P4802ACMC datasheet(176/212 Pages) TECCOR

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Overcurrent Protection

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SIDACtor® Data Book and Design Guide

Overcurrent Protection

In addition to protecting against overvoltage conditions, equipment should also be protected

from overcurrent conditions using either PTCs, fuses, power/line feed resistors, or

flameproof resistors. In all instances the overcurrent protector is a series element placed in

front of the overvoltage protector on either Tip or Ring for metallic (closed loop)

applications and on both Tip and Ring for longitudinal (grounded) applications.

PTCs

PTCs are positive temperature coefficient thermistors used to limit current. During a fault

condition, heat is generated at a rate equal to I2R. When this heat becomes sufficient, the

PTC increases its resistance asymptotically until the device simulates an open circuit,

limiting the current flow to the rest of the circuit. As the fault condition drops below the

PTC’s holding current, the device begins to reset, approximating its original off-state value

of impedance.

Advantages

Because PTCs are resettable devices, they work well in a variety of industrial applications

where electrical components cannot withstand multiple, low-current faults.

Restrictions

Although PTCs are well suited for the industrial environment and in many telecom

applications, they exhibit some limitations that have prevented them from being endorsed

by the entire telecommunications industry. Limitations include low surge current ratings,

unstable resistance, and poor packaging options.

Applications

PTCs are used in a variety of applications. In addition to protecting telecommunications

equipment, PTCs are also used to prevent damage to rechargeable battery packs, to

interrupt the current flow during a motor lock condition, and to limit the sneak currents that

may cause damage to a five-pin module.

Fuses

Due to their stability, fuses are one of the most popular solutions for meeting AC power

cross requirements for telecommunications equipment. Similar to PTCs, fuses function by

reacting to the heat generated due to excessive current flow. Once the fuses I2t rating is

exceeded, the center conductor opens.

Advantages

Fuses are available in both surface mount and through-hole packages and are able to

withstand the applicable regulatory requirements without the use of any additional series

impedance. Chosen correctly, fuses only interrupt a circuit when extreme fault conditions

exist and, when coordinated properly with an overvoltage protector, offer a very competitive

and effective solution for transient immunity needs.

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