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Sensing temperature

On numerous occasions, having the capability to detect temperature proves to be highly valuable. Industrial electricians will come across various instruments engineered to modify a set of connections in response to temperature fluctuations, as well as other tools employed to gauge temperature levels. The choice of method predominantly relies on the circuit's intended applications and the specific temperature measurements required.

Expansion of Metal One highly prevalent and dependable method for temperature sensing is through the expansion of metal. It has been widely recognized for a long time that when metal is heated, it undergoes expansion. The extent of this expansion is directly proportional to two key factors:

  1. The specific type of metal employed.
  2. The magnitude of temperature change. Let's take the metal rod depicted in Figure 1 as an example. As this rod is heated, its length increases, and when the metal is allowed to cool, it will contract. While the actual movement caused by contraction and expansion may be minor, a straightforward mechanical principle can be applied to amplify this movement.

Fig. 1 - Metal expands when heated

The metal bar is firmly secured at one end using mechanical means, allowing the expansion to occur in a single direction. When the metal undergoes heating and expands, it exerts force on the mechanical arm. Even a slight movement of the bar results in a significant displacement of the mechanical arm. This amplified motion in the arm can serve multiple purposes, such as indicating the temperature of the bar by attaching a pointer and scale, or actuating a switch, as illustrated.

It's important to note that these illustrations are employed to convey a fundamental concept. In practical applications, the switch depicted in Figure 2 would typically incorporate a spring-loaded mechanism to ensure swift and decisive contact action. Electrical contacts should never open or close gradually, as this can lead to insufficient contact pressure and may result in contact burning or erratic operation of the equipment they are meant to control.

Fig. 2 - Expanding metal operates a set of contacts

Hot-Wire Starting Relay

A widely used device that operates a set of contacts based on the principle of metal expansion is the hot-wire starting relay commonly found in the refrigeration industry. It earns its name from its use of a length of resistive wire connected in series with the motor to monitor motor current. A schematic of this relay type is depicted in Figure 3.

Fig. 3 - Hot-wire relay connection.

When the thermostat contact closes, electric current flows from line LI to terminal L of the relay. This current then traverses the resistive wire, the movable arm, and the normally closed contacts, eventually reaching the run and start windings. As current flows through the resistive wire, its temperature rises. This temperature increase causes the wire to elongate. With increased length, the movable arm is pushed downward, exerting tension on the springs of both contacts.

The relay is designed in a manner that prioritizes the opening of the start contact first, disconnecting the motor's start winding from the circuit. Under normal conditions, if the motor's current remains within acceptable limits, the wire never reaches a temperature high enough to trigger the overload contact to open. However, if the motor's current exceeds safe levels, the temperature of the resistive wire will rise sufficiently to make it expand to a point where it forces the overload contact to snap open, thereby disconnecting the motor's run winding from the circuit.

The Mercury Thermometer

Another highly valuable device that operates based on the principle of metal contraction and expansion is the mercury thermometer. Mercury is a metal that retains its liquid state at room temperature. When confined within a glass tube, as illustrated in Figure 4, it ascends the tube as it expands due to a rise in temperature. If the tube is appropriately calibrated, it furnishes a precise measurement of temperature.

Fig. 4 - A mercury thermometer operates by the expansion of metal.

The Bimetal Strip

Another device that relies on the expansion of metal is the bimetal strip, and it is possibly the most commonly used heat-sensing component in the manufacturing of room thermostats and thermometers. The bimetal strip is crafted by joining two dissimilar types of metals together, as illustrated in Figure 5. These two metals possess distinct rates of expansion due to their dissimilarity, causing the strip to flex or deform when subjected to heat, as seen in Figure 6. Frequently, a bimetal strip is shaped into a spiral form, as shown in Figure 7. This spiral configuration allows for the utilization of a lengthy bimetal strip within a confined space, which is advantageous because it results in a more pronounced movement in response to temperature variations.


Fig. 5 - 6. A bimetal strip warps with a change of temperature. 

When one end of the strip is mechanically secured, and a pointer is affixed to the center of the spiral, any temperature alteration will induce the pointer to rotate. If a calibrated scale is positioned behind the pointer, it functions as a thermometer. Alternatively, if the center of the spiral is anchored in place, and a contact is connected to the end of the bimetal strip, it transforms into a thermostat. A small permanent magnet is employed to achieve a snap action for the contacts, as depicted in Figure 8. When the moving contact approaches the stationary contact, the magnet attracts the metal strip, causing an abrupt closure of the contacts. As the bimetal strip cools down, it moves away from the magnet. Once the force of the bimetal strip exceeds that of the magnet, the contacts snap open.

Fig. 7 - A bimetal strip used as a thermometer.

