By J. A. "Sam" Wilson, CET
The resistivity of some materials vary widely according to the heat.
Examples are given of passive transducers that use thermistors to sense temperatures in industrial applications.
In Part 1, we defined active transducers as sensors that generate a voltage. By comparison, passive transducers undergo a change of resistance, capacitance, or inductance while they are sensing some form of energy.
Resistors used as passive transducers to sense temperatures will be discussed this month.
Laws and Effects of Resistor Transducers
Physical laws and effects needed for proper understanding of resistor transducers are shown in Figure 1.
Current flowing through a resistor always produces these two effects:
• a voltage drop occurs across the resistor; and
• heat is produced in the resistor.
The resistance of any material can be calculated from the basic equation: Resistance equals the length (in centimeters) divided by the area (in square centimeters) times the specific resistance (rated in micro-ohms per cubic centimeter). Also, the resistance of most materials changes with temperature.
Therefore, it is necessary to consider the temperature along with the value of resistivity.
When the resistance increases with rising temperature, the material is said to have a positive temperature coefficient. Or, if the resistance decreases from a rising temperature, the material has a negative temperature coefficient.
Metals usually have a positive temperature coefficient (PTC), while semiconductors often have a negative temperature coefficient (NTC).
Bolometers
Bolometers are passive tempera ture-sensitive transducers. They are made of materials that have a large change of resistance when their temperature is varied. There are two general types of bolometers.
A barretter (called a resistance thermometer) is made with a very thin thread of metallic wire that's suspended between two contacts.
This wire often is made of platinum.
A thermistor is made of semiconductor material, and usually has an NTC. However, Texas Instruments (and other manufacturers) makes a thermistor of heavily doped semiconductor material that has a PTC. It's called a sensistor.
Thermistors are most often found in industrial equipment. Figure 2 shows three types of thermistors.
Bead-type thermistors have glass envelopes filled with an inert gas or evacuated to a vacuum.
Problems Of Measuring Temperature With Thermistors
A simple thermometer circuit with a thermistor sensor is shown in Figure 3. As the temperature that's being measured increases, the thermistor resistance decreases, thus increasing the current through the meter. The scale of the meter is calibrated in degrees, either Fahrenheit or Centigrade.
If the voltage source is not regulated, changes of supply voltage will produce nearly equal changes of meter current and reading. One solution to the voltage-regulation problem is to connect the thermistor in a bridge circuit (described later), because the supply voltage does not change the accuracy of bridges.
Another problem is that the current flowing through the thermistor increases the temperature of the thermistor. Therefore, any in creased current from higher temperatures in turn increases the thermistor temperature a second time, causing a reading error.
Now, this self-heating temperature rise could be compensated for by a change of meter calibration.
Unfortunately, the effect is not constant.
For example, the amount of self heating depends on the heat conduction of the medium that surrounds the thermistor. The effect is increased by mounting the thermistor in a vacuum. And the tempera ture will be significantly lower in water than when the thermistor is surrounded by motionless air.
In addition, the calibration resistor of Figure 3 also has a temperature coefficient, and changes of the ambient heat varies its resistance and current.
There is even a small amount of coil resistance charge inside the meter, because the wire too has a temperature coefficient.
Of course, the self-heating effect of thermistors can be minimized by using the smallest current possible.
This also reduces the drifting of meter and calibration resistor.
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Figure 1 Current flowing through a resistance produces' a voltage drop
and heat, illustrated by A. The graph at B gives typical resistance versus
tempera- ture curves for PTC' and NTC materials.
Figure 2 The tiny thermistor at lower left can have either PTC or NTC,
depending on the material used, and of ten is encased in a glass bead.
A "washer" type of thermistor is shown at center top. This
one has NTC, and it is intended for use in automatic degaussing A "Globar" type
that resembles a conventional carbon resistor is pictured at the lower
right.
Figure 3 A thermistor, and a current meter can, be used to measure the
approximate temperature. The calibration control adjusts for minor variations
of individual thermistors (this one has NTC). Any variation of the supply
voltage changes the reading, so the stability, is not very good.
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Figure 4 Although the self-heating of thermistors from the current in
the circuit ordinarily is considered a shortcoming that limits the accuracy
and stability, the self-heating effect can be used to indicate whether
the thermistor is in liquid or air. When immersed in a liquid, the NTC
thermistor runs cooler (higher resistance) than after the tank runs dry
and the thermistor is surrounded by air.
