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Fuses vs PTCs(I)

2014-08-04 11:56:17 lipscall

This section provides guidance in choosing the best overcurrent circuit protection device...PTC or fuse...for the application. 

Many times choosing between using a Fuse or PTC is a matter of preference, though there are important considerations and common areas of applications where the use of one may be better than the other. 

For example, much of the design work for PCs, peripherals, and portable devices (smart phones, tablets, etc.) urges the use of PTCs because they are self-resettable; using a fuse that must be replaced each time an overcurrent condition occurs is unacceptable. 

In other cases, fuses may be more acceptable because they completely stop current in fault condition; this may be more desired if safety or avoidance of downstream circuit equipment is a premium concern. Fuses are also helpful for diagnostic purposes, aiding equipment designers and users in tracing the origin of the overcurrent faults.


Overcurrent Circuit Protection

The circuit designer has a choice of technologies when faced with the task of providing overcurrent protection. The traditional fuse and the Polymer based PTC (positive temperature coefficient) device represent the most common solutions employed. Understanding the differences between these two components simplifies choosing the best protection device for the application.

Fuses have been referred to as “one time” devices, in that the fuse will provide protection from the overload by opening only once and then it needs to be replaced. The heart of a typical fuse is a length of wire which is heated to its melting point by the excessive current. The circuit current flow decreases to zero as the wire melts open.

The PTC also reacts to the excessive current but is known as a “resettable” device. The polymer based unit can provide overcurrent circuit protection a number of times when reset by removing the overload. The conductive polymer increases in resistance when heated by the overload and limits the circuit current.


PTC Protection Function

The principles of operation for a fuse have been documented over the years and are generally well understood. The actual process by which the PTC provides overcurrent circuit protection is less clear and merits further discussion. So far the PTC has been identified as being polymer based, current limiting, and resettable.

The PTCs under discussion are conductive polymer based products. The polymer material used contains particles of carbon black as the conductive media. The resistance is controlled by the amount of carbon black introduced in o the mixture. Heat produces some expansion of the polymer which causes the carbon black to shift resulting in less conductivity or an increase in resistance.

The PTC functions by current limiting a potentially damaging overcurrent to a safe level. Specifically the excessive current through the device causes internal heating (I2R) which raises the temperature of the PTC and results in an increase in its resistance. The resistance of the PTC is generally a small part of the total circuit impedance until the heating takes place. The increase in resistance for a polymer based PTC is nonlinear as shown in the graph below and this relatively large increase in resistance will reduce or limit the circuit current to a safe level. The transition from low to high resistance is referred to as the trip point.

The heat generated by this limited current through the higher resistance value will maintain the temperature of the PTC at a level that will cause the resistance to remain high. This thermal equilibrium condition will continue until power is removed from the circuit which allows the PTC to cool down and the resistance will decrease. The resettable feature of the PTC is based on the fact that the increase in resistance resulting from the increase in temperature is reversible. The PTC is reset or returned to the lower resistance state by removing the power from the circuit which allows the device to cool down. The unit is then ready to react to future overloads. The resistance will remain low if the cause of the overcurrent has been corrected, but if the overcurrent recurs the device will again switch to the high resistance state.
 

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