17-12-2012, 02:26 PM
What is reason to select range of resistance according to given temperature?
1What is reason.docx (Size: 65.99 KB / Downloads: 49)
Thermistor:
A Thermistor is a temperature dependent resistor. When temperature changes, the resistance of the thermistor changes in a predictable way. Thermistors are inexpensive, easily-obtainable temperature sensors. They are easy to use and adaptable. Circuits with thermistors can have reasonable output voltages not the milli-volt outputs thermocouples have. Because of these qualities, thermistors are widely used for simple temperature measurements.
Characteristics of Thermistor:
The following are typical characteristic for the popular 44004 thermistor from YSI:
Basic reason of choosing resistance value is material of thermistor from which it made. It is according to given equation:- logR=A+B∕(T+θ). Because practically small physical size is applicable but if resistance value becomes out of range than size increase due to which response time becomes low & high resistance leads to self-heating errors.
A problem with the thermistor is the varying measured temperature resolution that is achieved over the temperature range. Usually the resolution is good at lower temperatures, but poor at higher temperatures. If the measuring device has a single scale, this can be an irritating characteristic. One way to simply fix this problem is to connect a resistor in parallel with the thermistor. The resistors value should equal the thermistor's resistance at the mid-range temperature.
Temperature Effects
The value of a resistor changes with changing temperature, but this is not as we might expect, mainly due to a change in the dimensions of the component as it expands or contracts. It is due mainly to a change in the resistivity of the material caused by the changing activity of the atoms that make up the resistor.
Materials which are classed as conductors tend to increase their resistivity with an increase in temperature. Insulators however are liable to decrease their resistivity with an increase in temperature. Materials used for practical insulators (glass, plastic etc) only exhibit a marked drop in their resistivity at very high temperatures. They remain good insulators over all temperatures they are likely to encounter in use.
The reasons for these changes in resistivity can be explained by considering the flow of current through the material. The flow of current is actually the movement of electrons from one atom to another under the influence of an electric field. Electrons are very small negatively charged particles and will be repelled by a negative electric charge and attracted by a positive electric charge. Therefore if an electric potential is applied across a conductor (positive at one end, negative at the other) electrons will "migrate" from atom to atom towards the positive terminal.
Conclusion:
Most conductive materials change specific resistance with changes in temperature. The resistance-change factor per degree Celsius of temperature change is called the temperature coefficient of resistance. This factor is represented by the Greek lower-case letter "alpha" (α). A positive coefficient for a material means that its resistance increases with an increase in temperature. Pure metals typically have positive temperature coefficients of resistance. Coefficients approaching zero can be obtained by alloying certain metals.