PTC Fuses

PTC Fuses: Resettable Protection for Electronics

PTC fuses, also known as resettable fuses, polymeric positive temperature coefficient (PPTC) devices, polyswitches, or polyfuses, offer unique protection in electronic circuits compared to traditional fuses. Here's a breakdown:

Function:

• Unlike traditional fuses that blow permanently, PTC fuses increase their resistance significantly when exposed to excessive current (overcurrent). This rise in resistance limits the current flow, protecting sensitive components from damage.

• As the current drops or the fault is cleared, the PTC fuse cools down and returns to its low resistance state, allowing normal operation to resume. This resettability eliminates the need to replace the fuse, making them convenient for circuits prone to temporary overcurrents.

Applications:

• Battery protection: In portable electronics and power supplies, PTC fuses guard against battery faults like short circuits or over-discharge.

• Motor protection: PTC fuses prevent motor burnout due to overloading or stalling.

• LED driver protection: They shield LED circuits from excessive currents or voltage spikes.

• Thermal protection: PTC fuses provide temperature-sensitive protection in various applications, like preventing overheating in transformers or power supplies.

• Current limiting: They can limit the maximum current in specific circuits, preventing damage from unexpected surges.

Limitations:

• Response time: PTC fuses have a response time that may not be suitable for very high-speed circuits or immediate protection needs.

• Holding current: They have a specific "holding current" that they can withstand continuously without tripping.

• Degradation: Frequent tripping or exceeding the holding current can degrade the PTC fuse's performance over time.

• Limited voltage range: Most PTC fuses have a specified voltage range they can operate within.

• Not for high-power applications: They are generally not suitable for high-power circuits due to their current and voltage limitations.

Pre-trip resistance and post-trip resistance

Pre-Trip Resistance (Ri):

• This refers to the resistance of the PTC fuse when it's in its normal, untripped state. Usually measured at room temperature (23°C), it is typically very low, often in the milliohm (mΩ) range.

• This low resistance allows the fuse to conduct current freely without significantly impacting the circuit's operation.

Post-Trip Resistance (Rtrip):

• This is the resistance of the PTC fuse after it has tripped due to overcurrent and the fault then removed. It is measured one hour after the trip event under specific temperature conditions (usually 23°C).

• However, the post-trip resistance is not permanent. As the PTC fuse cools down, its resistance gradually decreases and eventually returns to a value close to its pre-trip state. This reforming process can take minutes or even hours depending on the specific fuse and its environment.

Implications:

• Circuit impact: The higher post-trip resistance can slightly increase the voltage drop across the fuse, depending on the current flowing through it. This impact is usually negligible in most circuits but should be considered for sensitive applications.

• Resettability: The ability of the PTC fuse to return to its low resistance state is crucial for its resettability. However, frequent tripping or exceeding the holding current can gradually increase the post-trip resistance over time, eventually leading to failure.

• Derating: The trip current and holding current of a PTC fuse may need to be derated depending on the operating temperature, as higher temperatures can lower the trip current and increase the post-trip resistance. Refer to the datasheet for specific derating factors.

PTC fuses can get quite hot during a trip event, but the exact temperature depends on several factors:

Factors affecting peak temperature:

• Tripped current: Higher currents cause the PTC fuse material to heat up more.

• Ambient temperature: Operating in a hotter environment leads to higher peak temperatures.

• Physical size and package: Smaller fuses have less thermal mass and reach higher temperatures.

• Cooling efficiency: Good airflow around the fuse helps dissipate heat faster and lowers peak temperature.

Typical and maximum temperatures:

• Typical peak temperature: While specific values vary, many PTC fuses have a typical peak temperature during trip between 200°C and 270°C.

• Maximum operating temperature: Datasheets generally specify a maximum operating temperature, often around 125°C. This is the temperature the fuse can withstand continuously without degradation.

Practical implications:

• Component damage: If surrounding components in the circuit are not rated for the PTC fuse's peak temperature, they can be damaged by the heat generated during a trip event.

• Ignition risk: In rare cases, if the PTC fuse gets excessively hot and the surrounding materials are flammable, there could be a small ignition risk. However, reputable manufacturers design their fuses with fire-resistant materials to minimize this risk.

• Circuit performance: The high temperature can slightly affect the operation of sensitive components nearby, but this is usually minimal in most circuits.

• Thermal derating: In applications where the ambient temperature is higher than normal, the trip current and holding current of the PTC fuse may need to be derated to account for the reduced cooling capacity and prevent premature tripping.

Safety and precautions:

• When selecting a PTC fuse, consider the peak temperature it can reach and compare it to the temperature ratings of surrounding components.

• Ensure proper airflow around the PTC fuse to facilitate heat dissipation.

• Choose fuses with fire-resistant materials for applications with potential flammability concerns.

• Refer to the datasheet for specific information on maximum temperature, cooling requirements, and potential de-rating factors.