Introduction :
Thermistor is a word formed by combining thermal with a resistor. Thermistors such as RTDs are temperature-sensitive resistors. Thermistors are non-linear devices. Their resistance will decrease with increase in temperature but at a much faster rate than RTDs. The resistance can change by more than 1000 times. As a result, thermistors can sense minute change in temperature which are undetected by RTDs and thermocouples.
Metals used in thermistors :
Thermistors are small and inexpensive devices which are most commonly made of metal oxides such as those of chromium, nickel, manganese and cobalt. The metal are oxidized through a chemical reaction, ground to a fine powder, than compressed and subjected to high heat. Theses oxides are semiconductor.
Types of thermistors :
1)Beads type
2)metallized surface-contact type
1)Beads type :
Bead types have platinum wires sintered into a ceramic body(bead)
2)metallized surface-contact type :
In contrast to bead types, lead are not sintered directly into the ceramic. Instead , the sintered ceramic is coated with a metallic contact. Either the chip manufacturer or user attaches leads to this contact. One advantage of chip thermistors over bead types is that chip are easily trimmed by cutting or grinding. Thus, they are easy to match and therefore, they are interchangeable. While matched bead thermistors are available, they more than interchangeable chip.
Application :
1)It is used in the measurement of thermal conductivity.
2)It is used in the measurement of composition
3)It is used in the measurement of flow and pressure of liquids
4) It is used in time delay circuits
