 |
NEWPORT®
Interchangeable Thermistor Applications
|
Figure 1
 |
Figure 2
 |
|
Thermistors
can be used in a variety of ways. Here are a few typical applications.
If you have questions concerning these or other thermistor uses,
we will be happy to discuss them.
TEMPERATURE MEASUREMENT—A thermistor in one leg of a Wheatstone bridge circuit will provide precise temperature information. Accuracy is limited in most applications only by the readout device. See Figure 1. Since lead length between thermistor and bridge is not normally a limiting factor, this basic system can be expanded to measure temperature at several locations from a central point. Thermistor interchangeability and large resistance change eliminate any significant error from switches or lead length. See Figure 2.
DIFFERENTIAL
THERMOMETERS—For accurate indication of temperature differential,
two thermistors can be used in a Wheatstone bridge circuit. Thermistor
interchangeability simplifies circuit design and reduces the number
of components. See Figure 4. To measure heat loss in a
piping network, thermistors can be placed at various points and
the difference between these temperatures and the original temperature
monitored at a convenient location. Measuring air temperatures
at different elevations with reference to ground temperature is
useful for temperature inversion data and geological studies.
See Figure 5.
TEMPERATURE MEASUREMENT—A thermistor in one leg of a Wheatstone bridge circuit will provide precise temperature information. Accuracy is limited in most applications only by the readout device. See Figure 1. Since lead length between thermistor and bridge is not normally a limiting factor, this basic system can be expanded to measure temperature at several locations from a central point. Thermistor interchangeability and large resistance change eliminate any significant error from switches or lead length. See Figure 2.
|
Figure
3
|
METER
COMPENSATION—The coil resistance of a meter movement
changes with temperature, making the meter temperature-dependent.
Using the thermistor’s property of a high negative temperature
coefficient, the coil can be compensated so total resistance
due to temperature rise is essentially constant, allowing
the meter to be used over a wide temperature range with
minimal error.
See Figure 3. |
|
Figure
4
 |
Figure
5
 |
|
 |
TEMPERATURE CONTROL—A system can be designed using
a thermistor with a known temperature/resistance curve
to form one leg of an AC bridge and a variable resistor
calibrated in temperature to form another leg. When the
resistor is set to a desired temperature, bridge unbalance
occurs. This unbalance is fed into an amplifier which
actuates a relay to provide a source of heat or cold.
When the thermistor senses the desired temperature, the
bridge is balanced, opening the relay and turning off
the heat or cold.
See Figure 6. |
|
MASTER-SLAVE CONTROL—Occasionally there is a need to control one temperature with respect to another, such as a product going through a series of baths. The first bath acts as a master and uses a thermistor to sense temperature. Succeeding baths, also using thermistors, are slaves. When these thermistors are placed in the controller bridge, the slave baths can be kept at a temperature relative to the master bath. The master bath can be controlled with the system described earlier. The master-slave controller can be used for as many baths as necessary. See Figure 7. |
Figure 7 
|
Copyright 2003,
NEWPORT Electronics, Inc. All rights reserved.