How to Test and Troubleshoot a PTC Thermistor with a Multimeter

Suspect a faulty PTC thermistor in your circuit? Whether it's a resettable fuse that won't reset or a heater that isn't warming up, a standard digital multimeter (DMM) is the perfect tool for a quick and effective diagnosis. This guide will walk you through the simple steps to test and troubleshoot a PTC thermistor.

Safety First!

• Disconnect Power: Always ensure the device is completely disconnected from any power source and that any capacitors are discharged before testing.

• Isolate the Component: For the most accurate reading, desolder at least one leg of the PTC thermistor from the circuit board. Testing it in-circuit can give false readings due to parallel paths with other components.

What You'll Need:

• A digital multimeter (DMM)

• The suspect PTC thermistor

• A heat source (e.g., a hairdryer, a hot air rework station on low, or a cup of hot water)

Step 1: The Room Temperature Resistance Test

This is the first and most basic check.

1. Set your multimeter to the resistance (Ohms, Ω) mode. Choose an appropriate range, usually the auto-ranging or the 2kΩ range is a good start.

2. Place the multimeter probes on the two terminals of the PTC thermistor. Polarity does not matter.

3. Read the value.

What to Expect:

• A good PTC should show a low resistance value. This can range from a few ohms to around 100 ohms, depending on its specific model and rating. Consult its datasheet for the exact typical value (R_min).

• Troubleshooting:

  • Infinite Resistance (OL): The PTC has failed open. It is blown and must be replaced. This is a common failure mode if it was subjected to a fault far beyond its maximum ratings.

  • Zero Resistance (0Ω): The PTC has failed short. This is very rare but indicates a catastrophic internal failure. It must be replaced.

Step 2: The Heat Test (The Functional Test)

This test confirms the component's core functionality: its ability to change resistance with temperature.

1. Keep your multimeter probes attached, watching the resistance reading.

2. Gently apply heat to the body of the PTC thermistor using your heat source. A hairdryer on a low setting works perfectly. Do not use a direct flame, as it can easily destroy the component.

3. Observe the multimeter's reading closely.

What to Expect:

• A good PTC will show a slow but steady increase in resistance as it warms up. If you heat it past its specific Curie or switching point, the resistance will increase dramatically, often jumping to hundreds of thousands of ohms (kΩ) or more, possibly causing the meter to read "OL".

• Troubleshooting:

  • No Change in Resistance: If the resistance stays stubbornly low regardless of how much you heat it, the PTC is failed and stuck in its low-resistance state. It has lost its PTC characteristic and must be replaced.

  • Resistance Drops: This is highly unusual and indicates the component is actually an NTC thermistor, not a PTC.

Step 3: The Cool-Down Test (Checking the Reset)

A key feature of a PTC is that it self-resets.

1. Remove the heat source.

2. Continue to watch the multimeter reading.

What to Expect:

• A good PTC will slowly cool down. As it does, its resistance will decrease steadily back to its original low value. This may take a minute or two.

• Troubleshooting:

  • If the resistance stays high and does not decrease after the component has clearly cooled to room temperature, the PTC has failed in its tripped state and will not reset. It needs replacement.

Interpreting Your Results

Conclusion

Testing a PTC thermistor is a straightforward process that can save you time and money in repairs. By systematically checking its resistance at room temperature, under heat, and after cooling, you can definitively determine if it's functioning correctly or if it's the cause of your circuit's malfunction. Remember, a working PTC should have a low cold resistance, a rapidly increasing hot resistance, and it must always reset itself after cooling down.