Temperature Sensing & Protection
Temperature Sensors#
RTD / PT100 / PT1000#
RTDs (Resistance Temperature Detectors) rank among the most accurate sensors available for industrial temperature measurement. They operate on the principle that the electrical resistance of a metal — typically platinum — changes linearly with temperature. PT100 and PT1000 are the most widely used standardized RTD elements, covering a measurement range of approximately −200 °C to +850 °C.
Key advantages include excellent long-term stability, high repeatability, and strong linearity. These characteristics make RTDs the preferred choice wherever high accuracy and traceable calibration are required. PT100 probes remain the most commonly specified RTD assemblies in industrial OEM applications.
Thermocouples#
Thermocouples exploit the Seebeck effect: two dissimilar metal wires are joined at one end, generating a millivolt-level voltage proportional to the temperature difference between the measuring junction and the reference end. Their construction is simple, response is fast, and the measurable temperature range is exceptionally wide. Noble-metal thermocouples such as type B, R and S can measure temperatures approaching 1700 °C. Common calibration types include K (nickel-chromium / nickel-aluminium, also known as Chromel-Alumel), J, T, and E, each optimized for a different temperature range and accuracy class.
| Type | Conductors | Range | Notes |
|---|---|---|---|
| K | NiCr / NiAl | −200 to +1260 °C | most widely used, general purpose |
| J | Fe / CuNi | 0 to +760 °C | reducing atmospheres, lower cost |
| T | Cu / CuNi | −200 to +370 °C | cryogenic and high-accuracy low-temp |
| E | NiCr / CuNi | −200 to +900 °C | highest sensitivity |
| N | NiCrSi / NiSiMg | −200 to +1260 °C | better long-term stability than K |
| S | Pt-10%Rh / Pt | 0 to +1480 °C | high-temperature, lab reference |
| R | Pt-13%Rh / Pt | 0 to +1480 °C | similar to S, slightly higher output |
| B | Pt-30%Rh / Pt-6%Rh | +250 to +1700 °C | very high temperatures |
The relative drawbacks are the need for cold-junction compensation and somewhat lower absolute accuracy compared to RTDs.
NTC Thermistors#
NTC (Negative Temperature Coefficient) thermistors are manufactured from semiconductor ceramic materials whose resistance drops sharply as temperature rises, resulting in very high sensitivity. Their main advantages are small size, low cost, and fast response. Most commercial NTCs are used below 150 °C, although glass-encapsulated designs can operate at substantially higher temperatures.
Most NTC thermistor failures occur in the packaging materials rather than in the ceramic sensing element itself. Glass encapsulation provides excellent hermetic sealing, electrical insulation, and chemical stability at high temperatures, allowing certain thermistors to operate at temperatures up to 250 °C.
Thermal Protection Components#
The common logic of this component family is that they do not output temperature data. Instead, they actively intervene in the electrical circuit when a temperature threshold is reached, protecting equipment or limiting current without requiring any external control logic.
Thermal Fuse / Thermal Cutoff (TCO)#
A thermal fuse is a one-shot over-temperature protection device. Its internal element — low-melting alloy or organic pellet — permanently opens the circuit when the ambient temperature exceeds the rated functioning temperature (Tf). The principal advantages are extreme simplicity, very low cost, and inherently reliable operation. The key limitation is that the device is non-resettable: once triggered, the entire component must be replaced. Rated temperatures typically range from 60 °C to 240 °C.
Thermal Protector / Temperature Switch#
Most thermal protectors employ a bimetal element. As temperature rises, the differential thermal expansion of the bonded metals causes the element to snap, opening or closing a set of electrical contacts. Thermal protectors are available in either automatic-reset or manual-reset configurations.
The enclosure form factor of a thermal protector is determined by the installation environment and the technical requirements of the application. Widely used standard series include:
KSD301: disc type, surface mounting, max current 10-16A, for temperature limiting in power circuit.
TB02 & 17AM: miniature embedded protectors for motors, transformers and control circuits.
PPTC Resettable Fuse#
PTC devices belong to the broader thermistor family, whose resistance changes with temperature. Depending on the material system, PTC thermistors can be divided into ceramic (CPTC) and conductive polymer (PPTC) types. PPTC is often used as resettable thermal fuse for overcurrent and overtemperature protection in electronic circuits due to its low cold state resistance (a few mΩ).
The polymer matrix undergoes a phase transition at a characteristic switching temperature, causing the resistance to increase dramatically. For standard commercial PPTC components, this threshold typically falls in the range of 85 °C to 140 °C. This physical characteristic limits its suitability for elevated-temperature applications.