Fuseology Selection Guide Page 5 Fuseology Selection Guide

www.littelfuse.com Fuseology Selection Guide Fuse Selection Checklist The application guidelines and product data in this guide are intended to provide technical information that will help with application design. Since these are only a few of the contributing parameters, application testing is strongly recommended and should be used to verify performance in the circuit/application. Many of the factors involved with fuse selection are listed below. For additional assistance with choosing fuses appropriate to you requirements, contact your Littelfuse products reprentative. Selection Factors 1. Normal operating current 2. Application voltage (AC or DC) 3. Ambient temperature 4. Overload current and length of time in which the fuse must open 5. Maximum available fault current 6. Pulses, Surge Currents, Inrush Currents, Start-up Currents, and Circuit Transients 7. Physical size limitations, such as length, diameter, or height 8. Agency Approvals required, such as UL, CSA, VDE, METI, MITI or Military 9. Fuse features (mounting type/form factor, ease of removal, axial leads, visual indication, etc.) 10. Fuseholder features, if applicable and associated rerating (clips, mounting block, panel mount, PC board mount, R.F.I. shielded, etc.) 11. Application testing and verification prior to production 1. NORMAL OPERATING CURRENT: The current rating of a fuse is typically derated 25% for operation at 25C to avoid nuisance blowing. For example, a fuse with a current rating of 10A is not usually recommended for operation at more than 7.5A in a 25C ambient. For additional details, see RERATING in the previous section and AMBIENT TEMPERATURE below. 2. APPLICATION VOLTAGE: The voltage rating of the fuse must be equal to, or greater than, the available circuit voltage. For exceptions, see VOLTAGE RATING. 3. AMBIENT TEMPERATURE: The current carrying capacity tests of fuses are performed at 25C and will be affected by changes in ambient temperature. The higher the ambient temperature, the hotter the fuse will operate, and the shorter its life. Conversely, operating at a lower temperature will prolong fuse life. A fuse also runs hotter as the normal operating current approaches or exceeds the rating of the selected fuse. Practical experience indicates fuses at room temperature should last indefinitely, if operated at no more than 75% of catalog fuse rating. Ambient temperature effects are in addition to the normal re-rating, see example. Example: Given a normal operating current of 2.25 amperes in an application using a 229 series fuse at room temperature, then: Normal Operating Current Catalog Fuse Rating = - - - - - - - - - - 0.75 - or - 2.25 Amperes ------- = 3 Amp Fuse (at 25C) 0.75 This charts shows typical ambient temperature effects on current carrying capacity of Littelfuse products. For specific re-rating information, please consult the product data sheet at www.littelfuse.com or contact a Littelfuse representative. Curve A: Thin-Film Fuses and 313 Series (.010 to .150A) Curve B: FLAT-PAK , TeleLink , Nano 2 , PICO , Blade Terminal and other leaded and catridge fuses Curve C: Resettable PTC's A robust web-based tool to help circuit designers identify the optimal electronic fuses for their products. To use the Littelfuse iDesign tool, simply register to create a free online account at the iDesign Login. 5

Previous Page
Next Page