Fuseology Selection Guide Page 9 Fuseology Selection Guide

www.littelfuse.com Fuseology Selection Guide Leakage Current: When a PTC is in a "tripped state" it protects the circuitry by limiting the current flow to a low leakage level. Leakage current can range from less than a hundred milliamps (mA) at rated voltage up to a few hundred milliamps (mA) at lower voltages. Fuses on the other hand completely interrupt the current flow when tripped, and this open circuit results in no leakage current when subjected to an overload current. Interrupting Rating: PTCs are rated for a maximum short circuit current at rated voltage also known as "breaking capacity" or Imax. This fault current level is the maximum current that the device can withstand safely, keeping in mind that the PTC will not actually interrupt the current flow (see Leakage Current above). A typical Littelfuse PTC short circuit rating is 40A; or for the battery strap PTCs, this value can reach 100A. Fuses do in fact interrupt the current flow in response to the overload and the range of interrupting ratings, vary from tens of amperes (A) up to 10,000 (A) amperes at rated voltage. Operating Voltage Rating: General use Littelfuse PTCs are not rated above 60V while fuses are rated up to 600V. Hold Current Rating: The hold (operating) current rating for PTCs can be up to 14A, while the maximum level for fuses can exceed 30A. Resistance : Reviewing product specifications indicates that similarly rated PTCs have about twice (sometimes more) the resistance of fuses. Agency Approvals: Littelfuse PTCs are Recognized under the Component Program of Underwriters Laboratories to UL Standard 1434 for Thermistors. The devices have also been certified under the CSA Component Acceptance Program. Time-Current Characteristic : Comparing the time- current curves of PTCs to time-current curves of fuses show that the speed of response for a PTC is similar to the time delay of a Littelfuse Slo-Blo fuse. Temperature Rerating: The useful upper limit for a PTC is generally 85C, while the maximum operating temperature for fuses is 125C. Ambient temperature effects are in addition to the normal rerating. PTCs hold and trip rating must be rerated when applied at conditions other than room ambient. For example, any rise in ambient temperature will decrease the hold current rating as well as the trip current. A reduction in ambient temperature will increase the trip current as well as the hold current. The temperature rerating curves in the table below compare PTCs to fuses and illustrate that more rerating is required for a PTC at a given temperature. Temperature Rerating Curves Comparing PTCs to Fuses Chart Key Curve A Thin-Film Fuses and 313 Series (.010 to .150A) Curve B FLAT-PAK , Nano 2 , PICO , Blade Terminal, Special Purpose and other leaded and cartridge fuses (except 313.010 - .150A) Curve C Resettable PTCs -60C -76F 20 40 60 80 100 120 140 -40C -40F -20C -4F 25C C A B AMBIENT TEMPERATURE PERCENT OF RATING A B 0C 32F 20C 68F 40C 104F 60C 140F 80C 176F 100C 212F 120C 248F PTC Characteristics and Terms (continued) 9

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