How to Test PTC Thermistor with Multimeter
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How to test PTC thermistor with multimeter: take the part out of its circuit, let it reach room temperature, set the meter to ohms and compare the reading with the resistance and tolerance the datasheet states for 25 degrees C. That cold reading is the one check the datasheets we opened support. A PTC thermistor reads low when cool and rises steeply near its switching temperature, so a part that was just running will read high until it cools.
Electrical safety first
Work on mains circuits can kill, and OSHA’s fact sheet on working safely with electricity warns that if the ground path is broken, fault current may travel through a worker’s body, causing electrical burns or death. A meter’s CAT rating has to suit the circuit; Fluke’s category-rating page says equipment with a rating that does not match the task can explode if used improperly. Panel and service work belongs to a licensed electrician, and OSHA’s rule for general industry allows only qualified persons to work on parts that have not been de-energized.
Many PTC thermistors work in mains products. TDK lists motor-start versions for 120 and 230 volt compressors, and a Vishay heater series is rated up to 265 volts. This guide covers a loose part on the bench, in the ohms function only, with every power source disconnected. It gives no steps for working inside refrigerators, heaters or any powered equipment. TDK also warns that a PTC’s surface temperature can be very high during operation, so let a used part cool before handling it. A meter’s CAT rating comes only from its listing or manual; if the listing does not state one, do not treat the meter as suitable for household mains.
Quick answer
- Isolate the part and let it cool to room temperature.
- Read ohms and compare with the datasheet’s resistance at 25 degrees C and its tolerance.
- For parts under about one ohm, correct for lead resistance or use a milliohm meter.
- Do not expect hand warmth to change a switching PTC; its resistance stays low until near the switching temperature.
Which kind of PTC do you have?
The name covers several families. TDK describes ceramic PTC thermistors as parts whose resistance rapidly increases when a certain temperature is exceeded, used as self-resetting fuses, motor starters, temperature sensors and heating elements. A Vishay datasheet adds that NTC thermistors sense and measure temperature over a defined range, while PTC thermistors switch at one particular temperature. A third family is the PTC resettable fuse, which Bourns lists with its own initial and post-trip resistance limits. The table shows the stated cold values we found, with our own arithmetic for the pass range: nominal × (1 − tolerance) to nominal × (1 + tolerance).
| Family and example | Stated at room temperature | Pass range (our arithmetic) |
|---|---|---|
| TDK motor-start, 120 V version | 4.7 ohms ±20% | 3.76 to 5.64 ohms |
| TDK motor-start, 230 V version | 33 ohms ±20% | 26.4 to 39.6 ohms |
| TDK motor-start, 230 V version | 15 ohms ±30% | 10.5 to 19.5 ohms |
| Vishay heater PTCHP12S | 1,200 ohms ±35% | 780 to 1,620 ohms |
| Vishay sensing PTCSL20 and PTCSL03 | R25 of 20 to 100 ohms and 20 to 120 ohms across each series | Use the value for your own part |
| Bourns MF-R010 resettable fuse | 2.50 to 4.50 ohms at 23 °C | Stated window |
| Bourns MF-R050 resettable fuse | 0.41 to 0.77 ohms at 23 °C | Stated window |
| Bourns MF-R250 resettable fuse | 0.025 to 0.048 ohms at 23 °C | Stated window |
The TDK figures are rated resistance values specified at a measuring voltage of 2.5 volts or less, and the Bourns windows come from its MF-R series sheet. Use the datasheet or markings for your own part, since the examples are only a guide to scale. If you cannot identify the part, our guide to checking a fuse with a multimeter covers ordinary fuses, which are a different device.
Take the cold reading
Fluke’s guide to measuring resistance gives the sequence. Turn off power and discharge any capacitor, set the dial to ohms, and expect OL until the leads touch the part. Best results come when the component is removed from the circuit, because components in parallel affect the reading, and the human body becomes a parallel resistance path, so keep your fingers off the metal tips. Use the range button to fix a range.
Handle the part with care. TDK’s cautions say the ceramic and metallization of its PTC thermistors must not be touched with bare hands and that gloves are recommended. They also say the parts must not be dropped and that chip-offs must not be caused during handling.
