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Bourns® Telefuse™

The latest addition to the Bourns®

Circuit Protection product offering

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Introduction

Bourns® Telefuse™•Part numbers: B1250T and B0500T

1.25 version most popular•10mm SMD body •Overcurrent protection compliance to:

UL 60950 - CPE equipment Telcordia GR-1089 - Central Office equipment

•Safely interrupts 600V/60A powercross•Withstands lightning surge without opening

Series Description Device Symbol Part Photo Applications Ampere rating

Voltage Rating

Peak Surge current 10/1000

B1250T Telecom line fuse Line Card 1.25 A 600 V 100

CPE

B0500T Telecom line fuse 500 mA 600 V 25

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Telecom Fuse Applications

The fuse is used as secondary overcurrent protection in central office and customer premises applications.The fuse is nearly always used in conjunction with a thyristor-based overvoltage protector (Bourns TISP product).

There are three design paths that a customer may take for building their overcurrent protection block.The goal of all options is to function properly after “first level” tests and fail safely during “second level” tests.

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Path One - Use of a PTC

A PTC gives a resettable solution, but the PTC has inherent properties of 8-10Ω resistance and an unpredictable resistance hyterisis as a result of overcurrent event.

These properties may not be allowed in the functionality of the design.

The PTC is the right product to use if resettability is a critical feature of the system.

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Path Two - Use of Line Feed Resistors (LFR)

An LFR offers very stable current limiting and very tight matching between tip and ring pair, even after an overcurrent event.

LFRs are very robust and can dissipate a lot of energy, but are also physically large.Many applications demand minimal resistance, and the LFR can realistically achieve only a minimum of around 4Ω and still meet GR1089 requirements. Some applications (POTS cards) require a higher resistance of >30Ω, so the LPM is the right choice.

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Path Three - Telecom Fuses

The fuse provides a no-resistance overcurrent protection.This solution can be used in most all of the telecom circuit protection applications, especially xDSL and T1-E1.

This is a distinct advantage, as the resistance of the PTC and LPM preclude them from being used.

With the development of 100A thyristor overvoltage protectors, a customer can eliminate the LFR which was needed in the past to protect the thyristor limited to 50A.

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Applications

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The End

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