Temperature Sensor Selection
Four dominant sensor technologies cover virtually all thermal measurement applications: NTC thermistors (high sensitivity, −55°C to +150°C, low cost, requires linearisation); Platinum RTDs PT100/PT1000 (best linearity and interchangeability, −200°C to +850°C, ±0.15°C Class A accuracy, 4-wire Kelvin connection eliminates lead resistance); Thermocouples (widest range, up to +1,260°C Type K, requires cold-junction compensation); Digital temperature ICs (factory-calibrated, I²C/SPI/1-Wire output, ±0.1°C available, no external ADC or linearisation firmware needed). For BOM cost below USD 0.15 with a spare ADC channel: NTC. For accuracy < ±1°C without ADC design effort: digital IC. And for temperatures above +125°C: RTD or thermocouple.
What Is a Temperature Sensor?
A temperature sensor is a transducer that converts ambient or contact temperature into a proportional electrical output — typically a resistance change, voltage, or digital code — enabling monitoring, control, and protection functions in electronic systems.
- Measurement range from −200°C to +1,800°C depending on technology
- Output formats: analog (resistance or voltage) or digital (I²C, SPI, 1-Wire)
- Accuracy classes from ±0.1°C (precision RTD) to ±3°C (low-cost thermistor)
- Response time constants from < 1 second (bare-die thermistor) to > 10 seconds (sheathed thermocouple)
- Operating supply voltages from 1.7 V to 5.5 V for digital variants; passive for analog types
Key Features and Advantages of Each Sensor Type
- NTC Thermistors: High sensitivity and resolution in narrow ranges (0°C to 100°C) due to large resistance changes. Ideal for battery monitoring and appliances without needing precision ADCs.
- Platinum RTDs (PT100/PT1000): Excellent linearity and interchangeability from −200°C to +850°C, simplifying second-sourcing and design qualification.
- Thermocouples: Widest range: up to +1,260°C (Type K) or down to −270°C (Type T). Standard for furnaces, kilns, and cryogenic applications. Requires cold-junction compensation.
- Digital Temperature ICs: Integrate ADC, reference, and interface into silicon, allowing MCUs to read temperatures directly via I²C and eliminating complex analog front-end design.
Technical Specifications
| Parameter | Symbol | Typical Range | Unit | Notes |
| Measurement Range (NTC) | Ta | −55 to +150 | °C | Varies by epoxy encapsulant rating |
| Measurement Range (PT100 RTD) | Ta | −200 to +850 | °C | IEC 60751 Class A/B |
| Measurement Range (Type K TC) | Tj | −200 to +1,260 | °C | ANSI MC96.1 / IEC 60584 |
| Measurement Range (Digital IC) | Ta | −40 to +125 | °C | Extended grade available |
| Accuracy (NTC) | — | ±0.5 to ±3 | °C | At 25°C, degrades with temperature |
| Accuracy (PT100 Class A) | — | ±0.15 to ±0.35 | °C | Per IEC 60751, 0°C to 100°C |
| Accuracy (Digital IC) | — | ±0.1 to ±1 | °C | TMP117: ±0.1°C max |
| B-Constant (NTC) | B25/85 | 3,000 to 4,500 | K | Higher B = higher sensitivity |
| TCR (PT100) | α | 0.00385 | Ω/Ω/°C | IEC 60751 standard |
| Supply Voltage (Digital IC) | Vcc | 1.7 to 5.5 | V | Device-specific |
| Thermal Time Constant (SMD NTC) | τ | 0.5 to 5 | s | Bare chip; lower in still air |
NTC Thermistor vs. Digital Temperature IC: Head-to-Head
| Parameter | NTC Thermistor | Digital Temperature IC |
| Measurement Range | −55°C to +150°C | −40°C to +125°C (typical) |
| Accuracy | ±0.5°C to ±3°C (uncalibrated) | ±0.1°C to ±1°C (factory calibrated) |
| Interface | Analog (voltage divider + ADC) | I²C, SPI, or 1-Wire digital |
| Power Consumption | Continuous (ADC dependent) | < 5 μA active; < 1 μA shutdown |
| Signal Conditioning | Requires linearisation firmware | On-chip; read register directly |
| Unit Cost (high volume) | USD 0.02–0.15 | USD 0.40–2.50 |
| Calibration Required | Yes (per-unit or lookup table) | No (factory trimmed) |
| Multi-sensor Topology | Requires ADC multiplexer | Up to 8 per I²C bus (address-selectable) |
Common Application Scenarios of Temperature Sensor
Automotive Battery Management Systems (BMS)
Li-ion cell stacks in EV and HEV applications require cell-level and pack-level thermal monitoring to prevent thermal runaway. NTC thermistors rated to AEC-Q200 with ±1% tolerance at 25°C are placed directly against cell surfaces. The BMS MCU reads resistance via a voltage divider and maps it to temperature using the Steinhart-Hart equation. Response time requirements under 2 seconds demand low-mass 0402 or 0603 SMD NTCs bonded with thermally conductive adhesive.
