piece-wise approximations maintain specified accuracy, depending temperature range interest. this respect, type thermocouple with lower Seebeck voltage advantage over type that significantly more linear over range. Generalized plots temperature output voltage types thermocouples shown Figure
Figure Generalized temperature output voltage plots type thermocouples shows pronounced nonlinearity about 0°C.
Note pronounced slope changes that occur around both
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curves. in-depth information thermocouples, check reference material available from manufacturers such Omega Engineering Inc. [3]. While there obvious variations, typical modern electronic thermocouple consists several basic building blocks. illustrated Figure these blocks consist thermocouple with secondary temperature sensor measure junction where thermocouple connecting wires join, signal conditioning block, converter (ADC).
Figure typical electronically compensated thermocouple consists these three building blocks.
Usually, thermocouple connected precision low-noise instrumentation amplifier that provides gain, offset, impedance adjustments necessary match low-level signal generated thermocouple input multibit ADC. turn converts conditioned signal from amplifier into digital format that sent microprocessor From cold junction sensor inputs, microprocessor computes actual temperature seen junction thermocouple. Some custom conditioning chips such AD594 from Analog Devices (Norwood, available that contain both instrumentation amplifier cold junction compensation circuitry particular thermocouple type such These chips replace first blocks plug directly into input.
DS2760
Originally designed monitor lithium battery pack, DS2760 from Dallas Semiconductor (see Photo offers several ways transform simple thermocouple into smart sensor [4].
Photo small chip left converts analog signal from thermocouple (off right) into digital 1-Wire signal. thermocouple small-gauge wire right. actual size board 1.75 0.20
chip directly digitize millivolt level output produced
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between cold junctions, while onchip temperature sensor continuously monitors temperature cold junction. unique address provides label that permits multiple units operate same twisted-pair cable. contains user-accessible memory storage sensor-specific data such thermocouple type, location, date into service [5]. This allows DS2760 used with thermocouple type because master uses stored data to-determine correct calculations make, based type thermocouple temperature cold junction reported onchip temperature sensor. complete signal conditioning digitizing solution with thermocouple, DS2760 contains 10-bit voltage input, 13-bit temperature ADC, 12-bit plus-sign current ADC. also provides bytes lockable EEPROM memory where pertinent user sensor documentation stored, minimizing probability error sensor mislabeling. present application, thermocouple directly connected current inputs that were originally designed read voltage drop developed across resistor lithium battery pack charged discharged. With full-scale range (LSB 15.625 µV), converter provides resolution better than 1°C, even with lower output type thermocouple.
Measuring Thermocouple over 1-Wire
Figure illustrates both simplicity ease with which DS2760 used convert standard thermocouple into smart sensor with multi-drop capability.
Figure output from 1-Wire thermocouple digitized 12-bit plus-sign ADC. onchip 13-bit temperature sensor provides cold junction compensation. provide power sensor. allows reading value omitted this function needed.
circuit, Schottky diodes form half-wave rectifier that provides power DS2760 "stealing" from during idle communication periods when This discrete implementation parasite power technique used internally 1-Wire devices provide their operating power. remaining Schottky diode package connected across DATA provides circuit protection restricting signal excursions that below ground about -0.4
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Without this diode, negative signal excursions >0.6 forward bias parasitic substrate diode DS2760 chip interfere with proper functioning. Under master control, DS2760, both monitors voltage developed between cold junctions thermocouple uses internal temperature sensor measure cold junction temperature. master uses this information calculate actual temperature junction. Adding optional resistor (R1) allows voltage available measured well. troubleshooting this useful verify that voltage available 1-Wire within acceptable limits. When mounting thermocouple board, care should taken connect close practical DS2760 that minimal temperature difference exists between these connections chip inside DS2760 package. maintain junctions same temperature requires copper pour lead placement create isothermal area around point where thermocouple leads attach copper traces PCB. Because temperature differentials generate voltage differentials, traces should routed together equal numbers junctions maintained each conductor.
Summary
This article described combine standard thermocouple with DS2760 lithium monitor chip convert into smart sensor that communicates with microcontroller over single twisted-pair cable. This cable, which serves cover distance between thermocouple master, effectively replaces expensive thermocouple extension cable normally used. chip digitizes millivolt-level signal produced between cold junctions dissimilar metals given temperature Seebeck effect, communicates necessary information local host that correct temperature junction calculated. onchip temperature sensor continuously monitors temperature cold junction minimize reference errors. addition, onchip memory store thermocouple type when where placed into operation, which minimize probability error mislabeling sensors. This information allows DS2760 used with different thermocouple types because master reads stored data determine correct calculations based type thermocouple cold junction temperature reported onchip temperature sensor. Finally, unique address that 1-Wire devices possess, multiple smart thermocouples placed where needed anywhere along net, greatly minimizing positioning cost installation. 1-Wire 1-Wire trademarks Dallas Semiconductor.
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References
Awtrey. Feb. 1997. "Transmitting Data Power over One-Wire Bus," Sensors:48-51. "1-Wire Design Guide." "Using Thermocouples." [PDF] DS2760 Data Sheet. [PDF] Tagging protocol. [PDF]
Awtrey Staff Engineer, Dallas Semiconductor, 4401 Beltwood Pkwy., Dallas, 75244-3292; 972-371-6297, 972-371-3715, dan.awtrey@dalsemi.com. information DS2760-based 1-Wire thermocouple modules, contact Aplicada), (442) 215-3166 (442) 215-3366.
further reading this related topics, these Sensors articles. "Designing Embedded Temperature Circuit Meet System's Requirements," September 2000 "Temperature Measurement," September 2000 "Choosing Using Temperature Sensor," January 2000 "Temperature Sensor Simplify Designs," January 2000 "Understanding Contact Temperature Sensors," January 1999 "Temperature Measurement: Making Sense All," June 1998
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SENSOR TECHNOLOGY DESIGN 1-Wire Thermocouple standard thermo

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