Thermistor Basics: NTC and PTC Selection Guide

This article explains operation principles, features and packaging options of thermistors. Thermistor basics begin with the distinction between NTC devices, whose resistance decreases with temperature, and PTC devices, whose resistance increases over their specified operating region. This article explains their materials, electrical and thermal behaviour, packages and failure mechanisms, then shows design and component engineers how to select thermistors for temperature measurement, inrush-current limiting and protection.

Key Takeaways

Thermistors serve different purposes depending on whether they are operated with negligible self-heating or deliberately heated by load current. Selection must therefore start with the circuit function, not just the resistance or package.

NTC thermistors support temperature sensing, compensation and inrush-current limiting, but sensing NTCs and power NTCs are not interchangeable.
The beta equation approximates an NTC resistance–temperature curve over a limited range. Higher-accuracy conversion uses the manufacturer’s resistance table or suitable Steinhart–Hart coefficients.
Measurement accuracy depends on thermistor tolerance, conversion error, excitation, self-heating, electronics and thermal coupling; high sensitivity alone does not guarantee accuracy.
Ceramic switching PTCs and polymer PTCs limit current by entering a high-resistance state. They do not provide the open circuit of a cleared fuse, and reset depends on the circuit and cooling conditions.
Compare ratings under matching temperature, mounting and test conditions. For protection devices, verify normal-load survival, fault response, residual current, voltage capability and recovery in the finished assembly.

Introduction to thermistors

Thermistors are temperature-sensitive resistors made principally from semiconducting ceramics or conductive polymer composites. Their resistance changes substantially with body temperature, whether that temperature is set by the surroundings or by electrical self-heating. [1–3]

NTC thermistors have a negative temperature coefficient: resistance decreases as temperature rises. Their main roles include temperature measurement, temperature compensation and inrush-current limiting.
Switching PTC thermistors have a pronounced positive temperature coefficient over a defined region. Ceramic and polymer implementations are used for current limiting, temperature detection and, for suitable ceramic types, self-regulating heating.

A PTC device trips because its body reaches a switching region, not because it measures current independently of temperature. Fault current produces heat, while ambient temperature and heat removal determine how quickly the transition occurs. This article concentrates on oxide-ceramic NTCs, switching ceramic PTCs and polymer PTCs rather than linear positive-temperature-coefficient sensing elements. [2,3]

Overview and classification

Temperature measurement and circuit protection are different selection problems. Table 1 separates the technologies by function without assigning universal accuracy, cost or temperature limits to an entire component family.

Technology	Main role	Characteristic	Main selection concern
NTC sensing thermistor	Temperature measurement and compensation	Strongly nonlinear resistance decrease	Specified R/T curve, accuracy, self-heating and coupling
NTC inrush limiter	Power-on current limiting	High cold resistance; lower resistance when self-heated	Cold resistance, continuous current, pulse capability and restart
Ceramic switching PTC	Temperature detection, current limiting and heating	Steep resistance rise over a specified transition region	Application-specific sensing, voltage, current and thermal ratings
Polymer PTC	Resettable current and thermal protection	Low-resistance and tripped high-resistance states	Temperature-derated hold current, trip time, fault limits and reset
RTD	Temperature measurement	Metal resistance–temperature characteristic	Accuracy class, range, lead resistance and excitation
IC temperature sensor	Temperature monitoring	Conditioned analog or digital output	Accuracy specification, interface, supply and package coupling
Fuse	One-shot over-current interruption	Opens the circuit when its element clears	Time–current behaviour, breaking capacity and voltage rating

Table 1. Temperature-sensing and protection technologies: functional distinctions and selection priorities. Thermistor operating principles are described in [1–4].

Construction and designs

Material composition establishes the basic resistance–temperature behaviour, while geometry, electrodes, coating and mounting determine how it can be used. An electrically suitable element can still be unsuitable mechanically, thermally or environmentally. NTC ceramics are manufactured from mixed metal oxides, formed into the required geometry and sintered. Contact metallization, leads, protective coatings and housings are then added as appropriate. Multilayer SMD construction can incorporate internal electrodes; arranging several conducting paths within the body allows resistance to be adjusted without simply increasing the external dimensions. [1] Common geometries include beads, discs, rods, leaded elements and SMD chips. Ceramic PTCs also appear as cylindrical surface-mount parts, heaters and probe assemblies. Polymer PTCs are available in board-mounted and battery-strap constructions. Coatings and housings must be assessed for the actual moisture, chemical, insulation and mechanical requirements rather than assumed to provide a hermetic seal. [1–4]

Construction diagram of a low-ohmic chip thermistor — Figure 2. Construction of a low-ohmic chip thermistor.