Industrial Motor Drive Thermal Protection
Three-phase AC motor drives monitor winding and heatsink temperatures to trigger derating before damage occurs. PT100 or PT1000 RTDs embedded in motor windings provide ±0.3°C Class B accuracy across −40°C to +150°C. The drive control board uses a dedicated RTD-to-digital converter IC (such as MAX31865) with 3-wire or 4-wire connection to eliminate lead resistance errors exceeding 0.1 Ω in long cable runs.
Server Inlet and Outlet Air Temperature Monitoring
Data centre servers sample ambient inlet and exhaust air temperatures to manage fan speed via IPMI thermal management. Digital ICs (I²C interface, address-configurable up to eight devices per bus) are mounted near PCIe slots and memory DIMMs. Mid-range devices such as the NCT7802Y or LM75A meet ±1°C accuracy requirements, while mission-critical systems specify TMP117-grade ±0.1°C sensors for precise thermal throttling thresholds.
Quick Selection Guide: Temperature Sensor in 60 Seconds
- BOM cost priority, spare ADC channels available, narrow measurement window (±30°C)? → NTC thermistor (USD 0.02–0.15; implement Steinhart-Hart in firmware)
- Accuracy < ±1°C required without ADC design or linearisation firmware? → Digital temperature IC (I²C/SPI, factory trimmed; TMP117 for ±0.1°C max)
- Temperature range > +125°C or < −40°C? → PT100/PT1000 RTD (−200°C to +850°C) or thermocouple (up to +1,260°C Type K)
- Industrial motor winding or long cable run (> 1 m)? → 4-wire (Kelvin) RTD connection eliminates lead resistance error; use MAX31865 or equivalent
- Multiple sensors on one bus? → Digital IC with address-selectable I²C (up to 8 devices per bus); avoids ADC multiplexer complexity
- Automotive or high-reliability application? → AEC-Q200 NTC thermistor or automotive-grade digital IC; verify HTOL and thermal shock test data
- Furnace, kiln, or cryogenic application (> +125°C or extreme cold)? → Thermocouple with cold-junction compensation IC (MAX31855, AD8497)
FAQ
Q: How should I derate an NTC thermistor’s self-heating error?
Self-heating occurs because excitation current through the thermistor dissipates power (P = I² × R), raising the sensor body above ambient temperature. This error equals P × Rth (thermal resistance to ambient), typically 0.5–2°C/mW for SMD NTCs in still air. Limit excitation current to below 100 μA for 10 kΩ devices — use a high-impedance analog input or duty-cycle the ADC sampling to minimise thermal error.
Q: What causes thermocouple cold-junction errors and how do I compensate?
A thermocouple measures the temperature difference between the hot junction and the cold junction (where thermocouple wire connects to copper PCB traces). If the cold junction is not at 0°C, an offset error proportional to the deviation is introduced. Cold-junction compensation ICs (MAX31855, AD8497) integrate an internal temperature sensor and correction DAC, compensating to within ±2°C. Without active compensation, errors of 10–25°C are common in non-climate-controlled environments.
Q: How do I select between 2-wire, 3-wire, and 4-wire RTD connections?
2-wire RTD connections include lead resistance directly in the measurement, introducing a 0.385 Ω/°C-equivalent error per ohm of cable resistance — acceptable only for cables under 1 metre in non-precision applications. 3-wire configurations cancel lead resistance if both leads are matched in length and gauge, suitable for industrial installations under 10 metres. 4-wire (Kelvin) connections force measurement current through one pair and sense voltage across the other, completely eliminating lead resistance error — mandatory for Class A accuracy systems or cable runs exceeding 10 metres.
