Polyfuse Seminar Report - Final Year

CHAPTER 1

INTRODUCTION

1.1 PROTECTIVE DEVICES

Equipment applied to electric power systems to detect abnormal and intolerable conditions and to

initiate appropriate corrective actions. These devices include lightning arresters, surge protectors,

fuses, and relays with associated circuit breakers, recloses, and so forth.

From time to time, disturbances in the normal operation of a power system occur. These may be

caused by natural phenomena, such as lightning, wind, or snow; by falling objects such as trees;

by animal contacts or chewing; by accidental means traceable to reckless drivers, inadvertent acts

by plant maintenance personnel, or other acts of humans; or by conditions produced in the system

itself, such as switching surges, load swings, or equipment failures. Protective devices must

therefore be installed on power systems to ensure continuity of electrical service, to limit injury to

people, and to limit damage to equipment when problem situations develop. Protective devices are

applied commensurately with the degree of protection desired or felt necessary for the particular

system.

1.1.1 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.

1.1.2 PROTECTIVE RELAYS

These are compact analog or digital networks, connected to various points of an electrical system,

to detect abnormal conditions occurring within their assigned areas. They initiate disconnection of

the trouble area by circuit breakers. These relays range from the simple overload unit on house

circuit breakers to complex systems used to protect extra high-voltage power transmission lines.

They operate on voltage, current, current direction, power factor, power, impedance, temperature.

In all cases there must be a measurable difference between the normal or tolerable operation and

the intolerable or unwanted condition. System faults for which the relays respond are generally

short circuits between the phase conductors, or between the phases and grounds. Some relays

operate on unbalances between the phases, such as an open or reversed phase. A fault in one part

of the system affects all other parts. Therefore relays and fuses throughout the power system must

be coordinated to ensure the best quality of service to the loads and to avoid operation in the no

faulted areas unless the trouble is not adequately cleared in a specified time. See Fuse (electricity),

Relay

1.1.3 ZONE PROTECTION

For the purpose of applying protection, the electric power system is divided into five major

protection zones: generators; transformers; buses; transmission and distribution lines; and motors

(see illustration). Each block represents a set of protective relays and associated equipment

selected to initiate correction or isolation of that area for all anticipated intolerable conditions or

trouble. The detection is done by protective relays with a circuit breaker used to physically

disconnect the equipment. For other areas of protection See Grounding, Uninterruptible power

system

1.1.4 FAULT DETECTION

Fault detection is accomplished by a number of techniques, including the detection of changes in

electric current or voltage levels, power direction, ratio of voltage to current, temperature, and

comparison of the electrical quantities flowing into a protected area with the quantities flowing

out, also known as differential protection.

1.1.5 DIFFERENTIAL PROTECTION

This is the most fundamental and widely used protection technique. The system compares currents

to detect faults in a protection zone. Current transformers on either side of the protection zone

reduce the primary currents to small secondary values, which are the inputs to the relay. For load

through the equipment or for faults outside of the protection zone, the secondary currents from the

two transformers are essentially the same, and they are directed so that the current through the

relay sums to essentially zero. However, for internal trouble, the secondary currents add up to flow

through the relay.

1.1.6 OVERCURRENT PROTECTION

This must be provided on all systems to prevent abnormally high currents from overheating and

causing mechanical stress on equipment. Overcurrent in a power system usually indicates that

current is being diverted from its normal path by a short circuit. In low-voltage, distribution-type

circuits, such as those found in homes, adequate overcurrent protection can be provided by fuses

that melt when current exceeds a predetermined value.

Small thermal-type circuit breakers also provide overcurrent protection for this class of circuit. As

the size of circuits and systems increases, the problems associated with interruption of large fault

currents dictate the use of power circuit breakers. Normally these breakers are not equipped with

elements to sense fault conditions, and therefore overcurrent relays are applied to measure the

current continuously. When the current has reached a predetermined value, the relay contacts

close. This actuates the trip circuit of a particular breaker, causing it to open and thus isolate the

fault. See Circuit breaker [5]

1.1.7 DISTANCE PROTECTION

Distance-type relays operate on the combination of reduced voltage and increased current

occasioned by faults. They are widely applied for the protection of higher voltage lines. A major

advantage is that the operating zone is determined by the line impedance and is almost completely

independent of current magnitudes.

