A Thermistor is a circuit element that is sensitive to temperature changes, wherein it reacts by changing in its resistance. There are two types of thermistors, Negative Temperature Coefficient (NTC) and Positive Temperature Coefficient (PTC). The NTC thermistor would respond to an increase in temperature by an equivalent decrease in resistance while the PTC thermistor will respond by an equivalent increase in resistance. In this lab you will be given an NTC thermistor, which will have a response as shown in figure 4.1. This response of NTC thermistors can be represented using a third-order approximation in equation (1), known as Steinhart-Hart equation. In this equation,
Tand
Rcorrespondingly represent the temperature and resistance associated with the thermistor and
A,B,Care modeling parameters.
(1)/(T)=A+Bln(R)+Cln----(Eq.1)But for practical purposes a simpler relationship is used that typically fits the prescribed operating range of the thermistor. This simple relationship is given in equation (2).
(1)/(T)=(1)/(\beta )ln((R)/(R_(0)))+(1)/(T_(0))
T_(0),T- temperature in Kelvin
R- resistance of thermistor at temperature
T
R_(0)- resistance of thermistor at temperature
T_(0)
\beta - Beta value or B value that is defined for a specific thermistor by manufacturer in datasheet Temperature estimation problem: Consider that you are given a bridge circuit as shown in figure 4.2, where
R_(1)=R_(2)=R_(3)=3k\Omega and the thermistor has resistance of
3k\Omega @25\deg C. Determine the following, a. Voltage measured
V_(ba)across the bridge at
25\deg Cb. At a temperature
T,V_(ba)=3.5V. Estimate at what temperature
Tthis could happen.