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In the operation cycle of electronic devices, the power-on moment is the most critical.When power is applied, a large current rushes into the filter capacitors, generating an inrush surge that can be tens of times higher than the normal operating current.This sudden surge can damage components such as rectifier bridges, fuses, and relay contacts, while also accelerating system aging and shortening overall equipment lifespan.

Inrush current NTC thermistors are the key components that protect against this surge current.But why must they always be connected in series?Let’s explore this through four aspects: principle, working process, advantages, and limitations.

I. Core Principle — Negative Temperature Coefficient and Series Logic.

NTC (Negative Temperature Coefficient) thermistors exhibit a key characteristic:their resistance decreases as their temperature increases.

At room temperature (cold state), their resistance is relatively high.Once current begins to flow, the device heats up, its temperature rises, and its resistance drops sharply — forming the foundation of dynamic current limiting.In any circuit, the most straightforward way to limit current is by adding a series resistor.An Inrush current NTC combines this principle with temperature sensitivity, acting as a “smart variable resistor” when placed in series with the main power line.It provides current-limiting protection during startup while introducing almost no loss during steady operation.

II. Series Operation — From “Current Limiting” to “Low Resistance Running”.

When the device is first powered on, the NTC is cold and highly resistive. This limits the inrush current, protecting the rectifier bridge, filter capacitors, and other downstream components.

As the NTC warms up due to current flow, its resistance rapidly decreases to just a few tenths of an ohm or less, allowing normal current flow with minimal power loss.This process involves two distinct stages:

• Cold (high resistance): limits inrush current at startup.
• Hot (low resistance): ensures efficient, low-loss operation afterward.

This automatic self-adjustment requires no external control circuit, providing a simple and reliable soft-start protection mechanism for electronic systems.

III. Key Advantages of Series Configuration.

• Effective Inrush Current Suppression.
  Series-connected NTC thermistors can reduce peak inrush currents by 5 to 15 times, significantly enhancing system safety.
• Simple Structure & Low Cost.
  A single component achieves reliable current limiting — no complex circuitry required, offering outstanding cost-performance benefits.
• High Reliability & Long Lifespan.
  With no moving parts or semiconductor control sections, NTCs operate with minimal interference and can last as long as the entire device.
• Fully Automatic Operation.
  Protection activates immediately upon power-on — no external signal, control, or user intervention required.

IV. Limitations and Design Considerations.

(1) Recovery Time
After shutdown, the NTC must cool to regain its high resistance.If the device restarts too quickly, the thermistor remains in a low-resistance state, losing its surge protection ability.
→ Solution: Use a relay bypass design — once the system stabilizes, the relay short-circuits the NTC, improving reliability and eliminating steady-state power loss.

(2) Heat Loss and Power Design
Even during steady operation, a small amount of heat is generated.Designers should ensure that the NTC operates within its rated power range and include adequate ventilation or PCB heat dissipation.

(3) Ambient Temperature Sensitivity
Initial resistance changes significantly with temperature.In very cold environments, startup may be difficult due to high resistance; in hot conditions, resistance may be too low to limit current effectively.
→ Solution: Select an NTC with a suitable B-value, or use a constant-current startup circuit for wide-temperature applications.

V. Summary

The series connection of power-type NTC thermistors is fundamental to their surge-limiting performance:

• High resistance in the cold state provides effective protection at startup.
• Low resistance in the hot state ensures efficient operation.

This combination makes NTC thermistors one of the most economical and reliable soft-start solutions in modern electronics.

Nanjing Shiheng Electronics Co., Ltd. specializes in the R&D and production of NTC thermistors and temperature sensors.Our full range of power-type products — MF72, MF73T, MF74, MF75— covers sizes from Φ3 mm to Φ40 mm, meeting demands from small household appliances to kilowatt-level industrial systems.

With high precision, excellent stability, and broad adaptability, Shiheng Electronics delivers safe, efficient, and long-lasting surge protection solutions for customers worldwide.