1.1.8 OVERVOLTAGE PROTECTION

Lightning in the area near the power lines can cause very short-time overvoltages in the system

and possible breakdown of the insulation. Protection for these surges consists of lightning arresters

connected between the lines and ground. Normally the insulation through these arresters prevents

current flow, but they momentarily pass current during the high-voltage transient to limit

overvoltage. Overvoltage protection is seldom applied elsewhere except at the generators, where

it is part of the voltage regulator and control system. In the distribution system, overvoltage relays

are used to control taps of tap-changing transformers or to switch shunt capacitors on and off the

circuits. See Lightning and surge protection

1.1.9 UNDER VOLTAGE PROTECTION

This must be provided on circuits supplying power to motor loads. Low-voltage conditions cause

motors to draw excessive currents, which can damage the motors. If a low-voltage condition

develops while the motor is running, the relay senses this condition and removes the motor from

service.

1.1.10 UNDERFREQUENCY PROTECTION

A loss or deficiency in the generation supply, the transmission lines, or other components of the

system, resulting primarily from faults, can leave the system with an excess of load. Solid-state

and digital-type under frequency relays are connected at various points in the system to detect this

resulting decline in the normal system frequency. They operate to disconnect loads or to separate

the system into areas so that the available generation equals the load until a balance is

reestablished.

1.1.11 REVERSE-CURRENT PROTECTION

This is provided when a change in the normal direction of current indicates an abnormal condition

in the system. In an ac circuit, reverse current implies a phase shift of the current of nearly 180°

from normal. This is actually a change in direction of power flow and can be directed by ac

directional relays.

1.1.12 PHASE UNBALANCE PROTECTION

This protection is used on feeders supplying motors where there is a possibility of one phase

opening as a result of a fuse failure or a connector failure. One type of relay compares the current

in one phase against the currents in the other phases. When the unbalance becomes too great, the

relay operates. Another type monitors the three-phase bus voltages for unbalance. Reverse phases

will operate this relay.

1.1.13 REVERSE-PHASE-ROTATION PROTECTION

Where direction of rotation is important, electric motors must be protected against phase reversal.

A reverse-phase-rotation relay is applied to sense the phase rotation. This relay is a miniature three-

phase motor with the same desired direction of rotation as the motor it is protecting. If the direction

of rotation is correct, the relay will let the motor start. If incorrect, the sensing relay will prevent

the motor starter from operating.

1.1.14 THERMAL PROTECTION

Motors and generators are particularly subject to overheating due to overloading and mechanical

friction. Excessive temperatures lead to deterioration of insulation and increased losses within the

machine. Temperature-sensitive elements, located inside the machine, form part of a bridge circuit

used to supply current to a relay. When a predetermined temperature is reached, the relay operates,

initiating opening of a circuit breaker or sounding of an alarm.

1.2 WHAT IS FUSE?

Fig 1.1 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.

1.2.1 HISTORY

In 1847, Breguet recommended use of reduced-section conductors to protect telegraph stations

from lightning strikes; by melting, the smaller wires would protect apparatus and wiring inside the

building. A variety of wire or foil fusible elements were in use to protect telegraph cables and

lighting installations as early as 1864.

A fuse was patented by Thomas Edison in 1890 as part of his successful electric distribution system

1.2.2 CONSTRUCTION

A fuse consists of a metal strip or wire fuse element, of small cross-section compared to the circuit

conductors, mounted between a pair of electrical terminals, and (usually) enclosed by a non-

combustible housing. The fuse is arranged in series to carry all the current passing through the

protected circuit. The resistance of the element generates heat due to the current flow. The size

and construction of the element is (empirically) determined so that the heat produced for a normal

current does not cause the element to attain a high temperature. If too high a current flows, the

element rises to a higher temperature and either directly melts, or else melts a soldered joint within

the fuse, opening the circuit.