Geometry or package	Typical implementation	Typical use	Design consideration
Bead	NTC	Small sensors and probes	Element protection and lead strain relief
Disc or rod	NTC or ceramic PTC	Inrush limiting, protection or heating	Pulse capability, hot surfaces and spacing
SMD chip	NTC, ceramic PTC or PPTC	PCB sensing or protection	Board coupling, land pattern and assembly process
MELF cylinder	Selected ceramic PTC families	Board-mounted protection	Termination and mounting requirements
Radial leaded	NTC, ceramic PTC or PPTC	Through-hole sensing, limiting or protection	Lead forming and heat transfer through leads
Axial leaded or strap	Leaded NTC or battery-strap PPTC	Remote sensing or battery protection	These constructions have different attachment requirements
Probe assembly	NTC or PTC sensor	HVAC, appliances and industrial measurement	Housing, insulation, sealing and installed response

Table 2. Common thermistor geometries and application considerations. Package availability depends on the product family. [1–4]

NTC thermistor basics

NTC thermistors use semiconducting ceramics whose conductivity increases with temperature. Their R/T characteristic is nonlinear, and the mathematical model must match the required measurement range and accuracy. [1]

The beta approximation

Over a restricted temperature range, resistance R at absolute temperature T can be approximated by Eq. (1). Here R is in Ω, T is in K, A is a fitted resistance prefactor in Ω and B is the beta value in K. A is not the resistance at a conventional reference temperature. [1]

R(T) \approx A\exp\left(\frac{B}{T}\right) \tag{1}

The reference-resistance form is usually more useful. In Eq. (2), R₀ is resistance at reference temperature T₀. When the reference is 25 °C, R₀ is R₂₅. All temperatures used in reciprocal-temperature expressions must be converted to kelvin. [1]

\ln\left(\frac{R(T)}{R_0}\right) \approx B\left(\frac{1}{T}-\frac{1}{T_0}\right) \tag{2}

The beta value determined between two temperatures is given by Eq. (3), where R₁ and R₂ are the zero-power resistances at T₁ and T₂. A designation such as B_25/85 identifies the temperature pair used. B_25/50, B_25/85 and B_25/100 are not automatically interchangeable because a real thermistor does not follow a constant-beta model exactly. [1]

B_{T_1/T_2}=\frac{\ln(R_1/R_2)}{1/T_1-1/T_2} \tag{3}

Common oxide NTC materials have beta values spanning approximately 2000 K to 5000 K, but this is not a product-selection limit. Compare the actual curve, the specified temperature pair and the tolerance of the candidate component. [1]

Steinhart–Hart and manufacturer R/T tables

The beta relation is not the Steinhart–Hart equation. A commonly used three-coefficient Steinhart–Hart form is Eq. (4), in which a, b and c are fitted coefficients. The resistance unit used inside the logarithm must match the coefficient convention; Eq. (4) explicitly normalizes resistance to 1 Ω. Use coefficients supplied for the selected thermistor and temperature interval, or interpolate the manufacturer’s R/T table. [1,5]

\frac{1}{T}=a+b\ln\left(\frac{R}{1\,\Omega}\right)+c\left[\ln\left(\frac{R}{1\,\Omega}\right)\right]^3 \tag{4}

A more elaborate equation does not remove sensor tolerance, self-heating or installation error. Its purpose is to reduce the conversion error between measured resistance and estimated temperature.

NTC tolerances and R/T curves

Nominal resistance tolerance describes only part of an NTC sensor’s interchangeability. Away from the reference temperature, curve-shape variation becomes increasingly important. [1]

Manufacturing control, stabilization, selection and, where applicable, trimming establish the final resistance and curve tolerances. These are product-specific: a broad-tolerance power limiter and a precision sensing thermistor should not be assigned the same “standard” tolerance. Compare the guaranteed limits rather than assuming that all precision parts are produced by the same machining process.

R₂₅ tolerance shifts the reference resistance, while beta tolerance changes the curve away from that reference. Figures 3 to 5 illustrate their separate and combined effects. Temperature-interchangeability specifications can instead define a permitted temperature deviation over a stated range, which is often more directly useful for measurement design. [1]

Figure 3. Thermistor resistance tolerance at the reference temperature.

Effect of beta tolerance on thermistor resistance–temperature curves — Figure 4. Effect of beta tolerance on the thermistor R/T curve.

Combined effect of reference resistance and beta tolerances — Figure 5. Combined effect of R₂₅ and beta tolerances.

For small deviations, Eq. (5) converts a relative resistance error into an approximate temperature error. ΔT is temperature error in K, ΔR is resistance error in Ω and α is the relative temperature coefficient in K⁻¹. This local approximation does not replace a full worst-case curve calculation. [1]

|\Delta T|\approx\frac{|\Delta R|/R}{|\alpha|} \tag{5}

NTC slope and temperature coefficient

The relative temperature coefficient describes the sensitivity at a particular temperature. It must be distinguished from beta, which is expressed in kelvin and describes a resistance ratio over a temperature interval. [1]

Equation (6) defines α. Its second expression follows from the constant-beta approximation, so it inherits that model’s range limitation.

\alpha=\frac{1}{R}\frac{dR}{dT}\approx-\frac{B}{T^2} \tag{6}

For an illustrative beta value of 3950 K, Eq. (6) gives approximately −4.44 %/K at 25 °C. Equation (5) then maps a 1 % resistance deviation to approximately 0.225 K of local temperature error, before other error contributions. These are calculated model values, not specifications for a particular component.

Resistance ratios such as R₂₅/R₅₀ or R₂₅/R₁₂₅ also characterize the curve. Ratios are meaningful only with the associated temperatures and tolerances.

Parameter	What to compare	Why it matters
Reference resistance	R₂₅ or resistance at another specified temperature	Sets signal level, bias current and sensitivity to leakage or lead resistance
Beta value	Value, temperature pair and tolerance	Determines the approximate curve shape
R/T characteristic	Manufacturer table or coefficient set	Controls resistance-to-temperature conversion
Interchangeability	Guaranteed resistance or temperature deviation over the required range	Determines sensor-to-sensor error without individual calibration
Temperature coefficient	Local α at the operating temperature	Converts electrical errors into temperature errors
Long-term stability	Drift after specified thermal and environmental tests	Determines calibration retention

Table 3. NTC sensing parameters to compare instead of relying on universal “typical” ranges. [1]

Example of a thermistor temperature-tolerance butterfly curve — Figure 6. Alternative tolerance specification using a butterfly-shaped temperature-deviation envelope.

