The Versatility of Thermistors: Common Applications in Industry

Thermistors, or thermal resistors, are small, inexpensive devices that have a wide range of applications in industry. These devices are used to measure temperature, control heating and cooling systems and provide feedback for process control. They are also used in a variety of other applications such as industrial automation, medical equipment and consumer electronics. The versatility of thermistors is due to their ability to quickly and accurately respond to changes in temperature making them a valuable tool for industrial applications.

Thermistor Applications

The typical applications of thermistors are the following:

1. Temperature measurement

The thermistor is a sensitive heat detector and can be made as the sensing element in an electrical thermometer or pyrometer. The small bead thermistor may easily be installed in the nose of a temperature probe. Bead and disc thermistors, like thermocouples, may be attached directly to surfaces for the monitoring of temperatures. Figure A shows three circuits for temperature measurement. In the figure, the thermistor is connected in series with a constant DC voltage source E, calibration rheostat R, and DC milliammeter M. A microammeter may be employed for higher sensitivity. The thermistor resistance, and consequently the meter deflection, is a function of temperature. The meter scale may be calibrated in degrees with the aid of an accurate temperature potentiometer or laboratory thermometer and the sensitivity control rheostat R.

In Figure B an ohmmeter is used to measure the resistance of the thermistor directly. The ohmmeter circuit should pass the smallest practicable current through the thermistor, to minimize self-heating. A resistance versus-temperature calibration may be made for the ohmmeter scale, or the latter may be calibrated to read directly in degrees. In Figure C, a resistance bridge circuit has been substituted for the ohmmeter to check the resistance of the thermistor more accurately. At null, the thermistor resistance Rt = (R₁.R₃) /R₂. For direct reading, the dial of R₂ may be calibrated in degrees.

2. Heater control

A thermistor may be placed inside an oven or heat chamber to serve as a temperature sensor for automatic control of the heaters. The figure below shows a control circuit. The thermistor current is adjusted, by means of the sensitivity control-rheostat R, so that the relay is actuated a few tenths of a degree above the desired operating temperature of the oven. This opens the relay contacts and disconnects the heaters. As the temperature drops, the thermistor resistance increases, lowering the current and dropping the relay. This closes the contacts and reconnects the heaters to the power line.

3. Vacuum gauge

A bridge circuit, comprised of the four arms T₁, T₂, R₁., and R₂, is used in the figure below. One thermistor (T₁) is mounted inside the vacuum system; the other one (T₂) is supported in still air outside the system. Initially, the bridge is balanced to null by adjustment of R₂, before starting the evacuation. As the evacuation progresses, the resistance of T₁, changes since this thermistor cannot get rid of its heat fast enough in the thinning air in the chamber to maintain the original balance. The bridge then may be rebalanced by the readjustment of R₂. At any point, the resistance of T₁ is proportional to the extent of the vacuum. Assuming the resistance of T₂, remain constant, the dial of R₂ may be calibrated in vacuum units. A less accurate method is to use R₂, as a zero-set control for the initial balancing of the bridge, and then to interpret the unbalance current reading of meter M as an indication of the progress of evacuation, calibrating the meter scale in vacuum units.

4. Flowmeter

A two-thermistor bridge circuit also is used in the below figure for flow measurements in pneumatic or hydraulic systems. One thermistor (T₁) is mounted in the direct flow of the liquid or gas; the other one (T₂) is supported in a quiet environment outside the flow. The bridge is balanced initially, by adjustment of R₂, with the flow quiet. Subsequently, during flow, both thermistors self-heat. Thermistor T₁ can get rid of its heat by dissipating it into the flow, but T₂, cannot dissipate its heat as rapidly. This causes the bridge to become unbalanced. The circuit may be calibrated, with the aid of R₃, so the unbalance current indicated by meter M may be interpreted in units of flow. A more accurate method consists of rebalancing the bridge for each new flow condition and calibrating the dial of R₂, inflow units.

5. Electronic anemometer

The same circuit shown in the above figure may be used as an electronic anemometer for reading wind velocity if T₁, is placed in the wind and T₂, is maintained in still indoor air at a constant temperature. Such an anemometer has the advantage of freedom from moving parts.

R – F Wattmeter

The thermistor is invaluable for measuring r -f power. Its very low reactance permits its use at ultra-high – frequencies and microwaves. The below figure shows a bridge circuit for power measurement. The bridge is balanced initially without r – f input, by adjustment of R₃. R – F energy then is applied to the thermistor through the coupling capacitor C. This energy heats the thermistor, lowering its resistance and unbalancing the bridge. Rheostat R, then is adjusted to restore null. At this point, the hot resistance of the thermistor is Rt = (R₁.R₃)/ R₂. The dial of R₃, accordingly may be calibrated for direct readings in watts. Meter M is a zero-center DC galvanometer or microammeter. The r-f chokes RFC₁, and RFC₂, keep r-f energy out of the DC measurement circuit.