The test voltage matters less than you might fear, but it is worth knowing. TDK specifies its rated resistances at a measuring voltage of 2.5 volts or less, and a Vishay resistance chart for the PTCSL20 series is drawn at 1.5 volts DC maximum. The Fluke 87 manual says its full-scale measurement voltage on ranges below 40 megohms does not forward-bias silicon diodes or transistor junctions enough to make them conduct, and it gives about 0.6 volts as the typical silicon forward voltage. By our reasoning that puts a meter of that kind under the datasheet ceilings, but the pages opened do not state any meter’s ohms-range voltage, so check your manual.
Low-resistance parts need a correction
The resettable fuses in the table have tiny resistances, and lead resistance can swamp them. Fluke’s guide puts lead resistance at typically 0.2 to 0.5 ohms and describes a relative mode that subtracts it, while the Fluke 87 manual gives 0.1 to 0.2 ohms and says to short the leads together to find the error. Apply that to the MF-R050 window of 0.41 to 0.77 ohms. A 0.2 ohm lead error turns it into a displayed 0.61 to 0.97 ohms, and a 0.5 ohm error into 0.91 to 1.27 ohms, so an uncorrected reading could pass a faulty part or fail a good one.
Resolution is the second limit. The Fluke 87 manual lists 0.01 ohm as the minimum display reading for resistance and points to its 4½-digit display mode for that figure. For the MF-R250 window of 0.025 to 0.048 ohms, one count of 0.01 ohm is 0.01 ÷ 0.048 = 21 percent of the upper limit. Parts like that call for a milliohm meter; our guide to milliohm meters explains the type.
What a hot or just-used part reads
A PTC’s resistance stays low until the part nears its switching temperature, then climbs steeply. The TDK overload-protection presentation defines the parts by RN, the resistance at 25 degrees C, and by Tref, the reference or Curie temperature, at which the resistance is 2 × Rmin. The Vishay PTCSL20 datasheet shows the steepness in numbers. For a version with a nominal working temperature Tn of 100 degrees C, the stated limits are 20 to 250 ohms from −20 degrees C up to Tn − 20 (80 degrees C), 50 to 550 ohms at Tn − 5 (95 degrees C), at least 1.33 kilohms at Tn + 5 (105 degrees C), and at least 4 kilohms at Tn + 15 (115 degrees C). Our arithmetic: 4,000 ÷ 250 = 16, so the resistance at 115 degrees C is at least 16 times the highest cold limit.
| Condition for a Tn = 100 °C part | Stated limit |
|---|---|
| −20 °C up to 80 °C | 20 to 250 ohms |
| 95 °C | 50 to 550 ohms |
| 105 °C | 1.33 to 50 kilohms |
| 115 °C | at least 4 kilohms |
Two practical consequences follow. First, a part that has just been running is above its cold band and will read high until it cools, so wait before you judge it. Second, hand warmth is far below the 80 degrees C in that table. On our assumption of a hand-warmed surface near 30 to 35 degrees C, the stated limit stays at 250 ohms or less, so the hand-warm test that works for an NTC part tells you little here. The pages opened give no heating method for a multimeter test of a PTC, so this guide does not offer one. It also does not suggest heating a part with a flame or a soldering iron, and TDK’s warning about very hot surfaces applies.
Resettable PTC fuses after a trip
A PTC resettable fuse shows its own pattern. Bourns lists both an initial resistance window at 23 degrees C and a maximum post-trip resistance measured one hour after tripping. By our reading, a part that has just tripped is hot and reads high, and the sheet applies its limit only after an hour. If a resettable fuse has tripped, give it time to cool before you read it, and then compare the result with the initial window for the exact model. The sources do not give a recovery time for any one part.
Reading the results
| What you see | What it may mean |
|---|---|
| Inside the stated cold window | The part passes the cold check only |
| High, but falling as the part cools | The part was recently hot or tripped; wait and read again |
| OL on a cool part | Open element, or a range that is too low |
| Far below the window | Parallel paths, a wrong part, or lead error on a tiny value |
| Wanders under your hands | Finger contact on the metal tips |
Mistakes to avoid
- Testing in the circuit. The Fluke 87 manual says the meter shows total resistance through all possible paths between the probes.
- Judging a warm part. The curves show the resistance rising steeply near the switching temperature.
- Ignoring lead resistance. On the MF-R050 window it is the same size as the quantity being measured.
- Handling the ceramic with bare hands. TDK’s cautions advise gloves.
- Treating a fuse and a PTC as the same part. An ordinary fuse is open or closed, and a PTC resettable fuse is a resistance that changes.
