SF139E SEFUSE Thermal Cutoff: Reliable Heat Protection for Electronics

The SF139E is a thermal cutoff (TCO) fuse manufactured by NEC SEFUSE, designed to provide over-temperature protection in a wide range of electrical and electronic devices. It is a one-shot, non-resettable safety component that acts as the last line of defense in case of excessive heat buildup. Once activated, it interrupts the electrical circuit permanently to prevent potential hazards such as fire, smoke, or damage to sensitive components.

Core Functionality

At the heart of the SF139E is a thermal-sensitive organic pellet enclosed in a compact, metal cylindrical body. The device works by holding a spring-loaded contact in place with this pellet. Under normal conditions, the pellet remains solid, and the circuit stays closed. When the ambient temperature reaches a specified threshold—around 139°C—the pellet melts. This triggers the spring mechanism to snap open, cutting off the electrical flow.

This simple but effective mechanism ensures reliable protection for appliances that may overheat due to electrical faults or mechanical failure.

Key Technical Specifications

  • Functioning Temperature (Tf): 142°C (±0 / -5°C)

  • Operating Temperature: ~139°C ±2°C

  • Holding Temperature (Th): Approx. 114°C

  • Maximum Temperature (Tm): Approx. 159°C

  • Rated Voltage: 250V AC

  • Rated Current: 10A

  • Case Type: Axial-leaded, metal-encased body

  • Reset Type: Non-resettable (must be replaced after activation)

These specifications ensure the SF139E performs reliably in systems requiring heat cutoff close to 140°C. It is particularly valued for its fast-acting, accurate temperature response.

Typical Applications

The SF139E is commonly found in both household and industrial products, including:

  • Rice cookers and electric kettles

  • Hairdryers and irons

  • Transformers and electric motors

  • Coffee machines and microwave ovens

  • Power supplies and battery packs

These devices often operate in thermally enclosed spaces or under high electrical load, making temperature protection essential.

Physical Features and Build

The SF139E features a hermetically sealed metal body to protect the internal components from humidity, dust, and corrosion. Its axial lead design makes it easy to mount in through-hole circuit boards or connect via crimping or soldering in wiring harnesses. The fuse is available in standard and long-lead versions depending on installation needs.

Certifications and Safety Standards

The SF139E is certified by leading global safety agencies, including:

  • UL (USA)

  • CSA (Canada)

  • VDE (Europe)

  • BEAB (UK)

  • PSE (Japan)

  • CCC (China)

These certifications confirm that the device meets stringent quality and reliability standards required for commercial and consumer electronics worldwide.

Conclusion

The SF139E SEFUSE thermal cutoff is a small but vital component in protecting electronic devices from overheating. Its precision-triggered design, compact build, and international safety approvals make it a trusted choice for manufacturers looking to ensure user safety and prevent equipment damage. Once the temperature threshold is exceeded, it disconnects the circuit permanently—providing peace of mind through dependable, one-time thermal protection.

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