NTC self-heating and V–I behavior

Electrical excitation changes the thermistor’s temperature as well as producing a measurable voltage. Sensing applications minimize this effect; inrush limiters and thermal flow sensors deliberately exploit it. [1,6]

For an NTC carrying current I with voltage V across it, electrical power P is given by Eq. (7). R is the resistance at the actual body temperature, not necessarily the ambient-temperature resistance.

P=VI=I^2R=\frac{V^2}{R} \tag{7}

At sufficiently low power, the body remains close to ambient temperature and the V–I curve is approximately ohmic. As current increases under steady-state conditions, self-heating lowers resistance. Beyond a voltage maximum, the V-versus-I curve can have negative incremental resistance: dV/dI is negative even though V/I remains positive. Circuit stability depends on the source impedance and thermal conditions. [1]

Bias topology matters. At constant current, falling resistance reduces I²R heating; at constant voltage directly across the NTC, falling resistance increases V²/R heating. A series resistor changes this feedback and must be included in the analysis. [1]

Self-heating effect on the voltage–current curve of an NTC thermistor — Figure 7. Self-heating effect on the V–I curve of an NTC thermistor.

NTC voltage–current diagram on logarithmic axes with power and resistance lines — Figure 8. NTC V–I diagram on logarithmic axes, with constant-power and constant-resistance lines.

Thermal time constant and dissipation factor

Thermal specifications describe an element in a particular mounting arrangement and medium. They are not universal properties of a case size, and installation can change them substantially. [1]

Thermal time constant τ, in s: for a first-order response, the time to complete approximately 63.2 % of the temperature change following a step in the surrounding temperature, with negligible electrical self-heating.
Heat capacity C_th, in J/K: energy required to raise the mean body temperature by 1 K.
Dissipation factor D, in W/K or mW/K: the steady-state power change per unit body-temperature rise under stated environmental and mounting conditions.

For a lumped first-order thermal model, Eq. (8) relates these quantities. Use matching power units: J/K divided by W/K gives seconds. Mixing J/K with an unconverted value in mW/K introduces a factor-of-1000 error.

\tau\approx\frac{C_{\mathrm{th}}}{D} \tag{8}

For small steady-state temperature rises, Eq. (9) estimates self-heating. ΔT_sh is the rise above the measured medium in K. D must describe the installed sensor environment closely enough for the estimate to be useful. [1]

\Delta T_{\mathrm{sh}}\approx\frac{P}{D} \tag{9}

A small bead can respond rapidly because of its low heat capacity, while a protective probe housing may dominate the response of the complete assembly. A PCB-mounted chip also exchanges heat through its terminations and board. Liquid exposure is permissible only for elements or assemblies specifically designed for it. [1]

Thermal step-response diagram showing an NTC thermistor time constant — Figure 9. Thermal time constant of an NTC thermistor; τ is used in the text for the time constant.

Construction	What controls response	What to request or measure
Bead	Bead size, coating, leads and surrounding medium	Time constant and dissipation factor under specified test conditions
SMD chip	Chip, solder joints, copper layout and board	Installed response to the relevant PCB or air-temperature change
Coated disc	Body mass, coating, leads and airflow	Heating and cooling behaviour for the intended electrical load
Probe assembly	Housing, potting, insertion depth and interface	Complete-assembly response in the actual medium or on the actual surface

Table 4. Thermal parameter comparison by construction. A package designation alone does not establish a valid response-time range. [1]

NTC application modes and type selection

NTC applications can be grouped by whether external temperature, self-heating or thermal delay supplies the useful information or function. The same material behaviour supports all three modes, but the component ratings and circuit design differ. [1,6]

Temperature sensing and compensation: operate at sufficiently low measurement power that resistance primarily follows the measured medium.
Self-heated operation: exploit the resistance change caused by electrical heating, as in power inrush limiters and thermal flow or level sensing.
Time-dependent operation: exploit heating or cooling delay in circuits where the thermal transient forms part of the intended behaviour.

Selecting an NTC sensing element

Define the temperature range, required error limit, response time and drift allowance.
Select the reference resistance and complete R/T characteristic, not just a nominal beta value.
Check coating, housing, humidity resistance, chemical compatibility and thermal-shock requirements.
Choose bead, chip, disc or probe geometry to suit the measured object and assembly process.
Specify soldering, welding, bonding or clamping requirements and prevent lead or termination stress.
Budget excitation self-heating, electronics error and installation gradients alongside thermistor tolerance.

