Temperature sensors are among the most commonly used technologies in modern products such as automotive systems, household appliances, and industrial equipment. Choosing the right temperature sensor is important for reliable measurements. Understanding the advantages and disadvantages of different sensor types helps make the correct choice before measurement. The most common types are thermocouples, thermistors (NTC/PTC), resistance temperature detectors (RTD), and chip-based temperature sensors.
1. Thermocouples
Thermocouples are passive sensing elements that produce a measurable voltage in response to temperature. These self-powered sensors require no excitation and can operate over a wide temperature range (up to about 2000°C). They have fast response times and introduce minimal system delay.
Structurally simple, thermocouples are made from two dissimilar metal wires. The resulting output voltage is small (for a K-type, roughly 40 μV/°C) and requires precise amplification. Otherwise external noise, especially with long wiring between the thermocouple and measurement circuit, can distort the signal.
Another issue is the cold junction created where the thermocouple leads connect to copper signal wiring, which effectively forms a second thermocouple. To compensate for the cold junction, the cold junction temperature must be measured and the voltage it produces added to the thermocouple output:
Vtc = Vout + Vcj
where Vtc is the voltage generated by the thermocouple sensor and Vcj is the voltage associated with the cold-junction temperature.
The typical compensation arrangement places a temperature sensor at the cold-junction location to monitor that temperature, and an ADC provides the required resolution for the compensation calculation.
2. Thermistors
A thermistor is a temperature-dependent resistor. The term combines "thermal" and "resistor." Thermistors fall into two main categories:
| Type | Description |
|---|---|
| PTC | Positive temperature coefficient — resistance increases with temperature |
| NTC | Negative temperature coefficient — resistance decreases with temperature |
Temperature compensation — NTC thermistors are often used to temperature-compensate quartz crystal oscillator frequency. Compensation reduces the effect of temperature variations on temperature-sensitive elements.
Thermal time constant — thermal time constant is the time a thermistor takes to change by a specified fraction of the temperature difference from its initial ambient temperature T0 to final temperature T1. The time constant τ (in seconds) is defined as the time required for the thermistor to achieve 63.2% of the total change between initial and final body temperatures.
Voltage withstand — the voltage that a thermistor can withstand in still air at 25°C for three minutes. The test method increases voltage from 0 V progressively.
B constant — the B constant indicates thermistor sensitivity to temperature changes, expressed as a rate of resistance change. It can be calculated from resistance values at two specified temperatures using:
B = ln(R/R0) / (1/T - 1/T0)
R is the resistance at temperature T (K). R0 is the resistance at temperature T0 (K).
PTC vs NTC
Both are passive resistive devices that change resistance with temperature.
NTC (negative temperature coefficient) thermistors
- Passive resistive device
- Resistance decreases as temperature increases
NTC resistance response to temperature is typically nonlinear but often treated as approximately linear over limited ranges. NTC thermistors suit applications needing a continuous, monotonic change of resistance with temperature, such as temperature compensation, temperature control systems, and inrush current limiting.
PTC (positive temperature coefficient) thermistors
- Passive resistive device
- Resistance increases as temperature increases
PTC resistors change slightly with temperature until a switching point is reached, after which resistance can increase by several orders of magnitude. PTC devices are often used for self-resetting fuses and heater control applications.
3. RTD Temperature Sensors
A resistance temperature detector (RTD) is a sensor whose resistance increases with temperature and decreases with cooling.
Considerations when using RTDs:
1) Material selection: Not all metals are suitable for RTDs. Materials used for RTDs should meet these criteria:
- Resistance versus temperature should be close to linear.
- Sensitivity to temperature changes should be relatively large, i.e., a high temperature coefficient.
- Good durability and resistance to fatigue caused by temperature cycling.
Common RTD materials include platinum (Pt), nickel (Ni), and copper (Cu).
2) Resistance and temperature relationship
R0 is the resistance at 0°C. RTDs are often specified by their resistance at 0°C, for example Pt100, Pt200, Pt500, and Pt1000. Pt100 means the sensor has 100 Ω at 0°C; Pt1000 means 1000 Ω at 0°C.
Pt100 vs Pt1000 selection
RTD resistance is typically measured by applying a constant current source to the RTD and measuring the voltage across it with an ADC to derive resistance. Under the same excitation current, a Pt1000 yields ten times the voltage of a Pt100, offering higher sensitivity but also increasing voltage range at the ADC input. The choice depends on the ADC and surrounding circuitry.
Temperature coefficient α
RTD resistance at different temperatures can be approximated by:
R = R0 (1 + A t)
where R0 is the resistance at 0°C, A is the temperature coefficient representing resistance change per degree, and t is temperature in °C. A larger temperature coefficient means higher sensitivity.
For platinum RTDs, more accurate resistance-to-temperature fitting follows standards such as DIN EN 60751.
When R0 = 100 Ω: A = 3.9083 × 10^-3, B = -5.775 × 10^-7, C = -4.183 × 10^-12
3) Operating temperature range and accuracy
Manufacturers provide calibration temperatures (often 0°C). As temperature moves away from the calibration point, tolerance typically increases. Accuracy is specified for a given temperature range and must be considered during design.
Example: TE NB-PTCO-011 datasheet specifies 0.15% accuracy over -30°C to 300°C.
Figure 1 Accuracy vs temperature range
4) RTD wiring configurations
RTDs are available in two-, three-, and four-wire configurations. Three- and four-wire configurations effectively eliminate lead-wire resistance from the measurement by separating the measurement and excitation paths. The measurement path carries negligible current, minimizing the effect of lead resistance.
5) RTD application example — ratio measurement
Ratio configuration is an economical and suitable approach for RTDs. Using a four-wire example, the excitation current flows through both the sensor resistor and a precision reference resistor. Voltages across the sensor and reference resistor are measured separately. The excitation source does not need to be highly accurate because the same current flows through both resistors, and the comparison is based on their voltage ratio.
Choosing an ADC that supports ratio measurements, such as certain sigma-delta ADCs with programmable gain amplifiers and excitation sources, can simplify RTD design.
Another benefit of sigma-delta ADCs is oversampling of the analog input, which simplifies anti-alias filtering; a simple single-pole RC filter is often sufficient.
Mains power can introduce interference at 50 Hz or 60 Hz and their harmonics. Some ADCs include filter options for 50 Hz/60 Hz rejection, though enabling these filters may affect ADC data throughput.
