Recreated polyfuse

SEMINAR REPORT ON POLYFUSE
SUBMITTED BY- MUMTAJ K. PAPPUWALE Page 1
INTRODUCTION
Current flow in a conductor always generates heat. Excess heat is
damaging to electrical components. Overcurrent protection devices are used to
protect conductors from excessive current flow. Thus protective devices are
designed to keep the flow of current in a circuit at a safe level to prevent the
circuit conductors from overheating.
A fuse is a one-time over-current protection device employing a fusible
link that melts (blows) after the current exceeds a certain level for a certain
length of time. Typically, a wire or chemical compound breaks the circuit when
the current exceeds the rated value. A fuse interrupts excessive current so that
further damage by overheating or fire is prevented. Wiring regulations often
define a maximum fuse current rating for particular circuits. Overcurrent
protection devices are essential in electrical systems to limit threats to human
life and property damage. Fuses are selected to allow passage of normal current
and of excessive current only for short periods.
Polyfuse is a resettable fuse that doesn’t need to be replaced like the
conventional fuse. Many manufacturers also call it PolySwitch or MultiFuse.
Polyfuse are designed and made ofPPTC material in thin chip form. It is placed
in series to protect a circuit. Polyfuse provide over-current protection and
automatic restoration.
Like traditional fuses, PPTC devices limit the flow of dangerously high
current during fault condition. Unlike traditional fuses, PPTC devices reset after
the fault is cleared and the power to the circuit is removed. Because a PPTC
device does not usually have to be replaced after it trips and because it is small
enough to be mounted directly into a motor or on a circuit board, it can be
located inside electronic modules, junction boxes and power distribution
centers.

OVERCURRENT PROTECTION
Polyfuse is a series element in a circuit. The PPTC device protects the
circuit by going from a low-resistance to a high-resistance state in response to
an overcurrent condition, as shown in Figure-1.This is referred to as "tripping"
the device. In normal operation the device has a resistance that is much lower
than the remainder of thecircuit. In response to an overcurrent condition, the
device increases in resistance (trips), reducing the current in the circuit to a
value that can be safely carried by any of the circuit elements. This change is
the result of a rapid increase in the temperature of the device, caused by I2
R
heating.
Figure 1- Overcurrent protection circuit using Polyfuse

PRINCIPLE OF OPERATION
Technically these are not fuses but Polymeric Positive Temperature
Coefficient (PPTC) Thermistors. Polyfuse device operation is based on an
overall energy balance. Under normal operating conditions, the heat generated
by the device and the heat lost by the device to the environment are in balance
at a relatively low temperature, as shown in Point 1of Figure-2. If the current
through the device is increased while the ambient temperature is kept constant,
the temperature of the device increases. Further increases in either current,
ambient temperature or both will cause the device to reach a temperature where
the resistance rapidly increases, as shown in Point 3of Figure-2.
Figure 2 – Operating curve as resistance varies with temperature
Any further increase in current or ambient temperature will cause the
device to generate heat at a rate greater than the rate at which heat can be
dissipated, thus causing the device to heat up rapidly. At this stage, a very large
increase in resistance occurs for a very small change in temperature, between
points 3 and 4 of Figure-2. This is the normal operating region for a device in
the tripped state. This large change in resistance causes a corresponding
decrease in the current flowing in the circuit. This relation holds until the device
resistance reaches the upper knee of the curve (Point 4 of Figure-2). As long as
the applied voltage remains at this level, the device will remain in the tripped
state (that is, the device will remain latched in its protective state). Once the
voltage is decreased and the power is removed the device will reset.

CONSTRUCTION & OPERATION
PPTC fuses are constructed with a non-conductive polymer plastic film
that exhibits two phases. The first phase is a crystalline or semi-crystalline state
where the molecules form long chains and arrange in a regular structure. As the
temperature increases the polymer maintains this structure but eventually
transitions to an amorphous phase where the molecules are aligned randomly,
and there is an increase in volume. The polymer is combined with highly
conductive carbon. In the crystalline phase the carbon particles are packed into
the crystalline boundaries and form many conductive paths, and the polymer-
carbon combination has a low resistance.
Figure 3 - Polymer film in semi crystalline phase and conducting chains of carbon molecules.
A current flowing through the device generates heat (I2
R losses). As long
as the temperature increase does not cause a phase change, nothing happens.
However, if the current increases enough so that corresponding temperature rise
causes a phase change, the polymer’s crystalline structure disappears, the
volume expands, and the conducting carbon chains are broken. The result is a
dramatic increase in resistance. Whereas before the phase change a polymer-
carbon combination may have a resistance measured in milliohms or ohms, after

