TEMPERATURE SENSOR READOUT CIRCUIT FOR MICROHEATER

PRESENTED BY SHYAMILI JOHN ROLL NO: 56 S7 F

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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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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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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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CONTND…

Fig 1. Temperature VS power consumption graph of microheater

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MICROHEATER PATTERNS

Fig 2. Types of Geometries Structure

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MICROHEATER PATTERN STRUCTURESS-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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HEATER DESIGN

Copper is the heating element. Resistance is calculated by , R= ρL/A , ρ is resistivity in

/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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MEANDER PATTERN HEATER

Fig 3. Temperature profile meander heater pattern

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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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CONTD…

Fig 4. Temperature vs voltage graph Fig 5. Current vs Voltage graph

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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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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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SELECTION OF NTC THERMISTOR

Minimum and maximum resistance values at the highest temperature.

4 Ω NTC 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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RESISTANCE VS TEMPERATURE GRAPH

Fig 6. Resistance VS temperature graph of 4 Ω thermistor

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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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WHEATSTONE BRIDGE CIRCUIT

V out = Vb – Vc = [R2/(R 1+R 2)] – [R 4/(R 3+R 4)]

Fig 7. Wheatstone bridge circuit

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RESULT

Fig 8. Circuit diagram of Wheatstone bridge

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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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OUTPUT VOLTAGE AND OUTPUT POWER

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

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OUTPUT VOLTAGE VS TEMPERATURE GRAPH

Fig 10. Voltage Output VS Temperature graph

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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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DISADVANTAGES

Since thermistors are semiconductor devices, their operation is highly non linear. 

 Limited temperature range due to which they are  rendered unsuitable for use at higher temperatures.

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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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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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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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THANK YOU…