Sensors

Temperature — thermistors, thermocouples, RTDs, and digital sensors

"Temperature sensor" covers four genuinely different physical principles, not four packaging options for the same underlying technology — which one belongs in a design depends heavily on the range, accuracy, and wiring budget the application actually has.

Thermistor: resistance that changes with heat

A thermistor is a resistor whose resistance changes sharply and predictably with temperature. NTC (Negative Temperature Coefficient) — resistance drops as it warms up — is by far the more common type; PTC (Positive Temperature Coefficient) is the less common opposite, more often used as a resettable fuse than a precision sensor.

That curve isn't a straight line, so getting a temperature back out of a measured resistance means either a lookup table or a curve fit — the Beta-parameter equation is the common simplified version datasheets provide:

1/T = 1/T0 + (1/B) * ln(R / R0)
// T, T0 in kelvin; R0, B from the datasheet (R0 typically the R25 value)

In circuit, a thermistor is almost always read as one half of a voltage divider against a fixed resistor, with the midpoint voltage sampled by an ADC — cheap, simple, and exactly why thermistors are the default choice for "just tell me roughly how hot this is" in consumer electronics.

Thermocouple: voltage from two dissimilar metals

A thermocouple is a junction of two different metals that produces a small voltage proportional to the temperature difference between that junction and a reference point — the Seebeck effect. Crucially, a thermocouple can only ever measure a difference, not an absolute temperature, so an accurate reading requires knowing the temperature of the reference ("cold") junction too — cold-junction compensation, usually done with a second, different kind of sensor (often a digital silicon sensor, see below) placed right at the connector where the thermocouple wire meets the measuring circuit.

Different metal pairs (Type K, J, T, and others) trade off temperature range, cost, and the exact — and non-linear — voltage curve, which real thermocouple amplifier ICs linearize internally so the rest of the system doesn't have to. The signal itself is tiny (tens of microvolts per °C), so amplification close to the junction matters for noise immunity. In exchange, thermocouples reach much higher temperatures than any of the other technologies here — well into furnace and industrial process territory.

RTD: precision resistance, usually platinum

A Resistance Temperature Detector uses a pure metal (almost always platinum) whose resistance rises with temperature in a nearly linear, extremely well-characterized way — the industry-standard PT100 (100 Ω at 0°C) and PT1000 (1000 Ω at 0°C) are named for that reference resistance. RTDs are the accuracy and stability leader of the four technologies here, at higher cost and real wiring complexity: because the sensing resistance itself is small, the resistance of the lead wires running to it is a real source of error, which is why precision RTD circuits commonly use 3-wire or 4-wire connections specifically to cancel lead-wire resistance out of the measurement rather than 2-wire, which simply adds it in.

Digital silicon sensors

Integrated sensors (LM75, TMP102, DS18B20, and many others) put a silicon bandgap temperature-sensing element and all the signal conditioning on one chip, and hand back a temperature directly over I2C or a similar digital bus (DS18B20 is a well-known 1-Wire example) — no external reference resistor, no ADC channel, no linearization math in application code. That convenience trades off against range (typically -55°C to +125°C) and accuracy (commonly around ±0.5°C) that fall short of what a good RTD achieves.

Comparison

Self-heating, response time, and placement

• Self-heating — any sensor that's read by passing current through it (thermistors, RTDs) dissipates a little power as heat itself; too much excitation current measurably warms the sensor above the actual ambient temperature, especially in still air with poor heat dissipation.
• Response time / thermal mass — a sensor with more physical mass (a large probe body) takes longer to reach the temperature of what it's touching — a fast-changing temperature can be under-read simply because the sensor hasn't caught up yet.
• Thermal coupling and placement — a sensor that isn't well thermally coupled to the thing it's supposed to measure (an air gap, a long unthermalized lead) will happily report a stable, accurate, and completely wrong number.

Real-world applications

• 3D printer hotends — thermistors are the near-universal choice for extruder and bed temperature feedback, cheap and fast enough for PID control loops.
• Industrial furnaces and kilns — Type K thermocouples handle the extreme temperatures thermistors and RTDs can't survive.
• Laboratory and calibration equipment — PT100 RTDs are the standard where traceable, high-precision temperature measurement actually matters.
• Battery management systems — digital silicon sensors (or thermistors) monitor cell temperature to prevent thermal runaway in lithium battery packs.
• Weather stations and HVAC — digital I2C sensors dominate here for their simplicity — no analog front-end, just a register read.

Gotchas