How to find the right Thermistors AI guide
Written by an AI model from its trained knowledge.
What the AI looks for in Thermistors
- NTC vs PTC
- Use NTC (resistance falls with heat) for inrush limiting and temperature sensing. Use PTC (resistance rises sharply when hot) for resettable overcurrent protection. The two behave in opposite ways and cannot be substituted for each other.
- NTC inrush limiter sizing
- Match the cold resistance at 25 °C to the inrush current you want to allow. Confirm that the maximum steady-state current covers the load, and that the energy or capacitance limit covers the bulk capacitor being charged. If undersized, the part overheats and cracks. If oversized in resistance, it wastes power and may stop the supply from starting.
- NTC sensor parameters
- Match R25 (such as 10 kΩ), the B value and the tolerance to the controller's curve or lookup table. A sensor with a different B value reads wrong across the range even when R25 matches.
- PTC resettable fuse ratings
- Hold current must exceed normal load at the highest ambient temperature. Trip current, maximum voltage and maximum fault current must all cover the circuit. Hold current derates with heat, and exceeding the voltage or fault limit can destroy the device instead of tripping it.
- Reset and recovery time
- Allow cool-down time. NTC limiters need to cool before they limit again, and PTCs reset only after power is removed and they cool. Rapid power cycling defeats an NTC limiter, and PTCs stay tripped while the fault voltage is present.
Brands that matter
- Ametherm: Specialist in NTC inrush current limiters.
- TDK (EPCOS): Broad NTC and PTC thermistor ranges.
- Littelfuse: PolySwitch PTC resettable fuses and NTC thermistors.
- Bourns: Multifuse PTC resettable fuses.
- Vishay: NTC sensing and inrush thermistors.
- Murata: Chip NTC and PTC thermistors for surface-mount boards.
What to avoid
- Replacing a cracked inrush NTC with one of different cold resistance or smaller body size.
- Using a PTC resettable fuse on a circuit whose voltage exceeds its maximum voltage rating.
- Mixing sensor NTCs with different B values on the same controller.
Questions
Can I bypass a burnt NTC thermistor in a power supply with a wire?
The supply may run, but the inrush current at switch-on will stress the rectifier, the capacitors and the upstream fuse or breaker. Replace it with a part of the same cold resistance and current rating.