Selecting a power NTC for inrush limiting

A power NTC starts with relatively high resistance and heats as load current flows, reducing its resistance and running loss. Selection requires cold resistance, maximum continuous current with temperature derating, and pulse capability. Where a datasheet specifies a maximum switched capacitance, compare it only under the stated voltage and test-circuit conditions. [6]

Account for both cold and warm starting conditions. A hot NTC has less resistance and therefore less inrush-limiting capability; a short off-time must not be assumed to restore cold-start protection. Check capacitor discharge, bypass operation where present, and the required restart sequence together. The running element can also be hot enough to require clearance from temperature-sensitive materials. [6]

Power NTCs are not short-circuit interrupters. Coordinate them with the appropriate fault-protection system. Do not parallel independent NTC inrush limiters to increase current or pulse capability: unequal heating can concentrate current in one element. This restriction does not describe the controlled internal electrode structure of a manufactured chip. [6]

NTC linearization and interface circuits

The interface must convert resistance into a useful signal while controlling self-heating and conversion error. Analog linearization remains useful for compensation and threshold circuits; digital measurement usually benefits from retaining the nonlinear sensor response and applying the correct R/T conversion. [1,5]

Analog linearization

Series and parallel resistors can reshape the resistance or divider-output characteristic over a limited temperature interval. A parallel resistor can be selected to reduce curvature around a chosen region, but the resulting network is not globally linear. Evaluate residual temperature error over the complete operating interval rather than relying only on a visually straight plot.

Resistor networks for reshaping an NTC thermistor response — Figure 10. Correction circuits for the R/T characteristic of NTC thermistors.

Equation for selecting a resistor connected in parallel with a thermistor to reduce response curvature — Figure 11. Original parallel-resistor linearization equation image.

Illustration of linearizing an NTC resistance–temperature curve — Figure 12. Linearizing the R/T curve of an NTC thermistor over a limited range.

Voltage-divider measurement

For the topology used here, fixed resistor R_S connects from excitation voltage V_EXC to the measurement node, and NTC resistance R connects from that node to ground. The measured node voltage is V_OUT. Equation (10) gives both the divider output and the inverse conversion to resistance.

V_{\mathrm{OUT}}=V_{\mathrm{EXC}}\frac{R}{R_S+R}, \qquad R=R_S\frac{V_{\mathrm{OUT}}}{V_{\mathrm{EXC}}-V_{\mathrm{OUT}}} \tag{10}

Choosing R_S near the NTC resistance at a critical temperature is a useful starting point, not a universal optimum. Calculate voltage span, temperature sensitivity, source impedance and self-heating across the required range. With the same source used for divider excitation and ADC reference, the measurement is ratiometric and the ideal conversion depends on a voltage ratio rather than the absolute excitation voltage. Resistor tolerance, ADC errors, leakage and thermal errors remain. [5]

Other interfaces include bridge circuits and comparators. A bridge does not automatically eliminate sensor drift, and a comparator needs an appropriate threshold and hysteresis. ADC circuits must satisfy input-range, acquisition-time, filtering and settling requirements. [5]

Step	Action
1	Select the thermistor R/T curve and required temperature interval.
2	Choose an initial fixed resistance near the NTC resistance at the critical temperature.
3	Calculate divider voltage, local sensitivity and thermistor power over the interval.
4	Check ADC range, input loading, acquisition time, noise and reference arrangement.
5	Apply R/T conversion, tolerance analysis and open/short diagnostics.
6	Validate installed accuracy and response, then adjust excitation or resistance if necessary.

Table 5. NTC voltage-divider design procedure. Interface considerations are discussed in [1,5].

Calculated divider and self-heating example

Consider an illustrative NTC with R₂₅ = 10 kΩ and constant beta B = 3950 K, a 10 kΩ fixed resistor and 3.3 V excitation. These are assumed design inputs, not a named component specification. Applying Eqs. (2), (7) and (10) gives approximately 33.621 kΩ and 2.543 V at 0 °C, 10 kΩ and 1.650 V at 25 °C, and 3.588 kΩ and 0.871 V at 50 °C.

At 25 °C, current is 165 µA and thermistor dissipation is 0.27225 mW. If the installed dissipation factor is assumed to be 1 mW/K, Eq. (9) estimates a self-heating rise of approximately 0.272 K. This first-order calculation uses the unheated resistance; a tighter analysis solves the electrical and thermal behaviour together. The power and dissipation-factor method follows [1].

Reducing excitation or using suitably timed switched excitation can reduce heating, but the measurement must settle before conversion. High resistance can make leakage and ADC loading more important, while low resistance makes lead resistance and excitation power more important. [5]

Diagnostics and error budgeting

With the divider orientation defined above, an open NTC tends to pull the node toward excitation voltage and a shorted NTC toward ground. Define diagnostic limits outside the valid temperature range, allowing for tolerances and leakage. Wiring faults, reference faults and ADC faults may require additional checks; a rail reading alone does not identify every failure mechanism. [5]

The temperature-error budget should include sensor interchangeability, model or interpolation error, fixed-resistor tolerance and drift, ADC error and noise, leakage, lead resistance, self-heating, and the difference between sensor temperature and the intended measurement point. Calibration can reduce selected errors but cannot compensate reliably for an uncontrolled thermal interface.

NTC failure modes and reliability

NTC reliability depends on the ceramic body, contacts, encapsulation and installation. Electrical overstress and mechanical damage must be separated from gradual material or contact aging. [1,6]

Improper lead forming, board strain or assembly stress can damage the body or lead–body interface. Damaged coatings can expose a non-hermetic element to moisture or chemicals. Glass encapsulation can improve environmental protection, but neither “glass” nor “SMD” alone establishes the reliability of the complete installed assembly.