In air it operates hotter, has a lower resistance for increased current, and the stronger current trips a relay which lights a warning sign.
Table 1 CONDITIONS OF FIGURE 4
Figure 5 The temperature meter of Figure 3 can be made immune to the
effects of a changing supply voltage by rewiring it as a bridge. As is
usual with all true bridges, the null (or zero-current point) first must
be found by watching the zero-center meter and varying the calibration
control. After the null is found, the temperature is read from the dial
markings of the calibration control. (Another version gives automatic
meter readout from a meter scale similar to that in Figure 3, but the
accuracy at extreme readings is affected slightly by any supply-voltage
changes. The schematic is the same as this one; the difference is whether
the meter or the calibration control provides the reading in degrees.)
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A Benefit from Self-Heating
The self-heating effect of Figure 3 can be used to an advantage in one kind low level When of circuit that indicates a of liquid (see Figure 4), the level of the liquid is continued on page 54 Industrial Electronics continued from page 53 high, the thermistor is immersed in the liquid. The temperature from self-heating is small because the liquid carries the heat away more quickly than if the thermistor were in air.
However, when the liquid level is low, exposing the thermistor to air, the thermistor temperature in creases because of the self-heating, thus decreasing the resistance and increasing the relay current enough to close the contacts and light the "Empty" warning sign. Table 1 summarizes the various actions.
ThermistorBridge Circuits
Figure 5 shows how a thermistor thermometer can be changed to a balanced-bridge circuit that is not sensitive to variations of supply voltage. When the bridge is balanced, the resistances conform to this equation: R1/R3=R2/R4, and the meter has no current.
If the temperature changes, the thermistor changes resistance, thus unbalancing the bridge and sending through the meter a current of an amount and polarity that depends on the temperature.
The formulas for the voltages at points A and B are given in Figure 5, and they show that those voltages are directly proportional to the supply voltage. In other words, if the supply voltage is increased or decreased, the voltages at points A and B both will be increased or decreased by the same amount. Therefore, calibration and accuracy of the circuit are not affected by any variation of the supply voltage.
In actual thermistor bridges, one of the other resistors in an adjustable type, so it can be varied to obtain correct calibration.
If greater sensitivity of the circuit is needed, the voltage between points A and B can be amplified.
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Figure 6 When the self-heating effect and the current both are large,
a significant amount of time must elapse before the thermistor current
stabilizes. The circuit is at A, and the current-versus-time graph is
at B. This can be used as a time-delay circuit.
Figure 7 Here is a practical application of the time-delay principle in Figure 6. The tube should not have plate voltage until the filament has operated for a certain time. The thermistor current builds up gradually, and the values are chosen so the relay trips just before the current stabilizes.
Figure 8 Temperature control in a small box, such as an oven for critical components, can be done by enclosing the heat-producing collector resistor and the NTC thermistor in the oven. Higher temperatures reduce the transistor bias and decrease the heat from R4.
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Time Delay by Thermistor
An appreciable amount of time is required for the self-heating of a thermistor to reach a stable point.
This effect can be used as a time delay. A graph of thermistor current versus elapsed time is shown in Figure 6, along with the circuit.
In practical instruments, the thermistor current trips a relay after it stabilizes at a high level (see Figure 7). The plate voltage is not applied until the self-heating in creases the current by a sufficient amount. Such circuits can be used with tubes requiring a filament preheating before the plate voltage and current are applied.
Closed-Loop Temperature Control
A thermistor and a transistor can be the special components of a closed-loop temperature control (Figure 8). Bias of the transistor is determined by the voltage divider made up of R1, R2, and R3. R2 prevents the forward bias from becoming so high that the transistor would be destroyed, and R1 is adjusted for the desired tempera ture in the small oven.
Heat for the oven comes from R4, the collector load resistor, which is inside the insulated box.
The oven heat determines the resistance of the thermistor, R3, that's also inside the oven.
Assuming that the oven tempera ture has reached the desired value, any increase of temperature de creases the resistance of R3 thermistor. The decreased resistance reduces the forward bias of Q1, thus reducing in turn the collector current and the heat emitted by R4. Sufficient heat cuts off the conduction of Q1.
If the oven temperature falls below the desired heat, the R3 resistance rises, applying more for ward bias to Q1, and causing increased collector current, which produces more heat inside the oven. These two heating and cooling actions operate gradually, without steps.
Next Month: Capacitive and inductive transducers will be explained next month.
(adapted from: Electronic Servicing magazine, Jul. 1977)
Also see: Part 3