Academic Workshop Lab

1
Measuring Temperature with Thermistors

• Academic Workshop Lab

2
Objective

• Exploit PSoC topology to build inexpensive
  digital thermometer.
• Understand the operation of a negative
  temperature coefficient (NTC) thermistor.
• Understand how to calculate Steinhart Hart
  constants for a specific thermistor.
• Calculate temperature using the Steinhart Hart
  equation.
• Calculate temperature using a look up table.

3
Hardware Overview

• CY8C3210-PSoCEval1 board.
• MiniProg
• Thermistor
• 10k resistor
• Breadboard wire

4
Reference Material

• AN2028 Ohmmeter
• AN2017 Thermistor Based Thermometer
• AN2239 ADC Selection

5
Measuring Resistance

• Unlike measuring voltage or current, measuring a
  a passive characteristic like resistance requires
  stimulus
• A Classic method is to push current into a
  resistor and measure the developed voltage.
• Only as accurate as
• Current Source
• ADC Gain and Offsets
• Resistance limited to ADC range.
• Requires different current values for wide range
  of resistors.
• Very popular when cost of accurate current
  sources was less than the cost of computation.

6
PSoC and Measuring Resistance

• For this circuit the following equation holds.
• Solving for Rtest results in
• Offset errors removed by difference
• Measurement offset voltages subtract out!
• Gain errors removed by quotient
• Measurement path errors divide out!
• Accuracy determined by an external reference
  resistor

7
PSoC and Measuring Resistance

• And ADC resolution
• For an n bit ADC the number of counts seen across
  aR is
• The reading is accurate to /- .5 counts.
• Overall resolution tolerance is
• For 14 bits and an attenuation of 15/16, the
  equation simplifies to

Tol () a
0.332 0.01
0.0399 0.1
0.013 1
0.0399 10
0.332 100
8
Thermistors

• A negative temperature coefficient thermistor
  (NTC) is a semiconductor device that becomes less
  resistive as its temperature increases. The
  change in resistance is roughly expressed by
  the equation below.
• Where
• A is some empirical value less than one for
  negative temperature coefficient (NTC)
  thermistors.
• T1 T2 are temperatures measured in Kelvin.
• R(T1) R(T2) are the thermistors resistances at
  these temperatures.

9
NTC Thermistors

• Roughly is defined as a good approximation for
  an academic introduction to thermistors.
• It shows the temperature/resistance relationship
  to be ideally exponential.
• It wont hold up for real world
  temperature-measuring application.
• But for small temperature differences the
  following holds

10
Steinhart-Hart Equation

• The Steinhart-Hart equation describes the
  resistance change of a thermistor as related to
  its temperature. The equation below shows it to
  be a 3rd order logarithmic polynomial using three
  constants.
• Where
• A, B, and C are empirical constants.
• TK is temperature in Kelvin.
• R is the thermistors resistance in Ohms .

11
Steinhart-Hart Equation

• Many thermistors come with these three parameters
  defined.
• For this particular thermistor they are in the
  datasheet
• If not they must be calculated.
• This is done by taking three points in the
  conversion table and solving for these constants.
  
• It makes most sense to use the minimum, maximum,
  and a middle value for the temperature range for
  which you are interested. From the Thermistor
  Table

Tc Resistance
0C 32,660 ohms
40 C 5,325 ohms
80 C 1,257 ohms
Note This is an example and not for the
thermistor we are using
12
Steinhart-Hart Equation

• Apply the three data points to the following
  equation.
• To get the three following equations.
• Solve to get
• A 0.11261637e-2
• B 0.23461776e-3
• C 0.85700804e-7

13
Thermistors

• The cost of thermistors is primarily determined
  by the accuracy of the thermistors resistance.
  This is where the exponential nature of
  thermistors works out to your advantage.
• A thermistors resistance tolerance shows up as a
  temperature shift. This can be calibrated out
  with a single point calibration.
• In test, bring the thermistor to 25C and measure
  its temperature.
• Suppose it reads 26.2C
• Software needs to store a 1.2offset in memory.