6. Limiter

The nonlinear resistance of the thermistor may be employed in simple circuits for signal limiting, peak compression, and voltage regulation. Resistor R and the thermistor form a voltage divider with the upper output terminal of the circuit connected to the tap between them. The thermistor and resistor are chosen so that, at the normal desired output voltage, the thermistor resistance is high. If the input voltage, then increases, current I increase and lowers the resistance of the thermistor. This, in effect, causes the tap to move down the voltage divider, reducing the output voltage to its original level. In this way, the output voltage is stabilized. Several of these single stages may be cascaded for increased voltage regulation or signal compression.

7. Expander

In the figure below the thermistor is connected ahead of resistor R. The resistance of the latter is chosen low with respect to the thermistor resistance so that the thermistor resistance will be the most effective in determining the current I. The output voltage is the voltage drop IR across the resistor. A small change in input voltage produces a large change in thermistor current, and this, in turn, produces a large change in the output voltage drop across resistor R. This circuit must not be misconstrued as an amplifier; it magnifies the ratio of change of input voltage but not the absolute voltage. The input voltage itself actually is reduced by the voltage-divider action of the resistor and thermistor in series. Several of these single stages may be cascaded for increased expander action.

8. Time-delay relay

The time-delay effect evidenced by a thermistor after switching on its current may be utilized to delay the pickup of a relay. The figure below shows the simple circuit. The thermistor is connected in series with the relay coil. A time interval follows the closure of switch S before the current I reaches a level high enough to actuate the relay. The delay interval with a given thermistor may be controlled over a reasonable range by adjustment of rheostat R. When switch S is opened, the relay drops out immediately.

9. Sequential switching circuit

After switch S is closed in the circuit shown in the Figure below, the flow of current into the various loads RL₂, to RL₅ starts at various times, depending upon the time-delay characteristics of the thermistors T₁ to T₄. By suitable choice of thermistors, this action may be made sequential. Thus, RL₁ is energized immediately, since there is no thermistor in this leg of the circuit; and if the time delay of each thermistor is longer than that of the preceding one, the other legs will operate in sequence (RL₂, then RL₃, next RL₄, and finally RL₅). If this circuit is employed for driving or triggering other circuits, RL₁, to RL₅, may be actual resistors, and output voltages taken from terminals a to e and the common terminal f.

10. Selector switching circuit

In Figure below, when any switch S₁ to S₄ is closed, the operating current will flow through the corresponding load RL₁ to RL₄, but the only negligible current (“zero current”) will flow through any other load if its switch also is closed. The on leg prevents any other legs in the circuit from being switched on; thus, only one leg can be on at a time. The thermistors are chosen so that their resistances with respect to the accompanying load resistance will permit maximum current flow. When one thermistor is conducting heavily, the voltage drop across the common series resistance R₀ reduces the voltage at all other thermistors to a level too low for any of the others to conduct heavily at the same time. Only when the switch in the conducting leg is opened will the starting condition be restored and another leg is operable.

Although mechanical switches are shown here for simplicity in illustration, they might be electronic switches.

11. R – C Oscillator stabilization

The Wien-bridge oscillator circuit is widely used in low-distortion, R-C-tuned audio, and supersonic signal generators at frequencies up to 1 megacycle. The figure below shows a portion of the oscillator circuit. In this arrangement, positive feedback for oscillation is transmitted through C₃, and the R-C circuit R₁– C₁– R₂– C₂. Negative feedback for stability is provided through C₃, R₂, and the thermistor. The cathode of tube V₁ is tapped to the junction of R₂ and the thermistor. The nonlinear resistance of the thermistor automatically regulates the amount of negative feedback and stabilizes the cathode voltage, since very large changes in the thermistor current fault in only small changes in the cathode voltage — the voltage drop across the thermistor.

12. Automatic gain control

The figure below shows one arrangement for automatic gain control of an audio amplifier. Here an input signal-voltage divider is formed by R₁ and a thermistor in series. This is an indirectly rated thermistor, and its heater element is connected to the low-impedance output of the amplifier. When the output signal rises above a predetermined level in response to an input -signal increase, the heater element is energized. This heats the thermistor and lowers its resistance, causing the voltage divider to lower e signal presented to the amplifier input terminals. The signal level at which this action occurs is governed by the setting of rheostat R₂. When the input and output signals fall, the opposite action takes place. In this way, the gain is stabilized at the desired level. The output characteristics of the amplifier must be such that connection of the heater element of the thermistor does not introduce distortion serious output-power loss.

13. Compensation of Transistor D – C Bias

To prevent thermal runaway and the possible destruction of transistors, and to maintain the proper operating point of a transistor circuit, the DC base bias must be stabilized. The transistor currents increase with temperature, so automatic temperature compensation is imperative. The figure below shows the use of a thermistor to stabilize a transistorized power amplifier against collector-current variations due to temperature. Here the thermistor is shunted across the lower leg R₁, of the base bias-voltage divider R₁, R₂. The thermistor is mounted close to the transistors so as to experience the same temperature environment. As the temperature increases, the resistance of the thermistor decreases. This lowers the total resistance of the lower leg of the voltage divider and reduces the DC base voltage, lowering the collector current to its initial, safe low value.