WYCP.FEB–wy
Circuit board protection–a variety of choices
A wide array of both resettable and non-resettable devices are available
to the circuit designer
BY WARREN YATES Associate Editor
The increasing complexity and increasing components count of
printed-circuit boards often makes some form of overcurrent protection
desirable from both a reliability and an economic standpoint. For the most
part, board-level protection is concerned primarily with preventing damage
to the sensitive parts on the board. These are the solid-state devices,
particularly ICs' fine board traces, and specialized components such as
precision resistors. The solid-state devices are the most sensitive to
current surges, while board-level traces can usually withstand a larger
surge. This situation is changing however, with today's emphasis on
very-high-density packaging of boards, which leads to narrower and
narrower board traces. And the inevitable further increases in component
densities are going to aggravate this problem even further. The ability
of devices to withstand current surges depends upon how rapidly the device
heats up with excessive current flowing. Most silicon devices fail rapidly
at temperature approaching 150 degreesC. Semiconductors, particularly ICs,
are not usually provided with heat sinks, so any energy applied to them
will result in rapid heating. Because current surges can be of different
intensity and duration, a standard term for energy absorbing capability
has been accepted in the protection industry. This term, known as “energy
let-through,” is expressed as I
current surge and t is its duration. Unfortunately, many manufacturers of
solid-state devices (and other board-level components) fail to specify the
capacity of their products to withstand current surges or the I
limits for their use. This lack of data on many devices can sometimes make
the selection of a protection device difficult for the circuit designer.
The implication of all this is that a very fast-acting protection device
with low-energy let-through is needed to protect sensitive on-board
devices like ICs and transistors from overcurrent faults that occur on the
board itself. But that's just one scenario, according to some
manufacturers of protection components. These manufacturers contend
overcurrent faults are more often created at, or beyond, a board's I/O
ports than on the board itself. They argue that overcurrent faults are
usually the result of pure user carelessness–plugging cables into the
wrong port, trying to mate incompatible connectors, and switching
equipment on and off under load, for example. Super fast-acting protection
devices are not needed to protect against these types of overcurrent
conditions, according to the manufacturers. Nevertheless, a wide range of
board-level circuit protection devices are at the system designer's
disposal. Major selection considerations include not only
circuit-interrupt performance capabilities and costs, but many other
factors, such as resettability versus non-resettability, automatic versus
manual insertion, and surface mountability.
Resettability vs. non-resettability Four broad types of protection
devices are described in this article–fuses, circuit breakers, PTC
thermistors, and Circuit Savers. All but the fuses are resettable after
tripping. Circuit breakers are manually resettable, while PTC thermistors
and Circuit Savers reset automatically after tripping. Although the
fuse's non-resettability may seem like the kiss-of-death, particularly
when soldered in or installed on relatively inaccessible pc boards or in
systems installed in remote locations, it's sometimes the protection
choice specifically because of its lack of resettability. This is so in
applications where serious electrical damage may result by continually
resetting a circuit without first correcting the fault of the overload. It
also goes a long way in discouraging “non-skilled” system users from
attempting repairs in many applications, particularly when the fuse is
soldered in. And then of course, in some applications the fuse's much
lower cost than that of the other protection devices may be the
over-riding factor in its selection. In some applications, however,
resettability is an important requirement. Automatic resettability, in
particular, can be important in applications where the protected equipment
is in isolated, hard-to-get-at locations because it may save a very
expensive service call to the site. Here, resettable devices like PTC
thermistors can be a big time and cost saver. However, there's the danger of
excessive damage due to continued on and off cycling of the circuit if the
current overload remains uncorrected. A circuit breaker, with its manual
resettability, can be a good compromise between automatic resettability
and non resettability in some applications. For example, nonskilled system
personnel can be instructed to try resetting an overloaded circuit, but
not to continually reset it if the overload persists. And on the other
hand, it does assist the service technician in determining if a
overcurrent fault was a onetime occurrence or if it persists.
