Direct answer: To test a PTC thermistor, power off the circuit, isolate at least one lead, set the multimeter to ohms, and measure the cold resistance near 20-25 C. Then apply gentle heat and watch the reading. A working PTC increases resistance as it warms and drops back after cooling. OL when cold usually means open; near-zero ohms usually means a short or wiring fault.

What This Test Can And Cannot Tell You

A multimeter test can tell you whether the PTC is open, shorted, responding to heat, resetting after cooling, or possibly not a PTC at all. It is useful for repair screening, incoming inspection and field diagnosis.

It cannot fully certify the rated switching temperature of a motor-protection PTC unless the test uses controlled temperature equipment and the correct datasheet limits. For safety-critical motor protection, use the multimeter test as a first check, then confirm the TK/TNAT and relay compatibility before replacement.

Tools And Safety Notes

Power off and discharge the equipment before measuring. If the PTC is still connected to a board, relay input or motor circuit, disconnect one lead or unplug the sensor where practical. In-circuit testing can confirm a gross open or short, but it can also hide the real PTC value behind parallel components.

Do not use a flame. Do not overheat a small bead or motor winding sensor just to force an OL reading. The goal is to confirm a rising resistance trend and reset behavior, not to destroy the part.

Four-Step Multimeter Test

For a small ceramic switching PTC, the cold value may be low ohms to a few hundred ohms depending on the part. For silicon PTC/KTY-style sensors, the room-temperature value is often around 1 kOhm. Always use the datasheet as the pass/fail reference.

Result Interpretation Table

Key diagnostic rule: cold resistance alone is not enough. It proves continuity and rough value only. A real PTC diagnosis needs the direction of resistance change with heat and the cool-down reset check.

Special Case: DIN 44081 / DIN 44082 Motor-Protection PTCs

Motor-protection PTCs are not precision temperature sensors. They are limit sensors designed to stay low-resistance below the rated response temperature and rise steeply near TNAT/TK so a thermistor relay can trip.

For DIN 44081 / DIN 44082 style motor-protection PTCs:

Example relay context: ABB CM-MSS thermistor motor-protection relays list overtemperature trip behavior around 2.83 kOhm total sensor-circuit resistance and reset after cooling around 1.1-1.2 kOhm, depending on model. Treat this as a relay-specific example, not a universal threshold. Always check the relay datasheet used in the equipment.

In-Circuit Testing Caveat

You can sometimes check a PTC in-circuit for obvious faults:

If the result affects a repair, replacement order or motor safety decision, isolate the PTC and compare with the datasheet before making the final call.

PTC vs NTC Identification Note

A PTC thermistor has a positive temperature coefficient: its resistance increases as temperature rises. An NTC thermistor does the opposite: resistance decreases as temperature rises.

If the unknown part drops in resistance when warmed, do not call it a failed PTC first. Treat it as an NTC candidate, check the part marking, and link the reader to the NTC vs PTC thermistor guide.

Focusens Product Route + CTA

If the test shows an open, shorted or non-resetting PTC, collect the application data before quoting a replacement:

CTA: Need a replacement PTC thermistor or MZ6 motor-protection assembly? Send Focusens the cold resistance, TK/TNAT value, sensor count, cable length, insulation requirement and relay model. Link this CTA to the PTC thermistor range and MZ6 product pages.

FAQ

What should a PTC thermistor read on a multimeter?
It should read its rated cold resistance at room temperature. Ceramic switching PTCs may read from low ohms to a few hundred ohms, while silicon PTC/KTY-style sensors are often around 1 kOhm. Use the datasheet value as the pass/fail reference.

How do I know if a PTC thermistor is bad?
OL when cold suggests an open sensor or broken lead. Near-zero ohms suggests a short. No resistance change with heat suggests the PTC effect is lost. Staying high after cooling suggests the part is damaged or still thermally tripped.

Can I test a PTC thermistor in-circuit?
Only for rough screening. In-circuit readings can be distorted by parallel resistors, relay inputs, controller circuits or cable faults. For a real pass/fail decision, isolate one lead or unplug the sensor and retest.

Does polarity matter when measuring a PTC with a multimeter?
For a two-lead ceramic PTC, polarity does not matter in resistance mode. For silicon PTC/KTY-style sensors, check the datasheet for operating polarity, but the basic resistance reading can still be used for identification.

Why does my meter show OL when I heat the PTC?
That can be normal for a switching PTC if it has reached the high-resistance region and the meter range is exceeded. It is a failure only if the part reads OL when cold or will not return after cooling.

Is cold resistance enough to verify a motor-protection PTC?
No. Cold resistance confirms continuity and approximate value. The protection function depends on the steep resistance rise near the rated response temperature TNAT/TK.

What is the DIN 44081 test voltage limit?
For room-temperature low-ohmic testing of DIN motor-protection PTCs, keep the test voltage at or below 2.5 V DC to avoid self-heating and misleading readings.

What if resistance falls when I apply heat?
That behavior points to an NTC thermistor, not a PTC thermistor. Re-check the part marking, application and datasheet before ordering a replacement.