Thermistor ceramics can also change with time through solid-state reactions, oxygen exchange and contact-area processes. Manufacturers use stabilization treatments to improve long-term behaviour. Assess the specified drift after relevant temperature, humidity and load tests rather than assigning a universal aging percentage or equating thermistors with unrelated passive-component families. [1]

Mechanism	Possible cause	Possible symptom or consequence
Mechanical damage	Lead forming, board strain, shock or handling	Resistance shift, intermittent contact or open circuit
Environmental exposure	Unsuitable or damaged coating, contamination	Drift, leakage or contact degradation
Material or contact aging	Time, temperature and environment	Change in reference resistance or R/T characteristic
Electrical or thermal overload	Excessive power, pulse loading or temperature	Permanent change, cracking or failure
Parallel power-NTC operation	Unequal resistance and self-heating	Current concentration and overload of one limiter

Table 6. NTC failure mechanisms and design checks. Symptoms are not unique diagnostic signatures. [1,6]

NTCs in special environments

Harsh-environment use must be qualified at the element and assembly levels. A general thermistor description is not evidence of suitability for radiation, vacuum, aggressive chemicals or prolonged liquid exposure.

Vacuum and changing gas or liquid conditions alter heat transfer and therefore self-heating and response. Use components whose encapsulation and electrical insulation are suitable for the medium. Radiation-sensitive applications require evidence for the selected material, dose, spectrum, operating temperature and permitted drift; a blanket claim of radiation immunity is not an appropriate selection criterion. The dependence of thermal behaviour on the surrounding medium is described in [1].

PTC thermistors: roles and principles

Switching PTC devices use a steep resistance increase to detect temperature or limit current. Their low-power sensing mode and self-heated power mode must be distinguished just as they are for NTCs. [2,3]

Ceramic switching PTCs use doped barium-titanate-based ceramics with a strong resistance change associated with the ferroelectric transition.
Polymer PTCs, or PPTCs, use a conductive network within a polymer matrix whose structure changes on heating.

In a current-protection circuit, load current generates heat until the device either reaches a stable low-resistance operating point or transitions into a higher-resistance state. The outcome depends on current, ambient temperature, voltage and heat transfer, not on a single universal trip-current threshold. [2,3]

Ceramic PTC thermistor technology

Ceramic PTC behaviour arises from both the semiconducting grains and the barriers at their boundaries. Material composition and processing tailor resistance and the useful transition region. [2]

Below the transition, the ceramic can exhibit a relatively low resistance and a modest negative temperature coefficient. Near and above the ferroelectric Curie region, changes in polarization and dielectric behaviour increase grain-boundary barriers, causing resistance to rise steeply. This rise applies over a finite temperature region: resistance does not continue increasing indefinitely, and excessive temperature or voltage can destroy the device. [2]

Datasheet reference temperature, sensing temperature and Curie temperature describe related but not necessarily identical quantities. For a temperature detector, use the specified resistance limits at the relevant sensing temperatures and the permitted measuring voltage. For a power limiter or heater, use the loaded electrical and thermal ratings. [2]

Applications include overload protection, motor starting, limit-temperature sensing, self-regulating heaters and inrush limiting. CRT degaussing is an established historical application rather than a main driver of current display design. Different PTC constructions serve these functions and are not interchangeable. [2]

Polymer PTC (PPTC) technology

PPTC devices contain conductive particles dispersed in a polymer matrix. At normal temperature, connected particle paths provide low resistance; heating changes the polymer structure and separates conductive paths. [3]

Heating can come from fault-current dissipation, an external heat source or both. The resulting resistance increase reduces current to a residual level rather than creating a galvanically open circuit. This distinction is fundamental when deciding whether a resettable limiter is acceptable in place of a fuse. [3]

PPTCs are widely used in electronics and battery protection because suitable constructions combine low normal-state resistance with resettable operation. Their suitability depends on voltage, prospective fault current, ambient temperature, mounting and recovery requirements; material family alone does not establish a universal cost or performance advantage. [3,4]

Ceramic vs polymer PTC: technology differences

Ceramic PTCs cover several distinct roles, from low-power temperature detection to heaters and power-circuit limiting. PPTCs are primarily selected as resettable protection devices, so a comparison must use equivalent applications and test conditions. [2,3]

Neither package size nor energy capability can be ranked solely from “ceramic” versus “polymer”. Compare available current ratings, resistance, fault-voltage capability, thermal response and assembly constraints in the actual product families. Miniature sensing elements should not be compared directly with power discs or battery straps.

Feature	Ceramic switching PTC	Polymer PTC
Active material	Doped barium-titanate-based ceramic	Polymer and conductive particles
Main roles	Temperature detection, current limiting, starting and heating	Resettable current and thermal protection
Selection data	Sensing or reference temperature; application-specific electrical ratings	Hold current, trip current, time-to-trip and fault limits
Loaded-state behaviour	Depends on voltage, temperature and heat dissipation	Depends on voltage, temperature and heat dissipation
Mechanical concerns	Ceramic stress, contacts, coating and hot surfaces	Mechanical damage, contamination and freedom for intended thermal expansion
Size, cost and energy capability	Compare the selected family and construction	Compare the selected family and construction

Table 7. Ceramic PTC and PPTC comparison without universal size or cost rankings. [2–4]

PTC manufacturing processes

Ceramic and polymer PTCs use different manufacturing routes. The following sequences describe their main stages rather than a single process that applies to every package.

Ceramic PTC process

For conventional bulk ceramic PTCs, the main sequence is powder preparation, forming and sintering, followed by contacting, connection elements and coating or enclosure. Attaching leads before the primary ceramic sintering step is not the general manufacturing sequence. [2]

Prepare and mix raw materials to obtain the required ceramic composition.
Form discs, rods or other required geometries.
Sinter the ceramic body to develop its electrical and mechanical properties.
Apply suitable contacts, attach connection elements and add the required protection or housing.

