TEMPERATURE SENSOR READOUT  CIRCUIT FOR MICROHEATER

1. TEMPERATURE SENSOR READOUT
CIRCUIT FOR MICROHEATER
PRESENTED BY
SHYAMILI JOHN
ROLL NO: 56
S7 F
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2. CONTENTS
 INTRODUCTION
 MEMS BASED MICROHEATER ARRAY ON SOI WAFER
 MICRO HEATER PATTERN STRUCTURE
 HEATER DESIGN
 TEMPERATURE SENSOR
 SELECTION OF NTC THERMISTOR
 INTERFACE CIRCUIT
 ADVANTAGES
 DISADVANTAGES
 CONCLUSION
 REFERENCES
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3. INTRODUCTION
 Micro heaters are used to elevate the temperature of gas
sensor.
 Micro heater array design can be more effective using
thin film and MEMS technology.
 Temperature measurement is recorded through resistive
sensor, NTC thermistor.
 For temperature readout circuit, NTC thermistor and
heater is combined together in Wheatstone bridge.
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4. MEMS BASED MICROHEATER
ARRAY ON SOI WAFER
 The thickness of the microheater is 0.1µm and total
length is 420µm.
 The microheater is made of platinum.
 Temperature achieved is 200 0C .
 Power consumption is 20mW .
 Uniform temperature distribution over the microheater.
 Minimum heat loss through the substrate.
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5. CONTND…
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Fig 1. Temperature VS power consumption graph of microheater

6. MICROHEATER PATTERNS
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Fig 2. Types of Geometries Structure

7. MICROHEATER PATTERN
STRUCTURES
 S-Shape – uniform temperature profile and sensing film
of small dimensions.
 Double spiral – to avoid radial temperature gradient of
conventional meander types.
 Honeycomb- redistribution of thermal energy.
 Meander shape- demonstrates undistributed hot spot at
high temperature.
 Plane plate- with central hole has a square hole in its
design.
 Fan shape- low power consumption and uniform
temperature profile.
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8. HEATER DESIGN
 Copper is the heating element.
 Resistance is calculated by , R= ρL/A , ρ is resistivity
in Ÿ/m, L for length of copper and A is a cross sectional
area of conductor.
 The resistance of copper at different temperature can be
measured by equation,
1. Rm = R0 [1+α 0 (T − T0)]
where R m : Resistance measured at T
R0 : Resistance given at To
α 0 : Temperature coefficient of metal
T0 : Room temperature
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9. MEANDER PATTERN HEATER
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Fig 3. Temperature profile meander heater pattern

10. CONTD…
 The single MEMS heater is designed with 3μm width,
209 μm length and 0.15 μm thickness.
 The maximum voltage from ambient temperature of
250C (300K) to 100 0C (375K) is 0.2V.
 The amount of current supply is 3.3mA .
 Power consumption is 0.6 mW .
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11. CONTD…
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Fig 4. Temperature vs voltage graph Fig 5. Current vs Voltage graph

12. TEMPERATURE SENSOR
Why Thermistor?
 Higher sensitivity.
 Less expensive than Resistance Temperature Detector.
 Better accuracy.
 Provides faster response.
 Reasonable output voltages.
 Very reliable and convenient to use.
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13. THERMISTORS
 Thermistors are of two types-Negative temperature
coefficient(NTC) and positive temperature
coefficient(PTC)
 NTC thermistor-used for temperature sensing applications
because of its high stability.
 PTC thermistor-used as heating element for small
temperature controlled ovens, circuit protection
applications etc.
 NTC thermistor results non linear measurement.
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14. SELECTION OF NTC THERMISTOR
 Minimum and maximum resistance values at the highest
temperature.
 4ŸΩŸNTC thermistor.
 According to Steinhart-Hart equation , the resistance of
thermistor is determined at different temperatures,
2. 1/T= A+ B (loge R) + C (loge R)3
 Dissipation factor should be maintained at low level.
 Excitation current calculated from,
3. Power dissipated= I2R = δ(T)
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15. RESISTANCE VS TEMPERATURE
GRAPH
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Fig 6. Resistance VS temperature graph of 4 Ω thermistor

16. INTERFACE CIRCUIT
 Wheatstone bridge is preferable than voltage divider.
 Used to connect the heater and thermistor for
temperature measurement.
 Supply noise is fully eliminated.
 Nonlinearity of R/T curve of thermistor leads to the use
of this circuit in order to linearize the signal.
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17. WHEATSTONE BRIDGE CIRCUIT
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V out = Vb – Vc = [R2/(R 1+R 2)] – [R 4/(R 3+R 4)]
Fig 7. Wheatstone bridge circuit

18. RESULT
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Fig 8. Circuit diagram of Wheatstone bridge

19. CONTD…
 Two voltage dividers.
 First voltage divider consists of two heaters with 8 Ω
resistance each.
 Total summed up voltages are 0.4 V.
 Current flow should minimize to avoid self heating
which is 0.1 A .
 Potentiometer of 4 Ω is used.
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20. OUTPUT VOLTAGE AND OUTPUT
POWER
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 Total current consumption of heater and
thermistor is 0.118A
 Constant value of heater and thermistor is selected
to have low power dissipation.
Fig 9. Reading of output voltage and output power

21. OUTPUT VOLTAGE VS
TEMPERATURE GRAPH
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Fig 10. Voltage Output VS Temperature graph

22. ADVANTAGES
 Bridge circuit offers the simplest configuration
 Low power consumption especially in micro heater gas
sensor array application.
 Number of parts can be reduced.
 Sensor measurement is more accurate as the distance is
close with heater.
 Proper thermal isolation between sensor element and
substrate.
 Ease of microheater array fabrication and small size.
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23. DISADVANTAGES
 Since thermistors are semiconductor devices, their opera
tion is highly non linear.
 Limited temperature range due to which they are
rendered unsuitable for use at higher temperatures.
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24. CONCLUSION
 Meander structure is suitable for heater .
 Resistance and current values to heat up the heater are
obtained from simulation of heater design.
 NTC thermistor resistance is selected based on the range
of temperature and power dissipation constant.
 Potentiometer is used to adjust the current and balance
the circuit.
 Maximum output voltage at 1000C is 0.4153V with
0.0049W requirement.
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25. REFERENCES
 Solehah Md Hashim,Umadevi Chandaran,
“Temperature Sensor Readout Circiut for
Microheater”,International Conference on Electronic
Design,2014
 A. Datta. Gupta, C. Roy Chaudhuri, “Design and
Analysis of MEMS Based Microheater Array on SOI
Wafer for Low Power Gas Sensor Applications”,
International Journal of Scientific & Engineering
Research, vol 3, pp. 1-8, 2012.
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26. CONTD…
 Avigyan Datta Gupta, Chiirashree Roy
Chaudhuri.“Design and Analysis of MEMS Based
Microheater Array on SOI Wafer Low Power Gas
Sensor Applications” International Journal of Scientific
& Engineering Research, vol 3. No 6. 2012.
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