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CHAPTER 1
INTRODUCTION
1.1 Need of Protective Devices
Current flow in a conductor always generates heat. Excess heat is damaging to
electrical components. Over current 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.
1.2 What is Fuse
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. Over
current 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.
1.3 What is a Polyfuse
Polyfuse is a resettable fuse that doesnt need to be replaced like the conventional
fuse. Many manufacturers also call it PolySwitch or Multi-Fuse .Polyfuse are designed and
made of PPTC 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.
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1.4 Over Current 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 over current
condition in fig1.1.
Fig.1.1: Over Current Protection Circuit Using Polyfuse device.
This refers to tripping the device. In normal operation the device has a resistance that
is much lower than the remainder of the circuit. In response to an over current 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 I2R heating.
1.5 What is a PPTC Device
A PPTC device is a form of thermistor. A thermistors is a type of resistor
whose resistance varies significantly with temperature, more so than in standard resistors.
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The word is a portmanteau of thermal and resistor. Thermistors are, widely used as inrush
current limiters, temperature sensors, self-resetting over current protectors and self-
regulating heating element.
Fig.1.2: A Type of PPTC Device
Thermistors differ from resistance temperature detectors (RTD) in that the material
used in a thermistor is generally a ceramic or polymer, while RTDs use pure metals. The
temperature response is also different; RTDs are useful over larger temperature ranges,
while thermistors typically achieve a higher precision within a limited temperature range,
typically 90 C to 130
.
Fig.1.3: Thermistor symbol
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Assuming, as a first-order approximation, that the relationship between resistance
and temperature is linear then:
Where,
R = change in resistance
T= change in temperature
k= first-order temperature coefficient of resistance.
Thermistors can be classified into two types, depending on the sign of k. Ifkis
positive the resistance increases with increasing temperature, and the device is called a
positive temperature coefficient (NTC) thermistor or posistor. Ifkis negative, the resistance
decreases with increasing temperature, and the device is called a negative temperature
coefficient (NTC) thermistor. Resistors that are not thermistors are designed to have a kas
close to zero as possible, so that their resistance remains nearly constant over a wide
temperature range. When a polymer film is attached to PTC thermistors these are known as
PPTC devices.
1.6 Resistance Temperature Characteristics
The resistance/temperature characteristics of the two types are shown in Fig.1. Theresistance the NTC falls following an exponential characteristic over a wide temperature
range. The NTC Thermistor shows a large increase of resistance over a small temperature
range of power dissipation within the component. When thermistors, especially the small
bead type, are used for temperature measurement, the power dissipation must be kept to a
low level to avoid inaccuracies due to self-heating. Fig1.3 shows the voltage-current
characteristic of an NTC thermistor. Initially the relationship is linear, since, at low power
levels, the dissipation is insufficient to raise the temperature above ambient. At higher power
levels.
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Fig.1.3: Resistance &Temperature Characteristics of NTC and PTC Thermistor
resistance falls and a value of voltage Emax is reached when further increases of current
cause a fall in potential across the thermistor. Dissipation factor and thermal time-constant
are two further properties frequently quoted. The first of these is the power expressed in mill
watts required to raise the temperature of the thermistor by 1 deg C. The time constant is the
time for the resistance of the thermistor to change by 63 % of the total change when
subjected to a step function change in temperature.
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CHAPTER 2
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
A of Figure. Point A is that point which shows that polyfuse works in normal working
conditions i.e. normal current flows through the circuit .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 thedevice to reach a temperature where the resistance rapidly increases as shown in fig 2.1.
Fig2.1: Temperature versus Resistance Characteristics of Polyfuse
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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 B and C of Figure2.1.Point C is that point at
which transition from low resistance state to high resistance state takes place. 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 C of Figure2.1). At this
point maximum resistance of the device can be obtained. 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.
2.1 Voltage-Temperature Characteristics
Thermistors can also be made with a positive temperature coefficient of resistance
but, as shown in Fig.2.2 their characteristic is not the inverse of the NTC type. These
thermistors are made from barium titanate. When used in its monocrystalline form this
material has a resistance which varies inversely with temperature. A polyfuse is not however
monocrystalline but rather numerous small crystals bonded together during the sintering
process. At a certain temperature, barrier layers form at the inter crystalline boundaries and
impedance to the electron flow. As the temperature rises, so does the resistance of these
barrier layers until, above a certain limit, the material resumes its normal negative
characteristics, but at a much higher resistance value. The nature of this resistance-
temperature characteristic prevents a simple mathematical relationship and manufacturers
usually quote a resistance at 25C together with resistance values at other temperatures.