Multilayer chip implementations instead use a laminated ceramic and internal-electrode structure. Their electrode materials, firing route and termination process are specific to the technology and must not be inferred from the process for a leaded bulk disc.

Polymer PTC process

The polymer route forms a conductive composite and integrates it with electrodes and the required terminations. Sheet-based processing, shaping and stabilization steps depend on the material platform and package.

Disperse conductive filler in the polymer system.
Form the composite into the required layer or body and apply the manufacturer’s stabilization process.
Integrate electrodes and board-mount, leaded or strap connections.
Apply protective materials where required and test electrical and thermal behaviour.

PTC package configurations

Package selection determines how a PTC exchanges heat and how it can be assembled. The permitted attachment method is part of the component specification, particularly for battery straps. [2–4]

Ceramic PTC configurations

Chip and cylindrical surface-mount parts for selected sensing and protection applications.
Radial or axial leaded components for current limiting, temperature detection or starting functions.
Rods, discs and housed elements for heating and power applications.
Probe and embedded-sensor assemblies for detecting temperature at a defined location.

Polymer PTC configurations

Radial-leaded parts for through-hole circuit protection.
Surface-mount parts for compact board-mounted protection.
Strap constructions intended for attachment to battery cells or pack interconnections.

Do not assume a battery-strap device is suitable for PCB reflow or that its identification tape provides electrical insulation. Follow the selected product’s welding, mechanical-clearance and contamination precautions. The Littelfuse VTP datasheet, for example, explicitly separates strap installation from reflow assembly and warns against restricting polymer expansion. [4]

PTC electrical behavior and datasheet parameters

Protection ratings describe behaviour under stated temperature and test conditions. PPTC terminology is useful, but ceramic PTC datasheets may use different current symbols or define application-specific sensing and switching parameters. [2–4]

Hold current I_HOLD is the specified current the device can carry without tripping under the stated conditions. Trip current I_TRIP is the specified minimum current that causes transition under the relevant conditions; it is not a guarantee of one fixed trip time. Between hold and trip current, a device may trip or remain in its low-resistance state depending on its characteristics and environment. [3]

Use the time-to-trip curve or guaranteed trip-time entry for the actual fault current. Check temperature derating, startup pulses and heat transfer through the PCB. A current-limited source may not supply enough heating to produce the expected trip, even though its overload is unacceptable to the load. [3]

Parameter	Meaning and selection check
I_HOLD	Non-tripping current under stated conditions; apply the relevant temperature derating.
I_TRIP	Specified tripping-current boundary; use separate time-to-trip information.
V_MAX	Maximum permitted voltage under the product’s specified operating or fault conditions.
I_MAX	Maximum fault current the PPTC is rated to withstand under specified conditions; not its normal current rating.
R_MIN, R_MAX	Initial resistance limits with the stated measurement temperature and conditioning.
R_1MAX	Maximum resistance after the defined trip-and-recovery test; often measured one hour after a trip.
P_D	Tripped-state dissipation under stated conditions, not normal hold-state loss.
Time-to-trip	Response at specified fault currents and temperatures; distinguish typical curves from guaranteed limits.

Table 8. PPTC datasheet parameters. Definitions and reference temperatures must be taken from the selected datasheet; the VTP series provides an example. [3,4]

Normal-state loss is calculated from load current and the applicable low-state resistance using Eq. (7). In the tripped state, residual current and dissipation depend on the applied circuit voltage and thermal equilibrium. Do not use R_1MAX as the hot tripped-state resistance. [3,4]

PTC selection and application examples

The same PTC label covers different circuit functions. Select and evaluate each device according to whether it carries load current, detects temperature or forms a controlled heating or precharge element. [2,3]

Low-voltage supply and battery protection

A PPTC can be placed in series with a supply rail to limit sustained overload or short-circuit current. The temperature-derated hold current must exceed the maximum intended load, while the fault response must protect the vulnerable wiring and components before damage occurs. Check voltage drop with recovery resistance as well as initial resistance. [3,4]

USB and battery applications require verification of the actual voltage, source behaviour, charging or load profile and prospective fault current. A PPTC does not by itself establish interface compliance or provide all battery safety functions. Strap devices also require the specified attachment and mechanical environment. [3,4]

Motor and transformer applications

A PTC temperature sensor embedded in a winding normally connects to a low-power sensing circuit or protection relay, not in series with the motor’s load current. Its task is to detect winding temperature and cause the control system to act. [2]

A current-carrying PTC protector is a different component. Likewise, a motor-start PTC controls the starting branch in a suitable motor circuit; it is not a universal substitute for winding-temperature sensing or overload protection. Transformer applications must make the same distinction between thermal sensing and current limiting. [2]

Degaussing, heating and power-circuit inrush limiting

In CRT degaussing, a PTC produces a decreasing current as it heats. In a heater, its resistance rise helps establish a thermal operating point, but the resulting temperature depends on supply and heat removal and remains subject to product limits. [2]

Dedicated ceramic PTC inrush limiters can also be used in power-circuit precharge arrangements. They are not ordinary PPTC rail protectors. Check the specified link voltage, charging energy, initial temperature, repetition conditions and the bypass or control sequence required by the circuit. [2]

