EDN -- 12.07.95 circuit linearizes, converts thermistor dat

-December 07, 1995

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Design Ideas:December 7, 1995

Circuit linearizes, converts thermistor data

Matthew Beckner,
Ball Corp
Boulder, CO

Although the response of thermistors is inherently nonlinear, you can make the devices' resistance over a selected range of temperatures linear. You can convert the thermistor value to a voltage for display on either a DMM or a panel meter. The circuit in Figure 1 performs linearization and provides gain and offset for a YSI 44031 thermistor, whose nominal resistance is 10 k[ohm] at 25ºC.

Resistor R1 in conjunction with the thermistor provides a 0 to 5V signal to the summing amplifier, IC1. R7, R9, and potentiometer R8 provide a variable offset that matches the thermistor response and establishes the proper output voltage for ambient temperature. Resistor R4 and potentiometer R3 provide variable gain that also matches the thermistor response.

fig 2 thumbnailFigure 2 shows a portion of an Excel spreadsheet designed to simplify component-value selection. Using this spreadsheet, you can enter thermistor values and modify the offset- and gain-resistor values to achieve the desired circuit response. You can find the spreadsheet, named THRMBIAS.XLS, in the compressed ZIP file attached to EDN BBS /DI_SIG#1763. The ZIP file also contains associated Excel-based graphs (with extension .XLC) illustrating the linearization performance.

The circuit in Figure 1 demonstrates accuracy within 1ºC over 0 to 501ºC and within 51ºC over -20 to +801ºC. The accuracy and range you achieve depend on the characteristics of the thermistor you choose and on your selection of the appropriate resistor values.

To calibrate the circuit, first adjust potentiometer R8 to achieve the required offset voltage from the spreadsheet at V1. Then, apply a known resistor at the thermistor input and adjust potentiometer R3 for the proper output (gain). Finally, connect the op amp's output to a DMM or to a voltage-input panel meter with a selectable decimal point. A ±2V meter displays temperatures from -199.9 to +199.91ºC, which is more than adequate for many applications. (DI #1763)

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