19-03-2012, 02:34 PM
Thermistor
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Thermistors are widely used for temperature sensing purposes (sensitivity, accuracy, reliability)
Thermistors are temperature dependent resistors
Most common: Negative-Temperature Coefficient (NTC) thermistors
NTC themistors have nonlinear R-T characteristics
Steinhart-Hart equation is widely used to model the R-T relationship.
Examples: thermistors in your car
Air conditioning and seat temperature controls.
Electronic fuel injection, in which air-inlet, air/fuel mixture and cooling water temperatures are monitored to help determine the fuel concentration for optimum injection.
Warning indicators such as oil and fluid temperatures, oil level and turbo-charger switch off.
Fan motor control, based on cooling water temperature
Frost sensors, for outside temperature measurement
Basic characteristics of thermistors
1) Operating temperature range
(2) Zero power resistance of thermistor
R=R0expB(1/T-1/T0), T, T0 are ambient temperatures, R, R0 are corresponding resistances and B is the B-constant (or β constant ) of the thermistor
Or B=ln(R/R0)/(1/T-1/T0)
(3) Since thermistor is a resistor, power dissipation
P=C(T2-T1), where C is the thermal dissipation constant (mW/ºC). This causes self-heating.
(4) Thermal time constant