Engineering B45

Thermistors

Short Answer

Fall, 2011

Name:

Procedure:

Attach the digital multimeter to each of the leads of the KNOWN (20 ) thermistor. Set up a stand and clamp in which to hold the thermistor. Use an appropriate scale on the digital multimeter to read about 400 .

Boil water in the Pyrex beaker on a hot plate. Using thermal gloves remove the Pyrex beaker from the hot plate and lower the thermistor (not the leads or alligator clips) into the boiling water. Put the thermometer in the water.

Measure the resistance and temperature beginning at about ~85oC and for each 5oC temperature drop to about 40oC. Record these measurements on the following data sheet.

KNOWN Thermistor	UNKNOWN Thermistor

Temp (oC)	Resistance ()	Temp (oC)	Resistance ()

Graph resistance (y) vs. temperature (oC) (x) for this thermistor.

Fit an exponential model to the data using Trendline in Excel. Print this regression equation along with the R2 value on your graph. Attach this graph to your report.

Repeat steps 1-5 with the UNKNOWN thermistor.

Set up circuits using two digital multimeters and both the KNOWN and UNKNOWN thermistors. Immerse both thermistors in a beaker of boiling water and measure the resistance of each thermistor. Record on the next page.

Resistance () of KNOWN Thermistor for Boiling Water	Resistance () of UNKNOWN Thermistor for Boiling Water

Questions to be addressed:

Base resistance is the resistance value of a thermistor at a specified temperature with negligible electrical power to avoid self-heating. Usually base resistance will be defined at 25oC. What is the accepted base resistance of your KNOWN thermistor?

Classify each thermistor as either PTC or NTC.

KNOWN Thermistor

UNKNOWN Thermistor

List the regression equation for each thermistor.

KNOWN Thermistor

UNKNOWN Thermistor

Using the regression equation, find the base resistance of the UNKNOWN thermistor. Show work in box.

Estimate the temperature of the boiling water using the resistance values measured and the regression equations recorded in (3).

UNKNOWN Thermistor KNOWN Thermistor

Find the percent discrepancy between the two estimates in (5). Show work in box.

What are some applications of thermistors?