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Fig2.2: Voltage versus Temperature Characteristics of Polyfuse
. The term 'switch temperature, Tsw' is introduced to denote the temperature at which the
resistance starts to rise rapidly. It is defined as that temperature at which the thermistor has a
resistance equal to twice its minimum value. Examination of the voltage-current
characteristic (Fig.2.2) shows the initial linear portion of the curve where voltage and
current rise together followed by the rapid drop in current that occurs once the thermistor
has changed to its high resistance state.
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CHAPTER 3
CONSTRUCTION & WORKING
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 conductor combination has a low
resistance.
Fig.3.1: Conductive paths and the Polymer Carbon
A current flowing through the device generates heat (I2R losses). As long as the
temperature increase does not cause a phase change, nothing happens. However, if the
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current increases enough so that corresponding temperature rise causes a phase change, the
polymers crystalline structure disappears, the volume expands, and the conducting carbon
chains are broken. The result is a dramatic increase in resistance. Whereas before in the
phase change a polymer-carbon combination may have a resistance measured milliohms or
ohms, after the phase change the same structures resistance may be measured in mega
ohms. Current flow is reduced accordingly, but the small residual current and associated I2R
loss is enough to latch the polymer in this state, and the fuse will stay open until power is
removed.
Fig.3.2: Polymer Molecules in Amorphous State
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Fig3.3: Transition of Molecules from Semicrytalline to Amorphous State
At normal working conditions, the molecules of the device are in low resistance
state, which is known as crystalline structure of the Polyfuse. When current starts to flow
through the device, the temperature of the molecules tends to increase and when the current
exceeds from a certain level the temperature increases and the resistance increases. So the
molecules of the material go into high resistance state so the current reduces accordingly in
the device. Due to leakage current and I2R losses the circuit is still open, until the power is
fully removed from the circuit then the molecules of the device cooled down and reforms in
its original structure so the Polyfuse resets.
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3.1 Operating Parameters
There are few operating parameters of the Polyfuse which are described below:
Initial Resistance: It is the resistance of the device as received from the factory ofmanufacturing.
Operating Voltage: The maximum voltage a device can withstand without damageat the rated current.
Holding Current: Safe current passing through the device under normal operatingconditions.
Trip Current: It is known as the value of current at which the device interrupts thecurrent of the device.
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 ithas switched to high resistance mode.
Trip Cycle: The number of trip cycles (at rated voltage and current) the devicesustains without failure.
Trip Endurance: The duration of time the device sustains its maximum ratedvoltage 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.
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3.2 Hold and Trip Current as a Function of Temperature
Fig.3.4 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 3.4: Hold current & Trip current variation with temperature
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3.3 Operating Characteristics
Fig.3.5 shows a typical pair of operating curves for a PPTC device in still air at 0oC
and 75oC. The curves are different because the heat required to trip the device comes both
from electrical I2R heating and from the device environment. At 75
oC the heat input from
the environment is substantially greater than it is at 0oC, so the additional I
2R 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).
Fig3.5: Operating characteristics of Polyfuse as Current Increases with Time
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CHAPTER 4
ADVANTAGES OF POLYFUSE
4.1 Utilities over Conventional 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 94C or higher fusing temperatures
often nuisance trip during normal operation or pack assembly. As we know that
conventional fuses use some protecting cover, this increases the size of the conventional
fuses while the Polyfuse are installed in a thin chip form so the size of the Polyfuse is much
less in comparison to traditional fuses. Polyfuses are considered as more safe than traditional
fuses as these are connected internally in series with the devices and reduces the arcing
probability in the circuit and there are much less power losses in Polyfuses as these requires
less amount of energy for its operation. The table for comparison of Polyfuse with some
other useful PPTC devices is given below:
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Table 4.1: Comparison between Different PPTC devices
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
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4.2 Typical Resistance Recovery by Polyfuse after a Trip Event
Figure 4.1 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.
Fig.4.1: Typical Resistance Recovery after a Trip Event
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CHAPTER 5
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-
5.1 In Transformer Protection
Fig.5.1: 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 5.1.
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5.2 In Speaker Protection:
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 will protect it
from over-current/overheating.
Fig.5.2: Speaker Protection by Polyfuse
5.3 In Motors, Fans and Blowers
If the motors are under overload, the extremely fine wire will be damaged by
overheating. Install of PPTC in motors and blowers to prevent from overheating .As in given
figure a Polyfuse (PPTC Device) is attached in series to the circuit instead of a conventional
fuse. This does not damage the circuit as this is a resettable device and protect it from
overheating. So the Polyfuses are widely used for motors. fans and blowers.
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Fig.5.3: Application of Polyfuse in motor protection
5.4 In Industrial Process Controls
As we know that different type of controllers are needed to control the different
process of any industry and these controllers require some overcurrent protecting devices to
be protectected from overheating.So polyfuses are best suitable devices for these controllers
as these are resettable devices and doesnt need to be replaced again and again.