14
Thermistors

• In consumer products this calibration is many
  times left to the user.
• The user interface allows access to the
  temperature offset register.
• The user sets this if they think the temperature
  is a bit low or a bit high.
• A good rule of thumb is that a thermistor
  resistance uncertainty of n works out to a
  temperature shift of approximately (n/3)C. This
  will help determine if any calibration is needed.
  Temperature calculations are only as accurate as
  the resistance measurement of the thermistor

15
Lets Get Started
PSoC

• Desired Topology
• Connect 10k Ohm from P05 to P01.
• Connect 10k Ohm thermistor from P01 to P03.
• Start Designer
• Name the Project Therm.

V0_Out
P05
REFHI
buf1
10k
InputAtten
P01
V1_In
Buffer
R
ADC
Therm
15R
REFLO
buf0
V2_Out
P03
16
EVAL1 Connections
10K
P05
Therm
P03
P01
Wire
17
Starting a New Project

• Open PSoC Designer
• Select Start new project

18
Starting a New Project

• Select Project Type

• Name The Project

19
Starting a New Project

• Select Device and Coding Method
• CY8C29466-24PXI
• C
• OK

20
Global Resource Settings
21
Select PGA UM

• Select PGA and name it InputAtten
• Insert into ACB00
• Set the PGA parameters to

• Atten Value set to 15/16
• Reference to AGND
• Input connected to column MUX to read all three
  points on the resistor string

22
Select Second PGA UM

• Select PGA and name it Buffer
• Insert into ACB01
• Set the PGA parameters to

• This UM generate API in multiplex the input
  lines.

23
Select AMUX4 UM

• Select an AMUX4 and rename it ADCMUX
• Set its parameter has shown.

24
Select ADCINC UM

• Select an ADCINC UM and rename it ADC.
• Select a single modulator and place it in ASC10.
• Select the clock to be VC2.
• Place the digital block in DBB0.

• Input connects to Buffer.
• PWM is not used

25
Select LCD UM

• Select and LCD UM and name it LCD.
• Connect to Port 2
• BarGraph is not needed.

26
Rename Buffers and Pins

• Connect the AnalogOutBuf_1 to P05.
• Rename this pin V0_Out.
• Connect the AnalogOutBuf_0 to P03.
• Rename V2_Out.
• Change PO1 to be an AnalogInput.
• Rename it V1_In.

27
Add Initialization Code
(Cut and Paste from File on CD)

• In the Initialization Section
• Add code to start Buffer, InputAtten, ADC, and
  LCD.
• Add code to connect REFHI to the column1 analog
  bus.
• Add code to connect REFLO to the column0 analog
  bus.
• Declare iV0, iV1, iV2, iRvalue to be global
  variables.
• Enable global interrupt.
• Declare bTempValue to be a global 8 bit
  variable.
• Add LookUp table.

28
LookUp Table Temperature Conversion

• The Steinhart-Hart equation requires using the
  floating point math library. Floating point is
  slow and uses buckets more ROM compared to
  integer math.
• An alternative is to use a look up table. For
  any particular thermistor, the manufacturer
  either supplies a R/T conversion table, or
  supplies the three Steinhart-Hart coefficients.
  If only the coefficients are supplied, a table
  can be generated from them. This particular
  thermistor has a R/T conversion table that
  supplied.

29
Create a Look Up Table

• Excel file ThermTable.xls contains the
  81resistance values for temperature for 0C to 80
  C.
• Calculate half values for ½C to 79 ½C using
  the following equation.
• ½ degrees are used for rounding.
• Add the value zero at the end.

30
Create a Look Up Table

• These values are used to make an ROM array
  WThermTable containing 81 values.

code is provided in lab file
31
Add Code Control Loop

• In the Control Loop
• Set ADCMUX to P05.
• Run ADC.
• Wait for data.
• Place in iV0
• Set ADCMUX to P01.
• Run ADC.
• Wait for data
• Place in iV1.
• Set ADCMUX to P03.
• Run ADC.
• Wait for data
• Place in iV2.
• Calculate Resistance.
• Display on LCD.

32
Add Code CalculateR

• If iV0lt-iV1 (Open Circuit)
• lRvalue -1
• Else If iV1 lt iV2 (Short Circuit)
• iRvalue 0
• Else
• Calculate Resistance
• Add half denominator tonumerator to
  implementround off.

33
Run

• Build the Project.
• Record RAM and ROM Usage.
• Download to the Eval board and run.
• Using the look up table determine the temperature.

34
Summary

• PSoC makes measure resistance a cost effective
  option.
• Temperature can easily be measure using
  thermistor with the Steinhart-Hart equation or
  look up table.

35
Questions