Application	Key selection focus
Supply or USB rail	Derated hold current, startup survival, fault-time coordination, voltage and residual current
Battery pack	Fault capability, thermal coupling, recovery resistance and approved attachment method
Winding-temperature detection	Sensing-temperature resistance limits and compatibility with the monitoring circuit
Motor starting	Starting-branch electrical and thermal requirements, including restart conditions
Degaussing or heating	Loaded behaviour, voltage, thermal limits and repetition duty
Power-circuit precharge	Dedicated limiter ratings, link voltage, pulse energy and control sequence

Table 9. Application-specific PTC selection priorities. [2–4]

PTC reset behavior, aging and limitations

Resettable does not mean that normal operation resumes immediately when an overload disappears. A device must lose enough heat to return to its low-resistance state, and the circuit may continue supplying the heat that holds it tripped. [3]

In many PPTC circuits, the fault must be cleared and power removed or reduced sufficiently before reset. Residual current can sustain the tripped condition. Cooling time depends on the package, board, enclosure and ambient temperature, while resistance recovery can continue after the device has resumed conduction. [3]

Use the specified post-trip resistance limit and relevant endurance data. Repeated operation, long exposure in the tripped state and environmental aging can affect behaviour, so validation should include the intended service cycle rather than only a single successful trip. [3,4]

PPTCs do not provide electrical isolation, and voltage can remain present downstream. Where restarting machinery could be hazardous, where a fault requires service inspection, or where positive current interruption is required, a fuse, breaker or controlled disconnect may be necessary. Reset behaviour is part of the system safety assessment. [3]

Packaging, mounting and thermal interface

The thermal interface links component specifications to actual system behaviour. A sensor measures its own body temperature, while a protector responds to the combined effects of current and heat transfer.

For measurement, provide repeatable coupling to the target medium while maintaining the required electrical insulation. Board copper, lead length, potting, housing, insertion depth and airflow can all affect the reading and response. A sensor near a power semiconductor measures a local package or substrate temperature; it does not automatically reproduce the semiconductor junction temperature. [1,5]

For current protection, additional copper can remove heat and change hold current and trip time. A nearby hot component can have the opposite effect. Validate the final land pattern, enclosure and assembly rather than relying only on a free-air test. [3]

High-temperature attachment

Higher-temperature sensing elements can require a different termination and attachment system. For example, TDK’s 2026 NTCSP expansion specifies operation up to 175 °C with AgPd terminals intended for conductive-glue mounting. The design lesson is to qualify the thermistor, termination, attachment material and substrate together; the ceramic temperature rating alone does not qualify a soldered assembly. [7]

Comparison with other technologies

Select temperature sensors by the required accuracy, range, stability, interface and installed response. Select protection devices by fault coordination and safe recovery rather than by the convenience of reset alone.

RTDs provide established metal-resistance characteristics and are useful where standardized behaviour and a broad measurement range are priorities. This does not mean every RTD system is more accurate than every precision thermistor system.
IC temperature sensors simplify analog or digital interfacing. Their response depends on die, package and mounting; it cannot be ranked universally as slower than a thermistor.
NTCs provide high resistance sensitivity and flexible small-element construction, but require excitation and nonlinear conversion. Precision performance depends on the complete error budget.
Fuses interrupt current after clearing, whereas PTC limiters retain residual conduction. Circuit breakers are resettable devices and must not be grouped with one-shot fuses as though both have identical recovery behaviour.

Thermistor measurement and interface trade-offs are discussed in [1,5]; the distinction between fuses, breakers and resettable PTC limiters is discussed in [3].

Reliability and standards

Qualification demonstrates performance under defined tests, while safety approval and end-equipment compliance address different requirements. Verify the exact component, construction and approval scope rather than treating a material or package designation as proof of suitability.

IEC 60539 is relevant to NTC thermistor characterization. Ceramic PTC documentation identifies IEC 60738-1 and, for applicable temperature-sensing types, DIN 44081 and DIN 44082. Use the requirements and editions applicable to the selected product and application; these references do not make every thermistor a safety-approved protective device. [1,2]

Automotive qualification and safety recognition must also be checked separately. The high-temperature NTC example in [7] is specified as AEC-Q200 compliant, while the battery-strap datasheet in [4] lists its own agency recognitions. Neither statement transfers automatically to another part, a custom probe or the completed equipment.

Design and purchasing checklist

Function: identify sensing, compensation, inrush limiting, temperature detection, overload limiting or heating.
Electrical definition: specify the complete R/T curve or protection-current limits, not just nominal resistance and package.
Thermal definition: record ambient range, installed coupling, self-heating allowance and required response.
Power and fault limits: verify continuous operation, startup pulses, prospective fault current, voltage and repetition duty.
Recovery: define permissible restart, cooling time, residual current and post-trip resistance.
Catalog versus custom: prefer a standard part when it meets the requirement; specify a custom probe or assembly when housing, sealing, insulation, cable, connector or response requirements demand it.
Custom acceptance: define the measured characteristic, test medium, mounting, insulation tests, environmental tests, tolerances and traceability for the complete assembly.
Qualification and safety: verify applicable test reports, recognition conditions and end-equipment requirements; distinguish material flammability from protective-device approval.
Second sourcing: compare curve identity, beta temperature pair, tolerance envelope, assembly process and protection behaviour. Matching R₂₅ and footprint alone is insufficient.
Validation: test measurement accuracy or fault protection on the final PCB or mechanical assembly, including realistic environmental and recovery conditions.

Original NTC thermistor characteristics table — Table 10. Original NTC thermistor characteristics overview.