Fig. 8 - A bimetal strip used to operate a set of contacts


In 1822, the German scientist Seebeck made a notable discovery: when two dissimilar metals are joined at one end and this junction is subjected to heat, it generates a voltage, as depicted in Figure9. This phenomenon is known as the Seebeck effect. A device created by combining two dissimilar metals with the intention of generating electricity through heat is called a thermocouple. The magnitude of voltage generated by a thermocouple depends on two key factors:

  1. The types of materials used in constructing the thermocouple.
  2. The temperature difference between the two junctions.

A thermocouple measures temperature using a pair of wires crafted from dissimilar metals. These wires are connected together at one end, often through welding. The distinct thermoelectric properties of these wires generate a minuscule voltage between their free ends, which can be used to derive the temperature at the connected ends.

Fig. 9 - Thermocouple.

No external power supply is required for a thermocouple, but it produces an extremely small voltage (measured not just in millivolts, but in microvolts). Furthermore, this voltage is highly nonlinear, necessitating hardware and/or software for conversion into a temperature value. Laboratory equipment or integrated circuit chips are available for this purpose.

Various types of thermocouples are accessible to measure different temperature ranges, and each type possesses its own unique characteristics, requiring appropriate conversion methods.

Fig. 10 - Thermopile

Figure 10 illustrates common types of thermocouples, showcasing the different metals employed in their construction and their typical temperature ranges. Typically, the voltage produced by a thermocouple is quite small, often on the order of millivolts (1 millivolt = 0.001 volt). The polarity of the voltage in certain thermocouples is temperature-dependent. For instance, a type "J" thermocouple yields 0 volts at approximately 320°F. At temperatures above 320°F, the iron wire becomes positive, and the constantan wire becomes negative. Conversely, at temperatures below 320°F, the iron wire becomes negative, and the constantan wire becomes positive. At a temperature of +300°F, a type "J" thermocouple generates a voltage of about +7.9 millivolts, while at -300°F, it produces a voltage of approximately -7.9 millivolts.

Fig. 11 - Thermocouple chart

Due to their low voltage output, thermocouples are often connected in series, as demonstrated in Figure 11. This arrangement is referred to as a thermopile. Thermocouples and thermopiles are primarily used for temperature measurements and are sometimes employed to detect the presence of a pilot light in appliances operating on natural gas. In this context, the thermocouple is heated by the pilot light, and the resulting current generates a magnetic field that keeps a gas valve open, allowing gas to flow to the main burner. If the pilot light extinguishes, the thermocouple stops producing current, causing the valve to close (Figure 12).

Fig. 12- A thermocouple provides power to the safety cut-off valve


Applications of Thermocouples

Thermocouples offer a broader temperature measurement range than any other type of contact temperature sensor, with certain types capable of accurately measuring temperatures as high as 1,800 degrees Celsius. The primary constraint lies in the joint between the thermocouple wires, which must endure high temperatures. Adequate insulation is essential, and if needed, ceramic tube segments are available to fulfill this role.

The remarkably low thermal mass of a thermocouple enables rapid responsiveness to temperature fluctuations. As thermocouples do not generate any self-heating, they consume no power. They are known for their simplicity and durability. However, their response curve is highly nonlinear, and the minute voltage signals they generate can be susceptible to interference from electrical noise.

Typically, the accuracy of thermocouples does not exceed plus or minus 0.5 degrees Celsius and may be less precise at lower temperatures. Thermocouples find common use in laboratories and various industrial applications, such as monitoring temperatures within a blast furnace or within the confines of an internal combustion engine.

They can also be employed for temperature measurements as low as -200 degrees Celsius. However, at temperatures below -100 degrees Celsius, the temperature coefficient decreases to the extent that voltage changes become less than 30µV per degree Celsius.

Resistance Temperature Detectors


The resistance temperature detector (RTD) is constructed from platinum wire, known for its significant variation in resistance with temperature. When platinum is heated, its resistance increases at a highly predictable rate, rendering the RTD an excellent device for exceptionally precise temperature measurements. RTDs are employed to gauge temperatures spanning from -328 to +1166 degrees Fahrenheit (-200°C to +630°C). RTDs are produced in various configurations to fulfill diverse functions.

Fig. 13 - Resistance temperature detector

Figure 13 illustrates a typical RTD used as a probe, where a minute coil of platinum wire is enclosed within a copper tip. Copper is chosen to ensure efficient thermal contact, allowing the probe to respond rapidly.

Fig. 14 - Temperature and resistance for a typical RTD

The resistance-temperature relationship for a standard RTD probe is portrayed in the chart in Figure 14, with temperature denoted in degrees Celsius and resistance expressed in ohms. Various case styles for RTDs are presented in Figure 15.



Fig. 15 - RTDs in different case styles.


The term "thermistor" is derived from the words of "thermal resistor." Thermistors are, in fact, thermally sensitive semiconductor devices, and they come in two primary types: those with a negative temperature coefficient (NTC) and those with a positive temperature coefficient (PTC). NTC thermistors exhibit decreasing resistance with rising temperature, while PTC thermistors see their resistance increase as the temperature climbs. NTC thermistors are the more commonly used variant.

