Understanding NTC Thermistors: The Hidden Guardians of Power Supply Circuits

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Thermistors may be small, but their role in electronic circuits, especially power supply boards, is massive. Among them, NTC (Negative Temperature Coefficient) thermistors are frequently used as inrush current limiters. This post walks you through how NTC thermistors work, why they’re wired in series with hotlines, and how to test or diagnose them.

 

 

What Is an NTC Thermistor?

An NTC thermistor is a temperature-sensitive resistor that decreases its resistance as temperature increases. When cold, it provides high resistance; as it heats up (due to current flow), its resistance drops, allowing higher current to flow safely into the circuit. This makes it perfect for power-up scenarios where large capacitors or switching components could otherwise cause a surge.

compact bridge rectifier pcb board module

Why Is It Wired in Series on the Hot Line?

The primary role of an NTC thermistor in a power circuit is inrush current suppression. It is always installed in series with the AC input "hotline." On startup, it restricts sudden surges of current, protecting the bridge rectifier, capacitors, and FETs downstream. Once the thermistor warms up and its resistance drops, it essentially becomes negligible, allowing normal operation.

 

 

Thermistor Characteristics

  • Symbol: Usually marked as TH1 or NTC on PCBs.
  • Typical Resistance: 5Ω to 100Ω at room temperature.
  • Behavior: Resistance drops rapidly as current flows and it heats up.
  • Failure Modes: Open circuit, short, or failure to decrease resistance (aging).

How to Test an NTC Thermistor

Use a multimeter in resistance mode to measure across the thermistor:

  • Cold Test: Room temperature should give you a few ohms of resistance (e.g., 10Ω).
  • Warm Test: Applying heat (e.g., hair dryer) should lower the resistance.
  • Open/Short: Infinite or zero resistance indicates failure.

 

 

Common Applications

  • Switching Power Supplies (SMPS)
  • Battery Chargers
  • LED Drivers
  • Motor Drives and Inverters

They’re especially useful in high-inrush situations—when a cold circuit is suddenly powered on and the filter capacitors demand a massive initial charge. Without thermistors, this can damage rectifiers or FETs almost instantly.

How to Identify a Thermistor on a PCB

Look for a small disc-shaped black or green component near the AC input or bridge rectifier, marked “NTC” or “TH1.” It’s almost always on the hotline, before the main filtering stage.

 

 

Final Thoughts

While they may seem insignificant, NTC thermistors play a critical role in the safety and durability of power circuits. If you're repairing or diagnosing a power supply, don't overlook this small component—it might be the silent protector that saved your circuit more times than you realize.