Overvoltage Protection Comparison
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.
© 2002 Teccor Electronics
5 - 11
http://www.teccor.com
+1 972-580-7777
®
SIDACtor Data Book and Design Guide
TECCOR [ TECCOR ELECTRONICS ]
