Resettable PTCs (positive temperature coefficient) feature fuses possessing internal resistance characteristics limiting further disturbances from a short circuit or electrical current overload. Unlike traditional fuses demanding replacement after such faults occur, it is not required with resettable PTCs. When the initial fault condition is dissipated or removed, the device cools, and its internal resistance returns towards the original value. This facilitates the resumption of normal equipment functioning. The solutions are designed for restricting unsafe power levels while permitting optimized charge flow.

Resettable PTCs accomplish this by warding against overcurrent faults arising in electronic circuits. Their operation is comparable to PTC thermistors. However, resettable PTCs rely on mechanical means, in contrast with charge carrier effects engaged in thermistors. The technology helps prevent damage to equipment of varying current values, voltages, and configurations. Their ability to reset without necessitating costly replacement makes them ideal for "always-on" powered devices, for example, computer power supplies.

They are deployed in the nuclear energy and aerospace industries to minimize repair, replacement, and overall maintenance expenses. In audio loudspeakers, the units protect speakers and tweeters from suffering damage when overdriven by excessively amplified currents. This type of product does not shut off the circulation. It regulates the maximum electric charge going to the tools, allowing them to operate without any impairment.

Types of Resettable PTCs

Types of resettable PTCs are differentiated by size, form, chemical, and electrical properties. These include:

• Low resistance PTCs ensure electric charge flows with minimal resistance during regular operation.
• Surface mount units are designed for processes requiring protection where space is limited.
• Radial leaded are appropriate for large spaces.
• Battery strapped units run with rechargeable battery cells.
• Resettable PTCs are widely used in telecommunications and networking to protect against fault conditions and ensure continuity of service.

Operation

PTC fuses reach increased resistance levels with nominal holding currents when a fault occurs. Their functioning is analogous to circuit breakers in permitting circuits to continue to run without necessitating opening of the chassis or replacing components. The elements possess current ratings. An electric charge passing through an instrument that exceeds these limits causes the PTC to warm up above a set threshold temperature. The electrical resistance rises by several orders of magnitude at amplified temperatures. The finite voltage of the energy source leads to the reduction in the charge flowing through the PTC. Ratings required to trip a PTC range between 20 mA and 100 A.

Resettable PTCs contain non-conductive crystalline organic polymer matrices containing carbon black particles. The arrangement of carbon in the crystalline structure makes the system conductive. When it is chilled, the polymer remains in a crystalline state, and the conductive carbon is constrained into gaps within the structure. The presence of carbon results in multitudes of conductive chains. This allows the transfer of flow known as the "hold current." When a charge of greater intensity, referred to as a "trip current," passes through the component, the resettable PTC begins to heat up.

The polymer expands as the trip current raises the temperature, changing its state from crystalline to amorphous. This expansion causes the carbon particles to separate and severs the conductive pathways influencing the resistance. The instrument then undergoes further heating and expands to a considerable degree. This process continues to intensify the element's resistance. This increase diminishes the current in the circuit by a substantial amount. A minimal charge continues to flow through the component with sufficient power to maintain the temperature at a level supporting the elevated resistance.

Once power is removed, heating caused by the holding current ceases, and the temperature begins to drop. As this happens, the original crystalline structure of the implement is regained, and it returns to a state of nominal resistance sufficient to hold the specified current. The cooling process takes only several seconds. However, a tripped instrument retains a slightly greater resistance over a period of hours while it approaches the preliminary indicator. If the operating current is above the holding current of the product after the fault is eliminated, complete resetting to the original value is delayed. In such instances, the resistance will remain above the desired level. Reverting to the early indicator can take several days, months, or years. The products feature superior resistance levels when compared to metallic fuses or circuit breakers at ambient temperatures.

Applications

Resettable PTCs are ubiquitous in electrical and electronic equipment, consumer, and industrial products. Areas of application include:

• Computers
• Aerospace
• Nuclear
• Audio
• Telecommunications
• Portable electronics
• Multimedia
• Game machines
• Mobile phones
• Automotive
• Industrial controls

Selecting Resettable PTCs

Resettable PTCs cover unique dynamic properties, dimensions, and shapes serving a plethora of specific functions. When selecting a product, confirm if a specialized version best suited for the planned operation is available.

PTC units are incapable of withstanding significant operating voltage reductions, which is a crucial consideration when evaluating solutions against alternative options, such as metallic fuses.

The amount of space available for the element, the expected resistance, and accessibility for maintenance form the basis of selection criteria. 

Image credits:

Mersen | Littelfuse, Inc.