Design engineers often give circuit protection
only cursory attention. Fuses, diodes,
and varistors are so well known that designers
select them with little thought as to the
nature of the overvoltage and overcurrent threats and the true
value of circuit protection to their application. But designers
should consider the costs of suboptimal device selection, many
of which go far beyond protecting just the design.
Circuit-protection devices fulfill two primary purposes:
safety and reliability. In terms of safety, disconnecting power
in a circuit during overcurrent eliminates the possibility of
electrocution as well as fire hazards. In addition, the right protection
may be necessary to comply with agency standards for
some end products. On the reliability side, dissipating transient
voltages, lightning, electrostatic discharge (ESD), electrical
fast transients (EFTs), and other dangers eliminates the risk of
damage while ensuring the functionality of the application.
So, circuit-protection devices can strongly affect users’ perceptions
of the end product. Warranty claims and replacement
costs hit the bottom line when technicians go on overtime and/or when the maker starts issuing and shipping replacement
units. In addition, these failures can affect a company’s top line
if consumers lose faith in the brand. When a digital camera or
MP3 player fails due to a user-generated ESD event, consumers
may consider an alternative brand when they make their
next purchase.
OVERCURRENT PROTECTION
Fuses, the most common overcurrent devices, come in fastacting
and Slo-Blo (time-lag) varieties. Slo-Blo fuses help
minimize the nuisance of repeated replacements when a circuit
experiences brief but recurring overcurrent spikes. But for personnel
safety, designers should employ a current-interrupting
fuse for critical ac equipment that requires frequent opening
for maintenance operations.
A current-interrupting fuse clears within one-half of an
ac cycle (0.00833 second). This limits let-through current
to a fraction of the peak available current, preventing conductor
and component overheating and reducing the severity of
arc-flash events.
Resettable devices offer an alternative
to fuses and circuit breakers. One
good example is the positive temperature
coefficient (PTC) thermistor. As
current increases, self-heating increases
PTC resistance and automatically
limits current. Polymer-based (PPTC)
materials are preferable as they exhibit
a pronounced knee in their resistance
versus temperature characteristics.
Usually, the trip current level is twice
the holding current. At 10 times the
holding current, a PPTC will typically
trip in less than 1 second. Once the overload
is gone, the PPTC cools, returning
the circuit to normal operation. This
prevents negative user perceptions that
may result from frequent fuse replacements
and circuit-breaker resets.
OVERVOLTAGE PROTECTION
A wide range of overvoltage protection
devices, each with unique capabilities
and characteristics, is available (see the
table). In keeping with the industry
trend to minimize harmful effects to the
environment, most of these devices are
fully lead-free or comply comply with the European Union’s Restrictions on
Hazardous Substances (RoHS).
CROWBAR VERSUS CLAMPING
The terms crowbar and clamping often are
used to describe how overvoltage protection
devices function during a transient
event. A crowbar device reduces the voltage
below the operating voltage of the system.
When the transient is complete, the
crowbar device resets and allows the circuit
to operate normally. During a transient
event, a clamping device holds the voltage
just above the operating voltage of the
system. It likewise resets after the transient
event terminates.
LIGHTNING PROTECTION
Gas discharge tubes (GDTs) typically protect
telecom and datacom lines, signal lines,
and customer premise equipment from
surge voltages. They are a good choice for
reducing lightning-induced transients
because they can handle surge currents up
to 40,000 A. GDTs dissipate surge energy
through a low-pressure gas plasma contained
within a small sealed tube.
With energy ratings from 0.1 to 10,000
Joules, metal oxide varistors (MOVs)
divert transient currents away from sensitive
circuit components in a wide range of
applications. They’re often used in surge
suppressors (TVSS) made for home office
equipment and are bundled with GDTs
and protection thyristors to protect the ac
input, cable TV (CATV)/satellite input,
and tip/ring circuit inputs.
Protection thyristor devices are a special
type of thyristor that suppresses overvoltage
transients in a wide assortment of telecom
and datacom equipment. They can
divert currents up to 500 A within nanoseconds
of reaching their break-over voltage.
Additionally, they’re qualified to help
customers meet various agency standards
such as GR 1089, K.20/21, IEC 60950,
and TIA-968-A for communication gear
and telecom equipment.
Transient voltage suppressor (TVS)
diodes form a general category of devices
that also protect a wide variety of circuits
and components from an assortment of
threats, including repetitive pulses in electrical
fast transients, inductive load switching,
commutative transients, and lightning
surges. Their p-n junctions have a much larger cross section than other diodes,
Schottky, and Zener, allowing them to conduct
large transient currents up to 10,000
A to ground without sustaining damage.
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