A PTC thermistor is a resistor whose resistance rises as it gets hotter. PTC stands for positive temperature coefficient. That single definition covers two devices that behave nothing alike and are bought for opposite reasons.

A silicon PTC changes by about 0.76% per kelvin across its whole range, smoothly, and you use it to read a temperature.

A ceramic switching PTC sits under 100 Ω all the way up to its rated point, then crosses 1330 Ω within five degrees and 4 kΩ within fifteen, and you use it to trip something.

Specifying one when you needed the other is the most common mistake in this category.

Published 10 September 2026. Reviewed by the Focusens technical department.

The two families side by side

Both are sold as "PTC thermistors". These are real figures from two Focusens series, one from each family.

Specifications from the Focusens product catalogue. The 60 s figure for the silicon series is quoted against different media on the series page and the element table, so confirm the medium against a part code before designing a control loop around it. A response time without its medium is not a specification.

Read the resistance rows together. A silicon PTC moves from 1000 Ω to about 1670 Ω across 75 degrees. A switching PTC moves from under 100 Ω to over 4000 Ω across fifteen. The first is a measurement.

The second is a switch that happens to be made of ceramic.

Built from the Focusens catalogue threshold figures. The switching curve carries no usable temperature information below its rated point.

Which one you actually need

Answer three questions in order.

Do you need a number or a decision?

If a controller has to display, log, or regulate against the temperature, you need a sensor, and that means a silicon PTC, an NTC or an RTD.

If something has to happen when a limit is crossed and nothing has to happen before that, a switching PTC does it with no conversion and no calibration.

Where does the device sit?

A sensor sits on a signal input.

A switching PTC for protection usually sits inside the thing being protected, embedded in a motor winding or clamped to a transformer core, wired to a relay that only reads two states.

Does it have to reset itself?

A switching PTC returns to its low resistance once it cools, which is why it is used where a fuse would have to be replaced.

That is a feature when the overload is transient and a hazard when it is not, because the machine can restart into the same fault. If the answer to the first question was "a number", the next decision is which sensing technology.

The NTC thermistor guide and the resistance temperature detector guide cover the two alternatives, and the short version is that NTC gives the most resolution near ambient at the lowest cost, a silicon PTC gives a flatter and more linear response, and an RTD gives a standardised curve over a much wider span.

How a switching PTC is actually specified

A switching PTC is not specified by a coefficient. It is specified by what its resistance must be at fixed offsets from its rated temperature. That rated point is written TK, and the thresholds are what a protection relay is built to detect.

These are the MZ6 series values for rated points between 90 and 160 °C, with a TK tolerance of ±5 °C.

From the Focusens MZ6 series datasheet, for rated points between 90 and 160 °C. The full threshold set, including the values for a series string and the different bands that apply below 90 °C and above 160 °C, is in the motor protection guide linked below.

One consequence is worth carrying away from those two rows. A switching PTC crosses roughly a factor of eight in twenty degrees, and outside that window it reports nothing useful at all.

Its resistance at 25 °C is specified only as a maximum, so a good element and a marginal one look identical on a bench meter at room temperature. Judge a switching PTC at its rated point or not at all.

The MZ6 series conforms to DIN VDE V 0898-1-401, the German standard that consolidated the older DIN 44081 and DIN 44082. The international generic specification for these devices is IEC 60738-1.

Where the parts go into a motor, the DIN 44081 and DIN 44082 guide covers the standard in detail and PTC thermistors for motor winding protection covers the wiring and the TK choice.

The silicon PTC as a linear sensor

Silicon PTCs, sometimes called silistors, are a different technology wearing the same label. A doped silicon element gives a resistance that rises smoothly and almost linearly, which makes it usable as a thermometer in a way a ceramic switching PTC never is.

The Focusens FTY series runs −40 to +150 °C with a temperature coefficient of 0.76 %/K at 25 °C, a rated working current of 5 mA and a maximum continuous current of 10 mA at 25 °C, and rated power dissipation of 50 mW.

Resistance ratios are specified rather than a single coefficient: R100/R25 is 1.65 to 1.69 with 1.67 typical, and R−55/R25 is 0.49 to 0.51. The 1 kΩ silicon PTC exists mostly because a generation of machinery was designed around the NXP KTY family, which is now discontinued.

If you are replacing one of those, the KTY83-110 cross-reference has the part-by-part mapping. Current is the parameter to watch on a substitution:

at 5 mA rated and 10 mA maximum, a silicon PTC has less headroom than most designers assume, and self-heating shows up as a reading that drifts upward after power-on.

Polymer PTC, and why it is a different purchase

A polymer PTC, usually sold as a resettable fuse or PPTC, uses conductive particles in a polymer matrix rather than a ceramic. It is a protection component in a power path, specified by hold current, trip current and voltage rating rather than by a temperature at all.

It shares the PTC label and almost nothing else. If you are searching for temperature sensing, a polymer PTC is not the part, and if you are searching for overcurrent protection, neither of the two families above is.

Testing, and what a meter can and cannot tell you

For a silicon PTC, a meter is useful: measure at a known ambient, compare against the rated R25, then warm the part and watch resistance rise smoothly. For a switching PTC, a meter at room temperature tells you only that the part is not open or shorted.

Confirming the switching behaviour needs the part taken through its rated point under controlled heat, which is a bench or oven job rather than a hairdryer job. The step-by-step procedure and the reading-interpretation table are in how to test a PTC thermistor with a multimeter.

If you are holding an unmarked part and do not yet know which type it is, start with how to identify NTC and PTC thermistors. The full range sits under PTC thermistors, with the silicon types under linear PTC silicon thermistors.