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How does the
Polymer-Resettable-Fuse effect work? |
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| A Polymer-Resettable-Fuse device comprises
a polymer matrix that is loaded with carbon black
particles to make it conductive. Since it is conductive
it will pass a given amount of current. If too much
current is passed through the device, the device
will begin to heat by I2R effect. As the device
heats it will expand. As it expands, the carbon
particles will separate and the resistance of the
device will increase. This will cause the device
to heat faster and expand more, further raising
the resistance. When the internal temperature of
the device reaches 125＜C, the change in resistance
increases dramatically. This increase in resistance
is sufficient to substantially reduce the current
in the circuit. A small amount of current will still
flow through the device sufficient to maintain the
temperature of the device and keep the Polymer-Resettable-Fuse
device at the high resistance level. When the power
is cycled off and the fault removed, the Polymer-Resettable-Fuse
device is allowed to cool. As the device cools,
it contracts to its original shape and reconnects
the carbon particles thus lowering the resistance
of the device to a level where it can hold the current
as specified for the device. This cycled can be
repeated multiple times. |
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How many times can you
trip a Polymer-Resettable-Fuse device at the maximum
voltage and interrupt currents? |
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| Each Polymer-Resettable-Fuse device is rated
to handle a specified operating voltage. Each device
can withstand a specified interrupt current as a
fault event. To obtain UL recognition, the device
must be tripped at least 6,000 times and still exhibit
PTC characteristics. For the telecom devices SN,SD,SF,
they have a rating for maximum surge voltage for
specific fault events that can occur in telecom
applications. This may be as few as ten times or
as many as several hundred times with the device
still meeting the original specification values.
Designers should keep in mind that the Polymer-Resettable-Fuse
device is intended to protect against faults and
failures and is not intended to be used in applications
where it will be expected to be tripped as the normal
mode of functioning |
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What are the basic differences
between a Polymer-Resettable-Fuse device, a fuse,
or other circuit protection device? How does
it work with overvoltage devices to provide protection?
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| The most obvious difference between a Polymer-Resettable-Fuse
device and a fuse is the feature of resettability.
While both products provide overcurrent protection,
a single Polymer-Resettable-Fuse device can provide
this protection multiple times, whereas after the
fuse has provided its protection, it must be replaced
for the circuit to function properly. The typical
performance of a Polymer-Resettable-Fuse device
is similar to that of a time delay fuse. Both devices
need to have the thermal derating of the device
taken into account, but the Polymer-Resettable-Fuse
device does not need to have an I2t derating since
it does not degrade like a fuse does under start-up
conditions. When comparing a Polymer-Resettable-Fuse
device to a bi-metallic circuit breaker the main
difference is not resettability, but a rather latching.
Both devices are resettable, but the bi-metallic
circuit breaker can reset itself even when the fault
is still present. This can lead to large EMFspikes
on resetting and when tripping and potentially reconnecting
a fault condition that could damage equipment and
be unsafe. The Polymer-Resettable-Fuse device will
latch in the high resistance state until the fault
is cleared and the power is cycled off and on. Polymer-Resettable-Fuse
devices differ from ceramic PTC devices in their
initial resistance, the time to react to fault events,
and size. Both products are resettable but the Polymer-Resettable-Fuse
device, compared to a ceramic PTC device of the
same hold current, will typically react (trip) much
faster than the CPTC, because it is a smaller device.
The most common application where Polymer-Resettable-Fuse
devices are used in combination with overvoltage
devices are in telecom applications. Here overvoltage
devices such as thyristors, gas discharge tubes,
MOVs, or diodes provide protection against lightning
and power cross faults. The Polymer-Resettable-Fuse
device protects the overvoltage protection device
in some of these fault events and can also provide
protection against other overcurrent events. |
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How quickly
do Polymer-Resettable-Fuse devices reset? |
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| The time it takes a device to reset to the low
resistance state depends on a variety of factors:
Which device it is How it is mounted or fixtured
The ambient temperature The nature and duration
of the trip event In general, most devices under
expected conditions will reset within a couple of
minutes although many will reset within seconds. |
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