Fuses–alive and well After many decades of protecting electronic
circuits, the venerable fuse is still the device of choice for many, and
probably most, applications. Although the fuse continues to function the
same as it did years ago, its performance has been significantly improved
and its packaging refined to keep up with the latest applications. In
particular, these packaging refinements have made fuses smaller and easier
to install on pc boards where space is almost always at a premium. Truly
small fuses are now available for pc-board applications in a wide range of
voltage and currents, trip speeds, and energy let-through ratings suitable
for just about any “non-resettable” protection application. Available from
a broad array of companies such as Littelfuse, Bussmann, Schurter, AVX,
Bel Fuse, and Wickmann,these fuses run from about $0.15 or $0.20 to about
$0.40 to $0.50 each in production quantities. It would take a compendium to
describe the features of the different types of fuses available. However,
some of the special features that have become available over the last few
years are described here. The availability of surface-mount types is
certainly one of the most important recent fuse advancements (see Fig. 1).
These fuses, available in tape and reel, are compatible with automatic
insertion and wave and reflow soldering. A wide selection of both axial-
and radial-lead fuses remain available from many manufacturers as well.
Obviously, fuses soldered in place on a printed board are not easily
replaced (a sometimes desirable feature, as noted previously). However,
surface-mount fuseholders are now available to simplify fuse replacement
(see Fig. 2). A newer type of fuse from Bussmann, called the solid-matrix
fuse, is of significantly different construction than the traditional
fusible element suspended in air. This fuse has certain advantages. As
shown in Fig. 3, wire leads are resistance welded to conductive pads on a
ceramic substrate and a wire or metallic-film fusible element is bonded to
the same pads. The fusible element is completely surrounded by a ceramic
material and the entire assembly is then encapsulated in a
high-temperature plastic body. Solid-matrix construction offers several
advantages over traditional subminiature axial-lead fuses. Among these are
a higher resistance to shock and vibration and greatly reduced
possibilities of failure due to fusible element fatigue. Furthermore, the
solid-matrix fuse's plastic encapsulation prevents moisture and cleaning
solvents from reaching and attacking the fusible element. And from the
standpoint of short-circuit performance, the let-through energy of the
solid-matrix fuse is claimed to be significantly less than that of the
traditional fuse under short-circuit conditions. In general, solid-matrix
fuses exhibit a somewhat higher resistance–and consequently a somewhat
higher voltage drop–than traditional fuses. However, maximum acceptable
voltage drop is highly application-specific so the higher voltage drop is
not a significant disadvantage in many applications. Solid-matrix fuses
are available in axial-lead, radial-lead, and surface-mount versions, are
available in bulk as well as in tape and reel format. They are compatible
with automatic insertion and wave and reflow soldering. Another problem
associated with fuses has been recently solved. It's often difficult to
see if the fuse element in a glass fuse has opened. Now, Littelfuse's 2AG
(glass) type indicating fuse goes a long way toward solving this problem.
The glass body of this fuse changes color to indicate that the fuse
element has opened (see Fig. 4). This feature can also be very useful
during system design testing because the fuse will show a discoloration of
the glass body if the current rating of the fuse selected is too low.
PTC thermistors–two types Positive temperature coefficient (PTC)
thermistors undergo a large, abrupt increase in their resistance when
heated above their transition temperature. Circuit designers can take
advantage of this unique characteristic to protect a circuit from damage
in the event of a dangerous current overload. Under normal circuit
conditions the resistance of a PTC thermistor is very low (see Fig. 5).
Should the circuit current rise above the thermistor's rated current, the
thermistor's resistance quickly increases, thereby decreasing circuit
current to a point where thermal equilibrium is achieved. Once power to
the protected circuit is removed, the thermistor's resistance drops to its
normal value. Thus, the PTC thermistor is an automatically resettable
device. That's one of its major advantages when compared to fuses, which
are non-resettable, and to circuit breakers, which are manually resettable
(assuming resettability is a desired feature for a specific application.)
Two types of PTC thermistors are available to the circuit designer–a
semiconductor ceramic type and conductive polymer type. The ceramic type
is available from a number of manufacturers, including Siemens, Murata
Erie, Thermodisc, and Cera-Mite. The second type, available from Raychem
only, is based on a proprietary conductive polymer technology. This
technology involves blending special formulated plastics with various
conductive materials. Both types come in a wide variety of voltage and
current ratings, trip times, initial resistance, and package styles, so
the designer should be able to find a PTC thermistor of one type or the
other that meets the application requirements if an automatically resettable
protection device is called for. Prices for both types run in a range
from about $0.20 to $1 or so, depending upon device ratings, package
style, and quantity.
