Fuses vs PTCs
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 into 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.
Choosing between a PTC and a Fuse
Overcurrent circuit protection can be accomplished with the use of either a traditional fuse or the more recently developed resettable PTC. Both devices function by reacting to the heat generated by the excessive current flow in the circuit. The fuse melts open, interrupting the current flow, and the PTC changes from a low resistance to a high resistance to limit current flow. Understanding the differences in performance between the two types of devices will make the best circuit protection choice easier.
The most obvious difference is that the PTC is resettable. The general procedure for resetting after an overload has occurred is to remove power and allow the device to cool down. There are several other operating characteristics that differentiate the two types of products. The terminology used for PTCs is often similar but not the same as for fuses. Two parameters that fall into this category are leakage current and interrupting rating.
Leakage current: the PTC is said to have “tripped” when it has transitioned from the low resistance state to the high resistance state due to the overload. Protection is accomplished by limiting the current flow to some leakage level. Leakage current can range from around a hundred milliamps at rated voltage up to several hundred milliamps at lower voltages. The fuse on the other hand completely interrupts the current flow and this open circuit results in “0” leakage current when subjected to the overload.
Interrupting rating: the PTC is rated for a maximum short circuit current at rated voltage. This fault current level is the maximum current that the device can withstand but the PTC will not actually interrupt the current flow (see LEAKAGE CURRENT above). A typical PTC short circuit rating is 40A. Fuses do in fact interrupt the current flow in response to the overload and the range of interrupting ratings goes from hundreds of amperes up to 10,000 amperes at rated voltage.
The circuit parameters may dictate the component choice based on typical device rating differences.
Voltage rating: general use PTCs are not rated above 60V while fuses are rated up to 600V. Current rating: the operating current rating for PTCs can be up to 11A while the maximum level for fuses can exceed 20A.
Temperature rating: the useful upper limit for a PTC is generally 85° while the maximum operating temperature for fuses is 125°C. Both devices require derating for temperatures above 20°C and a representative curve for that purpose is provided.
The PTC Rerating Curves located on data pages, should be consulted for the proper rerating of the various PTC series at ambient temperatures other than 20°C.
Additional operating characteristics can be reviewed by the circuit designer in making the decision to choose a PTC or a fuse for overcurrent protection.
Agency approvals: PTCs are recognized under the Component Program of Underwriters Laboratories to UL Thermistor Standard 1434. The devices have also been certified under the CSA Component Acceptance Program. PTCs can, in addition, be approved to IECStandard 730-1 (Automatic Electric Controls) with certification by TUV, VDE, etc. Approvals for fuses include recognition under the Component Program of Underwriters Laboratories and certification from the CSA Component Acceptance Program. In addition many fuses are available with full “Listing” in accordance with the new Supplementary Fuse Standard UL 248-14.
Resistance: Reviewing product specifications indicates that similarly rated PTCs have about twice (sometimes more) the resistance of fuses. Time-current characteristic: comparing the time-current curves of PTCs to fuses shows that the speed of response for a PTC is similar to the time delay of a Slo-Blo® fuse.
Overcurrent Protection Applications
The PTC material is supplied in both a radial leaded package as well as a surface mount type. The function of the resettable PTC has many design applications.
Plug and Play applications include both the mother-board and the many peripherals that can be frequently connected to and disconnected from the computer ports. The mouse, keyboard, audio, network, monitor and USB ports represent opportunities for a faulty unit or damaged cable to be connected and also possible misconnections. The ability to reset after correction of the fault is particularly attractive. Some of these applications use radial leaded while surface mount units are more appropriate for others.
Disk drive protection can be supplied by a PTC from the potentially damaging overcurrents resulting from excessive voltage from a power supply malfunction. Disk drive applications tend to use the surface mount PTC.
Power supplies are vulnerable to malfunctions in the circuits the power is being supplied to. Without protection the power supply will attempt to provide the current required by a low resistance fault. Individual PTCs can be used to protect each load where there are multiple loads or circuits. Typically the device is placed in the output circuit and can be either radial leaded or surface mount.
Motor overcurrents can produce excessive heat that may damage the winding insulation and for small motors may even cause a failure of the very small diameter wire windings. The PTC will generally not trip under normal start up currents. Motors are commonly protected by radial leaded PTCs.
Transformers can be damaged by overcurrents due to circuit faults and the current limiting function of a PTC can provide protection. The PTC is located on the load side of the transformer to minimize the effect of circuit faults. Various applications use either the radial leaded or surface mount units.