How we chose
Everything here comes from pages opened on 2026-10-21: the TDK EPCOS PTC product profile and its overload-protection presentation, three Vishay datasheets for sensing and heating PTC thermistors, the Bourns MF-R series sheet, the Fluke resistance guide, the Fluke 87 manual, two OSHA pages and a Fluke category-rating page. We kept the one check the sources support, a cold reading against a stated window, and drew on data-sheet tables for what hot readings look like. Our arithmetic is labeled. The licensed-electrician advice is a site rule, since OSHA speaks of qualified persons.
What we could not verify
What the sources do not say is the most useful limit here. They give no multimeter heating procedure for a PTC, no recovery time for a tripped resettable fuse, and no test voltage for any particular meter. The Bourns table in the text we read is laid out in columns, so only the initial-resistance windows for three models are quoted, read in order against the model list, and the datasheet for your own part should be checked directly. The motor-start and heater values are for specific parts, not for every PTC. The hand-warmth figure is our own assumption.
Frequently asked questions
What should a PTC thermistor read at room temperature?
The resistance its datasheet states at 25 degrees C, within the stated tolerance. Examples run from about 4.7 ohms for a 120 volt motor-start part to 1,200 ohms for a heater part, so the number depends on the part.
Why does my PTC read high right after use?
It is still hot. A PTC’s resistance climbs steeply near its switching temperature, so a part that has been running reads high until it cools toward room temperature.
Can I use the hand-warm test on a PTC?
It tells you little. A switching PTC stays within its low cold band until it is close to its switching temperature, which for the Vishay PTCSL20 sensing series starts at a nominal working temperature of 70 degrees C.
Does it matter that the meter’s test voltage is low?
Datasheets specify low measuring voltages, 2.5 volts or less for TDK and 1.5 volts DC maximum on a Vishay chart. A handheld meter in ohms mode should stay under those ceilings, but check your manual for the exact figure.
Is a PTC resettable fuse tested the same way?
The cold check is similar, but the windows are tiny. A low-resistance part needs lead correction or a milliohm meter, and a tripped part needs time to cool.
Bottom line
Read a cool, isolated PTC part against the window its datasheet states, correct for lead resistance on tiny values, and treat a high reading on a warm part as heat, not a fault.
Sources
- TDK EPCOS, PTC Thermistors product profile 2016: https://www.tdk-electronics.tdk.com/download/141178/22abc1c70cb555978705dd4188584463/ptc-thermistors-pp.pdf (opened 2026-10-21)
- TDK Electronics, Overload PTC Thermistors, Resettable Fuses presentation (July 2020): https://www.tdk-electronics.tdk.com/download/2852446/7f9d0d2f16a75be7663019da577a3727/ptc-ocp-presentation.pdf (opened 2026-10-21)
- Vishay BCcomponents, PTCSL20 datasheet (doc 29012), copy hosted by Farnell: https://www.farnell.com/datasheets/2245099.pdf (opened 2026-10-21)
- Vishay BCcomponents, PTCSL03 datasheet (doc 29160): https://wwwlegacy.vishay.com/docs/29160/ptcsl03.pdf (opened 2026-10-21)
- Vishay BCcomponents, PTCHP12S heating thermistor datasheet (doc 29093): https://wwwlegacy.vishay.com/docs/29093/ptchp12s.pdf (opened 2026-10-21)
- Bourns, MF-R series PTC resettable fuses datasheet: https://www.bourns.com/docs/Product-Datasheets/MF-R.pdf (opened 2026-10-21)
- Fluke, How to Measure Resistance with a Digital Multimeter: https://www.fluke.com/en-us/learn/blog/digital-multimeters/how-to-measure-resistance (opened 2026-10-21)
- Fluke, 87 True RMS Multimeter Users Manual: https://assets.fluke.com/manuals/87______umeng0800.pdf (opened 2026-10-21)
- OSHA, Working Safely with Electricity fact sheet: https://www.osha.gov/sites/default/files/publications/OSHA3942.pdf (opened 2026-10-21)
- OSHA, 29 CFR 1910.333 selection and use of work practices: https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.333 (opened 2026-10-21)
- Fluke, IEC Category Ratings: Use the Right Tools for the Job: https://www.fluke.com/en-us/learn/blog/safety/iec-category-ratings-use-the-right-tools-for-the-job (opened 2026-10-21)
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