P3602ACMC datasheet(173/212 Pages) TECCOR

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Overvoltage Protection Comparison

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transient to follow. Once the transient drops below the DC holdover voltage and current, the

gas tube returns to its off state.

Advantages

Gas tubes have high surge current and low capacitance ratings. Current ratings can be as

high as 500 A for 200 impulses, and capacitance ratings can be as low as 1 pF with a zero-

volt bias.

Restrictions

Gas tubes have a limited shelf life and their performance degrades with usage. Out of the

four devices discussed, gas tubes exhibit the slowest response time and highest peak

voltage measurement. (Figure 5.3)

Applications

Because gas tubes are large and require a substantial amount of time to reach full

conduction, they are rarely used as board-level components. Consequently, gas tubes are

not normally used in telecommunications applications other than station protection

modules.

Metal Oxide Varistors

Metal Oxide Varistors (MOVs) are two-leaded, through-hole components typically shaped in

the form of discs. Manufactured from sintered oxides and schematically equivalent to two

back-to-back PN junctions, MOVs shunt transients by decreasing their resistance as

voltage is applied.

Advantages

Since MOVs surge capabilities are determined by their physical dimensions, high surge

current ratings are available. Also, because MOVs are clamping devices, they can be used

as transient protectors in secondary AC power line applications.

Restrictions

Like gas tubes, MOVs have slow response times resulting in peak clamping voltages which

can be greater than twice the device’s voltage rating. (Figure 5.3) MOVs also have long-

term reliability and performance issues due to their tendency to fatigue, high capacitance,

and limited packaging options.

Applications

Although MOVs are restricted from use in many telecom applications (other than disposable

equipment), they are useful in AC applications where a clamping device is required and

tight voltage tolerances are not.

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