Fig. - Schematic symbols representing a thermistor. The letter 't' preceded by a plus (+) or minus (-) sign indicates whether the thermistor is of the PTC or NTC type, respectively.

Thermistors display highly nonlinear characteristics, making them challenging to employ for direct temperature measurements. Devices utilizing thermistors for temperature sensing must undergo calibration specific to the particular thermistor type being used. If a thermistor is ever replaced, it must be an exact match, or the circuit will cease to function accurately. Due to their nonlinear attributes, thermistors are often employed as set point detectors rather than instruments for precise temperature measurement. A set point detector is a device that triggers a specific process or circuit when the temperature reaches a predetermined level.

Fig. 16 - Solid-state starting relay.

For instance, consider a scenario where a thermistor is placed inside the stator winding of a motor. If the motor were to overheat, it could cause significant damage or complete destruction of the windings. The thermistor can detect the winding's temperature, and when it reaches a certain threshold, the thermistor's resistance changes sufficiently to deactivate the starter coil, disconnecting the motor from the power source. Thermistors can function in temperatures ranging from approximately -100°F to +300°F.

Fig. 17 - Connection of solid-state starting relay.

One common application of thermistors is in solid-state starting relays used with small refrigeration compressors (Figure 16). These starting relays are used with hermetically sealed motors to disconnect the start windings from the circuit when the motor reaches around 75 percent of its full speed. Thermistors are suitable for this application because they exhibit an extremely rapid change in resistance with temperature fluctuations. A schematic diagram depicting the connection for a solid-state relay is shown in Figure 17.

Fig. 18 - Constant current generator.

Fig. 19 - Field effect transistor used to produce a constant current generator.

Upon initial power application to the circuit, the thermistor is cool and has a relatively low resistance, allowing current to flow through both the start and run windings of the motor. The thermistor's temperature increases due to the current passing through it, causing its resistance to shift from a very low value of 3 or 4 ohms to several thousand ohms. This resistance increase is sudden and results in the opening of a set of contacts connected in series with the start winding. While the start winding is never entirely disconnected from the power source, the current flowing through it is minimal, typically 0.03 to 0.05 amps, and does not impact the motor's operation. This small leakage current maintains the thermistor's temperature, preventing it from returning to a low resistance state. After disconnecting power from the motor, a cooldown period of approximately 2 minutes is recommended before restarting the motor. This cooldown duration allows the thermistor to return to a low resistance state.

The PN Junction

Another device capable of measuring temperature is the PN junction or diode. The diode has gained popularity as a temperature measurement device due to its accuracy and linearity.

Fig. - A basic circuit for demonstrating the temperature sensitivity of a diode. Right, an NPN transistor can be substituted to emulate the diode.

When a silicon diode is utilized as a temperature sensor, a constant current is passed through the diode. Figure 18 illustrates this circuit configuration. In this setup, resistor R1 restricts the current flowing through the transistor and sensor diode. The value of R1 also determines the amount of current passing through the diode. Diode D1 is a 5.1-volt zener diode employed to maintain a consistent voltage drop between the base and emitter of the PNP transistor. Resistor R2 controls the current flowing through the zener diode and the transistor's base. D1, a standard silicon diode, serves as the temperature sensor for the circuit. When a digital voltmeter is connected across the diode, a voltage drop ranging from 0.8 to 0 volts can be observed. The magnitude of this voltage drop is dependent on the temperature of the diode.

Another circuit suitable for constant current generation is presented in Figure 19. In this setup, a field-effect transistor (FET) is employed to generate a current source. Resistor R1 determines the amount of current that flows through the diode, with diode D1 serving as the temperature sensor.

If the diode experiences a lower temperature, such as when it comes into contact with ice, the voltage drop across the diode increases. Conversely, if the diode's temperature rises, the voltage drop decreases because the diode exhibits a negative temperature coefficient. As the temperature increases, the diode's voltage drop diminishes.

Expansion Due to Pressure

Fig. 22. Bellows contracts and expands with a change of refrigerant pressure.

Another commonly employed method for detecting temperature changes involves the use of pressure changes in certain substances. For instance, refrigerants confined within a sealed container will experience an increase in pressure within the container as the temperature rises. If a straightforward bellows is connected to a line containing refrigerant (as depicted in Figure 21), the bellows will expand when the pressure inside the sealed system increases. Conversely, when the ambient air temperature decreases, the pressure within the system decreases, causing the bellows to contract. When the air temperature rises, the pressure increases, leading to the expansion of the bellows. If the bellows is connected to control a set of contacts, it functions as a bellows-type thermostat. Figure 22 illustrates a bellows thermostat along with the standard NEMA symbols utilized to represent a temperature-operated switch.










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