I. Differences in Core Principles
- One-time Fuses (Fast-acting / Slow-blow Glass or Ceramic Fuses)
These operate by having a metal wire melt due to heat generated by overcurrent, permanently breaking the circuit. Once blown, the circuit is completely interrupted and cannot be restored; manual replacement is required.
- Resettable Fuses (PPTC – Polymeric Positive Temperature Coefficient Thermistors)
Upon heating due to overcurrent, the internal polymer crystals expand, causing resistance to spike to a near-open-circuit state. Once the fault is cleared and the device cools, the crystals contract and resistance drops, automatically restoring conductivity; no replacement is needed, and they can be reused repeatedly.
II. Differences in Electrical Performance
The primary distinction between standard fuses and PPTC resettable fuses is the latter's ability to reset; however, there are other differences as well.
Leakage Current: During an overload, a resettable fuse transitions from a low-resistance state to a high-resistance state—a process often called "tripping." It limits the current to a specific leakage level to provide protection. This leakage current can range from around 100 mA at rated voltage to several hundred milliamperes at lower voltages. In contrast, when a standard fuse encounters an overload, it blows and completely cuts off the current, resulting in zero leakage current in the disconnected circuit.
Interrupting Current: This refers to the maximum short-circuit current a fuse is rated to handle at its rated voltage. While this represents the maximum fault current the device can withstand, a resettable fuse does not actually cut off the current completely (see "Leakage Current" above); the standard short-circuit rating for resettable fuses is 40 A. Conversely, a standard fuse physically interrupts the current in response to an overload. Their rated interrupting currents cover a wide range, extending from several hundred amperes up to 10,000 amperes.
Rated Current: The rated operating current for resettable fuses can reach up to 11 A, whereas the maximum rated operating current for standard fuses can exceed 20 A.
Rated Voltage: Conventional resettable fuses typically have a rated voltage of no more than 60 V, while standard fuses can have ratings up to 600 V. Resistance: Product specifications reveal that, for similar ratings, the resistance of a resettable fuse is double (or sometimes higher than) that of a standard fuse.
Temperature Rating: The typical upper temperature limit for resettable fuses is 85°C, whereas standard fuses have a maximum operating temperature of 125°C. Both types of devices require derating when operating in environments above 20°C.
Time-Current Characteristics: A comparison of time-current curves shows that the response time of a resettable fuse is comparable to the time delay of a "Slo-Blo" (time-lag) fuse.
Semiware resettable fuses comply with the UL 1434 standard for thermistors, and their PPTC products meet the IEC 730-1 standard (Automatic Electrical Controls).
III. Differences in Application Areas
- Prefer One-Time Fuses
Suitable for scenarios involving critical failures, high fire risks, or where automatic recovery is prohibited; faults require manual inspection to prevent secondary damage caused by automatic reconnection.
High-voltage mains circuits (AC 220V inputs, adapter power inlets, charging pile AC inputs, home appliance power cords):
Mains short circuits can cause fires or equipment explosions; power must be completely cut off upon a short circuit, and automatic reconnection is forbidden.
High-power heating equipment: Electric blankets, electric kettles, ovens, charger high-voltage sides, and switching power supply primary circuits.
Battery main circuit high-voltage protection (lithium battery pack master fuse): Cell short circuits are high-risk faults; automatic power restoration is not permitted to prevent fire or explosion.
Applications requiring precise, rapid disconnection: Audio amplifiers and precision instruments require instantaneous short-circuit cutoff; PPTC response times are too slow and could result in component burnout.
Equipment that cannot be left unattended: Industrial high-voltage control cabinets and high-power industrial power supplies.
- Prefer PPTC Resettable Fuses
Suitable for low-voltage, low-current DC scenarios characterized by frequent nuisance overcurrent events, where automatic retry is desirable and device disassembly for fuse replacement is inconvenient.
Consumer electronics low-voltage branch circuits: USB ports, headphones, keyboards, mice, smart bands, and power bank output ports. USB ports frequently experience short circuits (due to incorrect insertion or cable faults); using a one-time fuse requires dismantling the device to replace it after a single incident, whereas a PPTC limits the current and allows for immediate recovery once the faulty cable is removed.
Low-voltage, light-load automotive applications: interior ambient lighting, in-car USB charging, small window motors, and vehicle cameras. Vehicle disassembly is difficult; occasional motor stalls or short circuits can be resolved simply by turning the vehicle off and restarting it.
Motor stall protection: small toy motors, fans, and gimbal motors. Overcurrent occurs when the motor jams; operation resumes once the obstruction is cleared, without needing part replacement.
High-density protection for multiple circuits: power supplies for PC motherboards, hard drives, and fans. If one circuit experiences a temporary short circuit, the PPTC limits the current without affecting the entire system, and the device recovers upon cooling.
Portable and wearable devices: limited space makes disassembly and repair inconvenient; these devices can tolerate temporary current limiting.
IV. Product Limitations and Differences
- Disadvantages of One-Time Fuses
Rendered useless by temporary surges or accidental short circuits; requires replacement stock.
Prone to nuisance tripping (blowing) due to motor operation or frequent high-inrush loads.
- Disadvantages of PPTC Resettable Fuses
Must never be used with high-voltage AC (220V): high-temperature carbonization can cause resistance to drop drastically (latching in a low-resistance state) or result in permanent conduction, leading to a loss of protection and risks of fire or electric shock.
Cannot achieve a complete "open circuit": during a fault, it limits current via high resistance but allows a small leakage current; complete isolation is not possible.
Slow response to high currents; may fail to protect chips in time during high-power short circuits.
Higher ambient temperatures reduce the holding current, making the device more prone to nuisance tripping in summer.
V. Typical Application Schemes
Adapters/Chargers: use a slow-blow one-time fuse on the AC input side and a PPTC on the low-voltage USB output side; this ensures complete fire protection on the high-voltage side and protects against short circuits caused by plugging/unplugging on the low-voltage side.
Lithium battery devices: place a one-time fuse in series with the battery cell's positive terminal for ultimate explosion protection, and use PPTCs for individual low-load circuits.
Automotive systems: use a glass fuse for the main battery circuit and PPTCs for interior lighting and USB ports.
Precision industrial control boards: use a slow-blow fuse at the power input and PPTCs for branch protection at various onboard interfaces. More Information
Semiware offers a comprehensive range of PPTC resettable fuses. For more information on our product lines and application solutions, please visit: https://en.semiware.com/products/pptc/
About Semiware
Semiware is a comprehensive provider of circuit protection solutions, serving customers in the electronics, automotive, and industrial markets by leveraging its semiconductor expertise and deep understanding of end-product applications. We offer an extensive portfolio of circuit protection devices—including overvoltage protection, overcurrent protection, composite protection, discrete components, and advanced packaging technologies—to meet diverse customer needs. Supported by a global network of EMC laboratories and Field Application Engineers (FAEs), we deliver complete solutions and product portfolios to our customers. For more details, please visit https://en.semiware.com.
