1

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 doesn‟t 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.

2

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.

3

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

4

Assuming, as a first-order approximation, that the relationship between resistance

and temperature is linear then:

Where,

ΔR = change in resistance

ΔT = change in temperature

k = first-order temperature coefficient of resistance.

Thermistors can be classified into two types, depending on the sign of k. If k is

positive the resistance increases with increasing temperature, and the device is called a

positive temperature coefficient (NTC) thermistor or posistor. If k is 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 k as

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. The

resistance 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.

5

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.

6

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 the

device to reach a temperature where the resistance rapidly increases as shown in fig 2.1.

Fig2.1: Temperature versus Resistance Characteristics of Polyfuse

7

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 25°C together with resistance values at other temperatures.

8

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.

9

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

10

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 in the

phase change a polymer-carbon combination may have a resistance measured milliohms or

ohms, after the phase change the same structure‟s 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

11

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 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 known as the value of current at which the device interrupts the

current 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 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.

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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

14

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

15

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 94°C 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:

16

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

18

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.

20

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 doesn‟t need to be replaced again and again.

Fig.5.4: Application of Polyfuse in Industrial Controllers

21

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.

22

Fig.5.6: Application of Polyfuses in Hard Disk Driver

5.6 In Rechargeable 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

5.7.1 Automotive 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

5.8.1 Network 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.

25

Fig.5.10: Polyfuses used for Network Equipment in Telecom Sectors

5.9 EXAMPLES OF 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.

26

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

27

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: Surface–mount 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

28

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

29

CONCLUSION

Polyfuses 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 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 1–2 orders of

magnitude, but with Polyfuses, 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 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.

30

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