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ComponentsPassive Components

Thermistor (NTC / PTC)

Resistor (non-linear)

A temperature-sensitive resistor whose resistance changes significantly with temperature. NTC types decrease resistance with heat; PTC types increase resistance with heat.

Schematic symbol

Thermistor

How thermistor (ntc / ptc) appears in a circuit diagram.

Types & variants

NTC (Negative Temperature Coefficient)PTC (Positive Temperature Coefficient)Silicon-basedPolymer PTCGlass-encapsulatedEpoxy-coatedSurface-mount (SMD)Probe-type

Key specs

Resistance at 25°C

Ohm (Ω)

Nominal resistance value at room temperature. Common values: 1k, 10k, 100k (NTC), 50Ω (PTC).

B-value (Beta)

K

Material constant describing the relationship between resistance and temperature. Higher B = more sensitive.

Temperature coefficient

%/°C

How much resistance changes per degree of temperature change. NTC: negative, PTC: positive.

Max operating temperature

°C

Highest temperature the thermistor can handle without damage.

Dissipation constant

mW/°C

How much power raises the thermistor's temperature by 1°C. Affects self-heating.

Laws that govern it

  • Laws of resistanceR = ρ × L / A

    This part exists to exploit the temperature term: R = R₀(1 + αΔT) taken to extremes, with α strongly negative (NTC) or positive (PTC).

  • Ohm's lawV = I × R

    You never read resistance directly — you drive it with a known current or divider and read the volts, so Ohm's law is the measurement.

  • Wheatstone bridge condition

    Precision temperature work puts the thermistor in a bridge so tiny resistance changes come out as a null-referenced voltage.

Each law is stated in full — with its diagram, variables and worked meaning — on the electrical laws page.

How it compares

Markings

NTC: Often color-coded (green = NTC) or marked with resistance at 25°C (e.g. 10K). PTC: Usually marked with resistance and trip temperature. SMD: 3-4 digit code similar to resistors.

Standard values

NTC: 1k, 2.2k, 5k, 10k, 50k, 100k, 1M. PTC: 50Ω, 100Ω, 1k, with trip temperatures from 40°C to 120°C.

How to choose

1) Determine NTC or PTC based on application (sensing vs. protection). 2) Select nominal resistance at operating temperature. 3) Choose B-value for required sensitivity. 4) Check max temperature rating. 5) Consider package for mounting.

Pinout & package

2 leads (non-polar). Packages: Disc (radial), bead, glass probe, surface-mount (0805, 1206), epoxy-coated with wire leads.

Example circuits

  • Temperature measurement with voltage divider and ADC
  • Inrush current limiting (PTC)
  • Over-temperature protection circuit
  • Thermal compensation for other components

Common failures

Open circuit (internal damage), resistance drift (aging), cracked body from thermal stress. May become unresponsive after extreme temperature exposure.

How to test

Measure resistance at room temperature with multimeter. Should match nominal value. For NTC: gently warm with fingers; resistance should decrease. For PTC: apply heat; resistance should increase sharply at trip point.

Substitutes

RTD (more accurate, linear), thermocouple (wider range), temperature IC (linear output). Must match resistance characteristics and temperature range.

Where to buy

DigiKeyMouserLCSCRobuAmazon