the phase change the same structure’s resistance may be measured in
megaohms. Current flow is reduced accordingly, but the small residual current
and associated I2
R loss is enough to latch the polymer in this state, and the fuse
will stay open until power is removed.
Figure 4 - Polymer film in amorphous phase and broken carbon chains
The process is almost reversible, in that when the temperature falls, the
polymer returns to its crystalline structure, the volume decreases, and the carbon
particles touch and form conductive paths. However, the exact same conductive
paths never form so that the resistance after reset is slightly different from
before. The resistances of a PPTC fuse may triple or quadruple after the first
reset, but thereafter changes are relatively unimportant.

OPERATING PARAMETERS
Initial Resistance: It is the resistance of the device as received from the
factory of manufacturing.
Operating Voltage: The maximum voltage a device can withstand
without damage at the rated current.
Holding Current: Safe current passing through the device under normal
operating conditions.
Trip Current: It is the value of current at which the device interrupts the
current.
Time to Trip: The time it takes for the device to trip at a given
temperature.
Tripped State: Transition from the low resistance state to the high
resistance state due to an overload.
Leakage Current: A small value of stray current flowing through the
device after it has switched to high resistance mode.
Trip Cycle: The number of trip cycles (at rated voltage and current) the
device sustains without failure.
Trip Endurance: The duration of time the device sustains its maximum
rated voltage in the tripped state without failure.
Power Dissipation: Power dissipated by the device in its tripped state.
Thermal Duration: Influence of ambient temperature.
Hysteresis: The period between the actual beginning of the signaling of
the device to trip and the actual tripping of the device.

HOLD AND TRIP CURRENT AS A FUNCTION OF
TEMPERATURE
Figure 5 illustrates the hold- and trip-current behavior of Polyfuse devices
as a function of temperature. One such curve can be defined for each available
device. Region A describes the combinations of current and temperature at
which the Polyfuse device will trip (go into the high-resistance state) and
protect the circuit. Region B describes the combinations of current and
temperature at which the Polyfuse device will allow for normal operation of the
circuit. In Region C, it is possible for the device to either trip or remains in the
low-resistance state (depending on individual device resistance).
Figure 5 – Hold current & Trip current variation with temperature

OPERATING CHARACTERISTICS
Figure 6 – Operating characteristics of polyfuse as current increases with time
Figure-6 shows a typical pair of operating curves for a PPTC device in
still air at 0o
C and 75o
C. The curves are different because the heat required to
trip the device comes both from electrical I2
R heating and from the device
environment. At 75o
C the heat input from the environment is substantially
greater than it is at 0o
C, so the additional I2
R needed to trip the device is
correspondingly less, resulting in a lower trip current at a given trip time (or a
faster trip at given trip current).

Typical Resistance Recovery after a Trip Event
Figure-7 shows typical behavior of a Polyfuse device that is tripped and
then allowed to cool. Over an extended period of time, device resistance will
continue to fall and will eventually approach initial resistance. However, since
this time can be days, months, or years, it is not practical to expect that the
device resistance will reach the original value for operation purposes. Therefore,
when Polyfuse devices are chosen R1MAX should be taken into consideration
when determining hold current. R1MAX is the resistance of the device one hour
after the thermal event.
Figure 7 – Typical resistance recovery after a trip event

ADVANTAGES OVER TRADITIONAL FUSES
Conventional thermal fuses are not resettable and are therefore limited in
their ability to match the low temperature protection of PPTC devices. The
selection of a low fusing temperature in conventional thermal fuses is limited by
the need to avoid nuisance tripping in temporary high ambient temperature
environments, such as car dashboards on a hot day or high storage temperatures.
Even thermal fuses with 94°C or higher fusing temperatures often nuisance trip
during normal operation or pack assembly.
Figure 8 – Table showing a comparison between a PPTC polyfuse and types of fuses
Hence, the major benefits of polyfuse are as-
Low base resistance
Latching (non-cycling) operation
Automatic reset ability
Short time to trip
No arcing during faulty situations
Small dimensions and compact design
Internationally standardized and approved
No accidental hot plugging
Withstand mechanical shocks and vibrations and comply with the safety
norms

