Thermistor

The thermistor an electronic solid-state device known as a semiconductor and requires an electronic circuit to utilize its capabilities. It varies its resistance to current flow based on its temperature.

The thermistor can be very small and will respond to small temperature changes. The changes in current flow in the device are monitored by special electronic circuits that can stop, start, and modulate machines or provide temper­ature readout.

Thermistors are usually made of cobalt oxide, nickel, or manganese, and from a few other materials. These materi­als are mixed and milled in very accurate proportions and then hardened for durability. A thermistor can have either a positive or negative coefficient of resistance. If the ther­mistor increases its resistance as the temperature in­creases, it has a positive temperature coefficient (PTC) of resistance. If it decreases its resistance as the temperature increases, it has a negative temperature coefficient (NTC) of resistance. Also, some thermistors can double their re­sistance with as little as one degree of temperature change. Thermistors are used in solid-state motor protectors, motor-starting relays, room thermostats, electronic expan­sion valves, duct sensors, micron gages for vacuum pumps, heat pump controls, and many other control schemes.

At the instant of start, the PTC thermistor has a very low re­sistance. This allows a large inrush of current to help in­crease the starting torque of the motor. However, after a fraction of a second, the thermistor's resistance increases because its temperature increases from the high starting current. Now the thermistor is like an open circuit, and the motor is running like a normal PSC motor. Thermistors can also be used in a bridge circuit when measuring the vacuum in a refrigeration system. One of the thermistors is placed in the vacuum, while the other is placed in the outside ambient. As the vac­uum gets deeper and deeper, the heat cannot be transferred away as easily from the thermistor placed in the vacuum, because fewer molecules are surrounding it. This differ­ence in resistance of the two thermistors can now be sensed by the bridge circuit and calibrated into microns for vac­uum measurement.

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