THERMISTOR CALC HELP

 

Introduction

Thermistor Calc is the easiest way to design and analyze thermistor/resistor circuit networks for linearizing sensor signal vs. temperature response curves.

Options are provided for analyzing simple and complex thermistor/resistor circuits alone, or included as part of inverting and non-inverting op amp circuits.

Results are displayed as tables and graphs of the thermistor network, the op amp circuits, and linear regression lines calculated form the data. Statistical data is provided to judge the “goodness” of the circuit’s linearity.

Expanded results can be exported for more detailed analysis in any spreadsheet program.

Graphs can be printed directly from the program.

Projects can be saved and reloaded to/from project files.

A glossary is provided at the end of this document to explain terms used in this help file.

Quick Start

Click on the radio button to select either the inverting or non inverting circuit diagram.

Enter the component values and parameters in the “Circuit Component Parameters” table.

Press the CALC button.

Review the results in the “Results Table”.

Click on the note book tabs to view the graphs and the stats.

Component Value Table

 

 

TIP : The thermistor network is not designed to be used as is. It is a general purpose all inclusive circuit that can be used to analyze various simpler circuits of two or three components simply by setting some values to zero ohms or open circuit, essentially eliminating those components from the circuit.

 

There are a few simple rules for entering the values in the Component Parameter table:

1)       Any component can be a thermistor or a resistor.

2)       A nonzero entry for the “B Constant” makes the component a thermistor. A thermistor’s TC must be 0.

3)       To open circuit a component, leave the value field blank or type in “OPEN”.  Any other non-number gibberish will also open circuit the component.

4)       To short circuit a component set its value to 0 (zero ohms). Zero ohm components can have either a TC or B Constant but to no effect.

TIP : Some circuits have the thermistor and resistor network on the same circuit board, and some have the thermistor located remotely with the sensor head. If the thermistor is located remotely set all the TCs to zero, since they will not be subjected to the same temperature variations as the thermistor.

Calculated Results Table

 

Calculated results are tabulated over a 0 to 100^O C temperature range. Each component has a column to display its resistance at 5O C increments.

Additional columns show the thermistor network’s resistance (Rfb) and overall circuit gain vs. temperature.

More detailed tabular information is available from the export function ( FILE | EXPORT menus ).

Circuit Diagrams

 

Click on the radio button for either inverting or non-inverting circuits.

Changing the circuit type will erase any results previously displayed. Press the Calc button to display results for the selected circuit type.

Results Graphs

 

 

It is easy to visually judge the linearity, or shape, of the circuit results with the results graphs.

Select from the three notebook tabs to view graphs for circuit gain or the resistance of the feedback network (Rfb).

The red series on each graph is the calculated results for gain and Rfb vs. temperature.

The green line is a linear regression line calculated from the calculated circuit results vs. temperature.

Click and drag a box on the graph to get a closer look at an area of interest; for a limited temperature range for example.

Keep in mind that the graph will remain zoomed in, and if you change component values and re-calculate, your results may disappear from the graphs zoomed in view.

Double click anywhere on the graph to un-zoom

 

Results Statistics

 

 

Stats are provided to judge the linearity of the circuit gain and Rfb. The stats are calculated from the linear regression line that is displayed in the results graphs described above.

The slope for circuit gain is gain unit per degree C. The slope for Rfb is ohms per degree C.

The offset values are useful if you transfer the information to a spreadsheet for further analysis.

The correlation coefficient (Corr Coef) is the “goodness of fit” stat. A perfectly linear response would result on a correlation coefficient of 1.0000000.

The standard error estimate (SEE) is another “goodness of fit” stat.  A smaller number indicates a more linear of circuit result.

The maximum deviation is the largest deviation, between the regression line and the calculate circuit results, that occurs in the range of temperatures.

The maximum deviation percent (Max Dev %) is 100 times the maximum deviation divided by the value estimated by the regression line at the maximum value’s temperature.

The last stat is the temperature of the maximum deviation (T of Max Dev).

 

Reference Temperature

 

 

Most thermistors are specified with a 25O C reference temperature.

At the reference temperature all the resistors and thermistors in the circuit will have their nominal resistance value.

 

The File Menu

Open

Load a previously saved project.

Save As

Save the current project component values. Neither the circuit type (inverting or non-inverting) nor the calculated results are saved.

Export CSV File

This writes a CVS (comma separated value) file, of all project information and results.

CVS file results are more detailed and include data columns that are to extensive to display in the program screen.
 

TIP : Use the CVS file data to build a model of your system design including the response of the signal source and any additional signal processing. Make a multi-page spread sheet with one page dedicated to importing the CSV file. Other pages, with the model, would then reference the import page for subsets of the data.

 

Print Graphs

The Print Graphs menu item will print both the gain and Rfb charts. Use the printer setup menu to set the printer to portrait or landscape mode. In landscape mode the graphs are printed on two pages; in portrait mode both graphs are printed to a single page.

Glossary of Terms

TC :  Temperature Coefficient – Usually expressed in parts per million (PPM).

Rfb :  Resistance of the feedback network. The feedback network consists of components R1 thru R4. Rfb is included  so you can use the network in other types of circuit. Use the Rfb calculated results when you have a circuit other than those included with this program.

B Constant :  A component parameter specific to thermistors defines the resistance response curve with respect to temperature using the exponential model.
 

	The formula is:  R = R0 * exp( B * (1 / T - 1 / T0 ))
	where -	R is the thermistor's resistance at temperature T in degrees Kelvin  ( oK  ).
		R0 is the thermistor's nominal resistance at reference temperature T0 in degrees Kelvin
		B is the thermistor's B Constant as specified by the manufacturers data sheet
		T0 is thermistor's the reference temperature as specified by the manufacturers data sheet, normally 298.16 oK (273.16 + 25 oC)
		T is  temperature of interest in degrees Kelvin ( oK  = 273.16  + oC)

            

Correlation Coefficient :  This number, between plus and minus one (-1.0 to 1.0) measures the amount of agreement between two variables, meaning how close the graph drawn linking to two is to a straight line with 1.0 representing perfect linearity in this case.

Linear Regression :  A technique in which a straight line is fitted to a set of data points to measure the effect of a single independent variable. The slope of the line is the measured impact of that variable.

Slope : The rate at which the linear regression curve fit of the resistance Rfb or the gain changes over temperature.

Offset :  Indicates where the linear regression line intersects the axis (either Gain or Rfb).

SEE :  Standard Error Estimate is a measure of the quality of the curve fit coefficients.