Conclusion

NTC and PTC thermistors support different sensing, limiting and protection functions through the interaction of resistance and temperature. Reliable selection starts by separating low-power temperature measurement from deliberately self-heated operation and then matching the component’s electrical, thermal and mechanical specifications to that function.

For NTC measurement, use the correct R/T characteristic and an error budget that includes self-heating and installation. For power NTCs and PTC protectors, verify startup, fault and recovery behaviour in the completed assembly. Nominal resistance, package size and resettable operation are useful descriptors, but none is a substitute for application-specific validation.

FAQ

These questions address the distinctions that most often affect thermistor selection, measurement accuracy and protection behaviour.

What is the difference between NTC and PTC thermistors?

An NTC thermistor decreases in resistance as its body temperature rises. A switching PTC increases sharply in resistance over a specified temperature region. NTCs serve sensing, compensation and inrush-limiting applications, while ceramic PTCs and polymer PTCs serve different protection, sensing and heating roles. Select the construction for the intended function rather than treating either family as interchangeable.

Is the beta equation the same as the Steinhart–Hart equation?

No. The beta equation uses a simplified exponential resistance–temperature relationship over a limited interval. A common Steinhart–Hart form uses three fitted coefficients to describe the curve more accurately. Use kelvin in either model and follow the resistance-unit convention of the supplied coefficients. Manufacturer resistance tables are another suitable basis for conversion.

What determines thermistor measurement accuracy?

Accuracy depends on the sensor tolerance over the required temperature range, resistance-to-temperature conversion, excitation, electronics and thermal installation. Measurement power raises the sensor above its surroundings, while poor coupling creates a difference between sensor and target temperature. High sensitivity helps resolve small changes but does not remove these systematic errors or guarantee absolute accuracy.

Can a PPTC replace a conventional fuse?

Only when its behaviour satisfies the application requirements. A PPTC limits current by becoming highly resistive, but residual current and downstream voltage can remain. A fuse interrupts current after clearing. Compare fault capability, trip time, temperature derating, leakage and reset behaviour, and assess whether restoration of operation after cooling is acceptable for the equipment.

Does a PPTC reset as soon as the fault is removed?

Not necessarily. The device must cool sufficiently, and remaining circuit current may keep it hot and highly resistive. Many applications require clearing the fault and removing or reducing power before reset. Recovery time depends on mounting and ambient conditions, and the recovered resistance can differ from the initial resistance. Use the specified post-trip resistance limit.

Why can an NTC inrush limiter perform poorly during a hot restart?

An NTC inrush limiter relies on relatively high resistance when cold. After carrying load current, it is hot and its resistance is lower. A short off-time may therefore leave insufficient resistance to limit the next startup pulse as intended. Check cooling, capacitor discharge and any bypass sequence together under the required restart conditions.

How should I select a thermistor for a harsh environment?

Select the complete element or probe assembly for its temperature, moisture, chemical, mechanical and insulation requirements. Verify the specified tests and installed thermal response rather than relying on the package name. Liquid, vacuum and radiation applications require evidence appropriate to the actual conditions. A general statement about thermistor robustness does not qualify an individual component or assembly.

How to select and validate a thermistor

Use the following procedure to turn a temperature or protection requirement into a component specification and an installed validation plan.

Select the correct thermistor family, calculate its circuit behaviour and validate it in the final installation.

Define the function and operating conditions
Identify whether the device measures temperature, compensates a circuit, limits inrush, detects a temperature limit or protects against overload. Record the temperature range, electrical conditions, environment, response requirement and acceptable recovery behaviour.
Select the technology and construction
Choose a sensing NTC, power NTC, ceramic PTC or polymer PTC for the required function. Match the bead, chip, disc, leaded, strap or probe construction to the mounting method, thermal interface, insulation and environmental conditions.
Compare the complete datasheet limits
For sensing, compare the resistance–temperature curve, tolerance, drift, dissipation factor and response conditions. For limiting and protection, compare normal current, pulse or fault capability, voltage, derating, trip time, residual current and recovery resistance. Check qualification and recognition for the exact part.
Design and calculate the circuit
For measurement, calculate signal range, conversion error, self-heating, input loading and diagnostic limits. For power applications, check startup survival, fault-time coordination and restart conditions. Distinguish a low-power temperature sensor from a device intended to carry load current.
Validate the finished assembly
Test the actual board, enclosure or probe under representative temperature and environmental conditions. Confirm accuracy or protection performance, mounting effects, fault response and recovery. Include repeated operation where relevant, and document acceptance limits for catalog parts or custom assemblies.

References

The numbered documents support the operating principles, selection methods and specific product example cited in the text.

EPCOS AG, NTC Thermistors: General Technical Information, EPCOS, January 2018.
TDK Electronics AG, PTC Thermistors: General Technical Information, TDK Electronics, April 2026.
Littelfuse, Fundamentals of Resettable Functionality in PPTC Devices, Littelfuse technical paper, 2016.
Littelfuse, PolySwitch Resettable PPTCs: Battery Strap VTP Series, Littelfuse datasheet, revised September 27, 2021.
Jellenie Rodriguez and Mary McCarthy, Thermistor-Based Temperature Sensing System Explained, Analog Devices contribution reproduced by Passive Components Blog, 2024; published page updated 2026.
EPCOS AG, NTC Thermistors for Inrush Current Limiting, EPCOS application note, 2013 edition.
TDK Corporation, Temperature Sensors: TDK Launches High-Reliability Automotive NTC Thermistors Rated for +175 °C, TDK, February 17, 2026.

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