Circuit Saver: fast, self-resetting A relatively new circuit protection
device called the Circuit Saver claims the fastest speed and lowest
let-through (I
from InResco, the Circuit Saver consists typically of a magnetic reed
switch surrounded by two concentric coils (see Fig. 6). The sensing coil
has a very low resistance; resistance of the holding coil is more than 1,
000 times that of the sensing coil. During normal operation, current
flows through the sensing coil and the normally closed switch lead to the
load. When current exceeds a specified trip current, the sensing coil
generates enough flux to start opening the normally closed switch contact,
thereby switching the holding coil into the circuit to protect the load.
The flux generated by the holding coil completes the transfer to the
normally open contact, thereby shunting the current around the load. The
Circuit Saver remains tripped until the current is reduced to 25% of rated
current, at which point it automatically resets itself. The Circuit
Savers main claim-to-fame is its exceptionally fast trip time in response
to a current overload–much faster than that of other overcurrent
protection devices. Its fast trip time results in a very-low-energy
let-through (I
1-A Circuit Saver at 400% overload is 0.04 ms and I
0009. Circuit Savers are available in six current ratings of 50 mA, 100
mA, 250 mA, 500 mA, 750 mA, and 1a. Voltage ratings include 5, 10, 20, and
30 Vdc. The devices come in small epoxy-shell SIPs; one type that contains
an LED to indicate that the Circuit Saver has tripped is available in a
conformal-coated SIP.
effect on the performance of a Circuit Saver. However, the device should
not be located in strong external magnetic fields without appropriate
shielding. If your application call for resettable super fast overcurrent
protection with very low let-through energy, then Circuit Savers may be
the way to to. But they don't come cheap–typical prices range from $1.88
to $3.60 each/1,000 depending upon model.
Pc-board circuit breakers Circuit breakers designed specifically for
mounting on pc boards are few and far between. But many circuit breaker
manufacturers do offer circuit breakers that can be pc board mounted,
although really not designed for that specific purpose. One manufacturer
that does provide circuit breakers specifically for use on pc boards is
E-T-A Circuit Breakers. This company's recently introduced 1410 series
(see Fig. 7) comes with pc-board terminals for vertical or horizontal
mounting, and measures 1.0 x 0.75 x 0.5 in. Standard voltage ratings are
48 Vdc and 125 Vac; current ratings range from 0.63 to 6.3 A. The price
range for 1410 series relays ranges from about $2 to $3 in production
quantities.
CAPTIONS:
Fig. 1. Surface-mountable thin-film fuses like these from AVX are
available on tape and reel for high-voltage applications.
Fig. 2. Surface-mount fuseholders like this one from Schurter permits
easy replacement of fuses on a pc board.
Fig. 3. Solid-matrix construction techniques in Bussmann's subminiature
fuses result in high shock and vibration resistance and reduce the
possibility of fusible element fatigue.
Fig. 4. The glass body of this fuse from Littelfuse changes color to
indicate that the fuse has opened because of an overload.
Fig. 5. The resistance of both a ceramic semiconductor PTC thermistor and
a Raychem conductive polymer type rises abruptly when the thermistor is
exposed to a severe current overload.
Fig. 6. Circuit Saver employ a magnetic reed switch surrounded by two
concentric coils that operate to very quickly to open a circuit in the
event of a current overload.
Fig. 7. Series 1410 circuit breakers are designed specifically for use on
pc boards.
For further information on the products mentioned in this article, call
the company or circle the appropriate reader service number.
AVX Inc./Kyocera Myrtle Beach, SC Dennis O'Toole 803-946-0562
Bel Fuse, Inc. Jersey City, NJ Dennis Ackerman 201-432-0463
Bussmann Div./Cooper Industries St. Louis, MO Steve Whitney
314-394-2877
E-T-A Circuit Breaker Niles, IL John Palsir 708-647-8303
InResco, Inc. Manasquan, NJ G. Lloyd Martin 908-223-6330
Littelfuse, Inc. Des Plaines, IL Art Skwerski 708-391-0307
Murata Erie NA Syrma, GA Tom Kuhl 404-436-1300
Raychem, Inc. Menlo Park, CA Chuck Coffin 415-361-3333
Schurter, Inc. Petaluma, CA Diane Cupples 707-778-6311
Siemens Components Inc. Islin, NJ Ullrich von Seckendorff 908-906-4317
Thermodisc, Inc. Mansfield, OH Jim Holbrook 419-525-8500
Wickmann USA, Inc. Atlanta, GA Mark Beldon 404-699-7820
Advertisement