APPLICATIONS
PolyFuses are used in automobiles, batteries, computers and peripherals,
industrial controls, consumer electronics, medical electronics, lighting, security
and fire alarm systems, telecommunication equipment and a host of other
applications where circuit protection is required.
Some of its applications in protecting various equipments are discussed as
below-
 TRANSFORMERS PROTECTION
Figure 9 – Transformer protection by Polyfuse
The equipment powered by a transformer gets overheated due to
excessive current or short-circuit. A Polyfuse on the secondary side of the
transformer will protect the equipment against overload as shown in Figure-9.
 SPEAKER PROTECTION
Figure 10 – Speaker protection by Polyfuse
Nowadays speakers are designed and sold independently of amplifiers.
Therefore, there are possibilities of damage due to mismatches. The protection
choices for loudspeaker systems are limited. Fuses protect the speaker, but a
blown fuse is always a source of frustration. Using a Polyfuse in series with the

speaker as shown in Figure-10 will protect it from over-current/over-heating
damage. Choosing a correct trip-current rated Polyfuse is important to match the
power level of the speaker.
 BATTERY PROTECTION
Figure 11 – Battery protection circuit for Li-ion batteries
The Figure-11 below shows a schematic of a typical single-cell Li-ion
battery pack for cellular phone applications, using a Polyfuse. Batteries are
constantly charged and discharged over their life-cycle. Over-charge results in
an increase in the temperature of the electrolyte. This could cause either a fire or
an explosion. Polyfuse play a vital role in the charging and discharging cycles
of batteries. The Polyfuse low resistance overcomes the additional series
resistance introduced by the MOSFETs and the low trip temperature can
provide protection against thermal runaway in the case of an abusive
overcharge.

1. KEYBOARD/MOUSE PROTECTION
FIGURE 11 – Protection of keyboard/mouse through PolyfuseDevic
The operating current of keyboard/ mouse is usually from 200 to 500 mA,
but in a short circuit the current will increase many times. Using PPTC in series
between the connector and host power supply will limit the current cut the
keyboard/ mouse port to the specified maximum.

CONCLUSION
PPTC resettable fuses are designed for today’s demanding electronic and
electrical industries. The concept of a self-resetting fuse of course predates this
technology. Bimetal fuses, for example are widely used in appliances such as
hairdryers, but these are generally large current devices. PPTC resettable fuses
compete with another common overcurrent protection device, namely positive
temperature coefficient (PTC) ceramic thermistors. However, PPTC fuses offer
several advantages. First, they have lower resistance and therefore lower I2
R
heating, and can be rated for much higher currents. Second, the ratio between
open-resistance and close-resistance is much higher than with ceramic PTC
fuses. For example, the resistance change in PTC thermistors is generally in the
range of 1–2 orders of magnitude, but with PPTC fuses, the change may be 6–7
orders of magnitude. However, ceramic PTC fuses don’t exhibit the increase in
resistance after a reset.
The vast majority PPTC fuses on the market have trip times in the range
1–10 seconds, but there are PPTC fuses with trip times of a few milliseconds.
Generally speaking, however, these devices are considered slow-trip fuses. The
blow time depends on the overcurrent, so that a fuse that may open within a few
milliseconds with a severe overload, may take tens of seconds for a light
overload. They are ideal for all low voltage DC and AC application.

REFERENCES
Electronics For You, Edition- September, 2004
Raychem circuit protection products- Tyco Electronics
http://www.circuitprotection.com
http://www.wikipedia.com
http://www.inter-technical.com

ABSTRACT
A traditional fuse is a one time over current protection device employing
fusible link that melts after the current exceeds a certain level for a certain
length of time. Typically, a wire or chemical compound breaks the circuit
when the current exceeds the rated value. Like traditional fuses, polyfuse
limit the flow of dangerously high current during fault condition. Unlike
traditional fuses, it reset after the fault is cleared and the power to the circuit
is removed. It is the main advantage of polyfuse over other circuit protection
devices. Polyfuse is a new standard for circuit protection. They provide both
over current protection and automatic restoration.

INDEX
Sr.
No.
Topics Page
No.
01 Introduction 01
02 Over current production 02
03 Principle of Operation 03
04 Construction And Operation 04
05 Operating Parameters 06
06 Hold And Trip Current As A Function Of
Temperature
07
07 Operating Characteractics 08
08 Typical Resistance Recovery after a Trip
Event
09
09 ADVANTAGES OVER TRADITIONAL FUSES 10
10 Applications 11
11 Conclusion 14
12 reference 15

Recreated polyfuse

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Recreated polyfuse