Fig.5.4: Application of Polyfuse in Industrial Controllers
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5.5 In Computers
5.5.1. Keyboard/ mouse:
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 Polyfuse in series between the
connector and host power supply will limit the current cut the keyboard mouse port to the
specified maximum.
Fig5.5: Use of PPTC Device in Keyboard/Mouse
5.5.2 Hard Disk Driver:
Hard disk driver is a important tool for computers. So we require an efficient over
current protection device to protect the circuit .In hard disk driver the Polyfuse (PPTC
device) is connected in series with platon motor and head actuator when the over current
flows through the circuit, the operation of Polyfuse takes place and Polyfuse provide
protection from overheating of the elements.
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Fig.5.6: Application of Polyfuses in Hard Disk Driver
5.6 InRechargeable Battery Packs
PPTC in series within battery pack will avoid the followed faults occurring:
a. Shorting of the positive and negative terminals.
b. A runaway charging condition in which the charger during charging, fails to stop
supplying current to the package when it is fully charged.
c. Using the wrong charger or the pack is reverse changed.
Fig.5.7: Polyfuses in rechargeable Battery Packs
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5.7 In Automotive Sectors
5.7.1Automotive harness:
The conventional solution in wire harnesses is that groups similar circuits together
and protects them with a single fuse. In order to limit risk of fire, the wire high current
carrying capability, and the oversized wire is commonly used. If anyone circuit under the
same fuse short, the other circuits will all stop. PPTC devices can be installed to each circuit,
which allows the optimum wire to be selected. And the other hand, the circuits don't have to
be through the central fuse box, thus reducing the length of wire required.
Fig.5.8: Polyfuses in Automotive Circuits for the Solution of Wire Harness
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5.7.2 Automotive Electronics:
Automotive electronics is the electronics used in automobiles. Automotive
electronics or automotive embedded systems are the distributed systems. So there are some
types of polyfuses used for automotive electronics equipments for over current protection.
The following figure shows that a Polyfuse is connected in automotive electronics
equipments to protect the circuit.
10
Fig5.9: Use of Polyfuse in Automotive Electronics
5.8 In Telecom Sectors
5.8.1Network Equipment:
The telecom networks are potentially exposed to AC power crosses, thunder hazard,
induced over current in the networks. The PPTC devices which are in series with line feed
resistor and in paralleled with MOV will protect against these fault and prevent network
equipments from damage.
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Fig.5.10: Polyfuses used for Network Equipment in Telecom Sectors
5.9 EXAMPLESOF THE POLYFUSES CLASSIFIED ON THE BASIS OF THE
APPLICATIONS
5.9.1 Automotive Devices:
Polyfuse automotive devices are qualified and sold under PS400 specification which
is derived from AECQ200, the standard for electronic components used in the automotive
industry. These devices have successfully passed to meet the demanding environmental
conditions that can be found in automotive applications. In the following fig.5.11 the
polyfuses used in the automotive devices are shown. These devices have ratings according
to the devices.
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Fig.5.11: Automotive Polyfuse devices
5.9.2 Radial-Leaded Devices:
For design or volume applications,the polyfuse radial-leaded devices represent the
most comprehensive and complete set of PPTC available in the industry today.
Fig.5.12: Radial-Leaded Polyfuse Devices
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5.9.3 Surface Mount Devices:
These devices are preferred circuit protection method for computer, consumer
multimedia, portable and automotive electronics application. Surface mount devices are
shown in figure given below-
Fig.5.13: Surfacemount Polyfuse Devices
5.9.4 Strap Battery Devices:
Many materials platforms and device forms factors allowing the engineer greater
design flexiblility. Polyfuse devices for battery protection include SRP, LTP,LR4,VLP,VLR
and MXP series, disc and special application strap devices.
Fig.5.14: Strap Battery Polyfuse Devices
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5.9.5 Telecom and Networking Devices:
These devices, for telecommunication and networking applications, help provide
protection against power cross and power induction surge as defined in ITU, Telcordia, and
Ul, available in chip, surface mount and radial leaded configurations, these devices also
helps to improve the reliability of consumers.
Fig.5.15: Telecom and Networking Polyfuse Devices
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CONCLUSION
Polyfuses are designed for todays 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 over current
protection device, namely positive temperature coefficient (PTC) ceramic thermistors.
However, Polyfuses offer several advantages. First, they have lower resistance and therefore
lower I2R 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 12 orders of
magnitude, but with Polyfuses, the change may be 67 orders of magnitude. However,
ceramic PTC fuses dont exhibit the increase in resistance after a reset.
The vast majority PPTC fuses on the market have trip times in the range 110
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 over
current, 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.
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