Many industrial control systems use a device that changes one form of energy to another; this device is called a transducer. Typical transducers convert mechanical, magnetic, thermal, electrical, optical, and chemical variations into electrical voltages and currents. Then, these voltages and currents are used, either directly or indirectly, to drive other control systems.

Because of the wide variety of solid state devices available today, many types of electromechanical transducers are being replaced with solid state transducers. We'll discuss one type of solid state transducer here, the thermistor.

Thermistor basics

What is a thermistor? Basically, it's a thermally sensitive resistor that changes resistance with a change in temperature.

How does it work? A thermistor's operation is a function of the electron-hole pair theory. As the temperature of the semiconductor rises, the electron-hole pair generation increases (due to thermal agitation). Increased electron-hole pairs causes a drop in resistance and a resultant increase in current flow.

There are several types of thermistors. Because of their small size, they can be mounted in places inaccessible to other temperature-sensing devices. One popular application is thermal protection in a motor, where a thermistor is embedded in the motor's coil.

The resistance range of thermistors is from a few ohms to megohms. The change in resistance, as a function of temperature, can be linear or nonlinear. With a linear thermistor, the resistance changes the same amount for each degree of temperature change. With the nonlinear thermistor, the resistance will vary dramatically in different temperature ranges.

Thermistors are relatively sensitive devices and can provide fractional degree temperature control. For example, some thermistors can be accurate to [+ or -]0.1 [degrees] C.

Because they are resistive devices, they can operate on AC or DC systems.

Fig. 1 shows the symbols for a directly heated and an indirectly heated (externally heated) thermistor. Directly heated thermistors are used in voltage regulators, vacuum gauges, and electronic time-delay circuits. Indirectly heated thermistors are used in precision temperature measurement and compensation applications.

Thermistor classes

There are two classes of thermistors: positive temperature coefficient (PTC) and negative temperature coefficient (NTC). With NTC, an increase in temperature causes a decrease in thermistor resistance, as shown in Fig. 2.

With PTC, a relatively small increase in temperature at the switch temperature point causes an extremely large increase in thermistor resistance, as shown in Fig. 3. A PTC thermistor will have relatively low resistance levels during its ON state and relatively high resistance levels during its OFF state.

Most thermistors operate on an NTC, although some applications require operation on a PTC.

NTC applications

NTC applications include fire alarm circuits, where the resistance of the thermistor is high because the ambient temperatures are relatively low. This high resistance keeps the current to the control circuit low; thus, the alarm remains deenergized. When a fire occurs, the increased ambient temperature lowers the resistance of the thermistor, allowing the current to the alarm device to increase and activate it.

NTC thermistors are also used for incandescent lamp life extension. Here, the thermistor acts as a shock absorber for the lamp. When the circuit is energized, a high percentage of the available voltage is dropped across the thermistor. As the thermistor heats up, its resistance drops and more current passes to the lamp.

Another NTC thermistor application is temperature compensation. Here, the thermistor is wired in series with a resistor located within a thermostat. When the thermostat senses a decrease in outdoor temperature, the NTC thermistor increases its resistance; this causes current to flow in the circuit. The net result is reduced thermostat variations and improved operation of HVAC systems such as heat pumps.

Still another application involves flow measurement. Here, two thermistors are used. The first thermistor is installed in the path of flow; the second thermistor is shielded away from the flow. Both are directly heated. If the flow rate is large, the thermistor in the path of the flow is much cooler than its counterpart. Because of this temperature differential, the output signal of the thermistor in the flow path will be large. If the flow is not as rapid, heat is not carried away from this thermistor as rapidly. As a result, the magnitude of its output signal will be smaller.

PTC applications

One popular PTC application is motor starting, where the thermistor, having a low resistance, permits most of the line voltage to be applied to the starter winding. As the motor starts up, the PTC thermistor heats up until the switch temperature (the temperature at which the resistance begins to increase rapidly) is reached. Then the thermistor rapidly changes from a low-resistance device to a high-resistance one. At this point, no current flows through the thermistor or the starter winding, which can be considered removed from the circuit.

Another PTC application involves arc suppression. The thermistor switches from a low resistance to a high resistance when the circuit switch is opened; this provides effective arc suppression. In addition, the PTC switching action transfers almost all of the power supply voltage from the load directly to the PTC thermistor itself.

Testing thermistors

It's important to check thermistor connections in electronic circuits because loose or corroded connections will create a high resistance that is in series with the thermistor's resistance. When this happens, the control circuit will sense the additional resistance as the correct temperature reading, which, in reality, is a false reading.

The hot and cold resistance of a thermistor can be checked with an ohmmeter; however, one end of the thermistor must be disconnected from the circuit. The procedure is as follows.

* Connect the ohmmeter leads to the thermistor leads and place the thermistor and a thermometer in a mixture of ice and water.

* Record the temperature and resistance readings.

* Place the thermistor and thermometer in hot water (not boiling).

* Record the temperature and resistance readings.

* Compare the hot and cold readings with the thermistor manufacturer's specification sheet, or with a similar thermistor known to be good.

If the PC board on which the thermistor as well as other components are mounted cannot be kept away from the water, a temperature testing unit should be used as the standard temperature source.