What Is an NTC Thermistor? Working Principle, Applications, and Failure Detection

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An NTC (Negative Temperature Coefficient) thermistor is an essential component used in the power supply section of electronic devices. It is commonly connected in series with the AC input line and is responsible for suppressing inrush current at the moment of power-up.

 

 

Structure and Principle of NTC Thermistor

An NTC thermistor consists of a semiconductor ceramic element whose resistance decreases as the temperature rises. At room temperature, it maintains high resistance, limiting the inrush current when power is first applied. As the device warms up, the resistance rapidly drops, allowing stable current flow with minimal power loss.



Main Functions of NTC Thermistor

  • Suppressing inrush current: protects bridge diodes and capacitors from surge damage
  • Gradual startup of power supply: ensures smooth charging of electrolytic capacitors
  • Thermal protection: can act as a safety element if placed near heating elements

 

 

Where Is It Installed?

In most SMPS (Switched Mode Power Supplies), you will find the NTC thermistor installed between the AC input and the bridge diode. It is usually a small disc-shaped component, often black or dark green, labeled with codes like "NTC 5D-9" or "10D-11", indicating its resistance and size.

Symptoms of Thermistor Failure

  • Cracked or burnt appearance – indicates internal breakdown from excessive current
  • Infinite resistance – thermistor behaves like an open circuit, resulting in no power flow
  • Very low resistance – indicates a shorted thermistor; surge protection is disabled

 

 

How to Test an NTC Thermistor

You can test it with a multimeter:

  • At room temperature, the resistance should match the rated value (e.g., 5Ω to 10Ω)
  • If the thermistor reads infinite resistance – it’s open
  • If it reads under 1Ω – it may be shorted

Replacement Tips

  • Use the same rated resistance and size (e.g., 5D-9, 10D-9)
  • Consider thermal environment – larger ones handle more power
  • Solder firmly and avoid thermal damage during replacement

 

 

FAQ

  • Q. Can I run the circuit without an NTC thermistor?
    Yes, but it increases the risk of inrush current damage.
  • Q. What happens if I use a wrong-rated thermistor?
    It may not suppress current properly or cause slow startup.
  • Q. Does an NTC thermistor wear out over time?
    Yes, repeated surges can degrade its structure, requiring replacement.