The fuse element is made of zinc, copper, silver, aluminum, or alloys to provide stable and

predictable characteristics. The fuse ideally would carry its rated current indefinitely, and melt

quickly on a small excess. The element must not be damaged by minor harmless surges of current,

and must not oxidize or change its behavior after possibly years of service.

The fuse elements may be shaped to increase heating effect. In large fuses, current may be divided

between multiple strips of metal. A dual-element fuse may contain a metal strip that melts instantly

on a short-circuit, and also contain a low-melting solder joint that responds to long-term overload

of low values compared to a short-circuit. Fuse elements may be supported by steel or nichrome

wires, so that no strain is placed on the element, but a spring may be included to increase the speed

of parting of the element fragments.

The fuse element may be surrounded by air, or by materials intended to speed the quenching of

the arc. Silica sand or non-conducting liquids may be used.

1.2.3 CHARACTERISTIC PARAMETERS

RATED CURRENT IN

A maximum current that the fuse can continuously conduct without interrupting the circuit.

SPEED

The speed at which a fuse blows depends on how much current flows through it and the

material of which the fuse is made. The operating time is not a fixed interval, but decreases as

the current increases. Fuses have different characteristics of operating time compared to

current, characterized as fast-blow, slow-blow, or time-delay, according to time required to

respond to an overcurrent condition. A standard fuse may require twice its rated current to open

in one second, a fast-blow fuse may require twice its rated current to blow in 0.1 seconds, and

a slow-blow fuse may require twice its rated current for tens of seconds to blow.

Fuse selection depends on the load's characteristics. Semiconductor devices may use a fast or

ultrafast fuse as semiconductor devices heat rapidly when excess current flows. The fastest

blowing fuses are designed for the most sensitive electrical equipment, where even a short

exposure to an overload current could be very damaging. Normal fast-blow fuses are the most

general purpose fuses. The time delay fuse (also known as anti-surge, or slow-blow) are

designed to allow a current which is above the rated value of the fuse to flow for a short period

of time without the fuse blowing. These types of fuse are used on equipment such as motors,

which can draw larger than normal currents for up to several seconds while coming up to speed.

THE I2T VALUE

The amount of energy spent by the fuse element to clear the electrical fault. This term is

normally used in short circuit conditions and the values are used to perform co-ordination

studies in electrical networks. I2t parameters are provided by charts in manufacturer data sheets

for each fuse family. For coordination of fuse operation with upstream or downstream devices,

both melting I2t and clearing I2t are specified. The melting I2t, is proportional to the amount

of energy required to begin melting the fuse element. The clearing I2t is proportional to the

total energy let through by the fuse when clearing a fault. The energy is mainly dependent on

current and time for fuses as well as the available fault level and system voltage. Since the I2t

rating of the fuse is proportional to the energy it lets through, it is a measure of the thermal

damage and magnetic forces that will be produced by a fault.

BREAKING CAPACITY

The breaking capacity is the maximum current that can safely be interrupted by the fuse.

Generally, this should be higher than the prospective short circuit current. Miniature fuses may

have an interrupting rating only 10 times their rated current. Some fuses are designated High

Rupture Capacity (HRC) and are usually filled with sand or a similar material. Fuses for small,

low-voltage, usually residential, wiring systems are commonly rated, in North American

practice, to interrupt 10,000 amperes. Fuses for larger power systems must have higher

interrupting ratings, with some low-voltage current-limiting high interrupting fuses rated for

300,000 amperes. Fuses for high-voltage equipment, up to 115,000 volts, are rated by the total

apparent power (megavolt-amperes, MVA) of the fault level on the circuit.

RATED VOLTAGE

Voltage rating of the fuse must be greater than or equal to what would become the open circuit

voltage. For example, a glass tube fuse rated at 32 volts would not reliably interrupt current

from a voltage source of 120 or 230 V. If a 32 V fuse attempts to interrupt the 120 or 230 V

source, an arc may result. Plasma inside that glass tube fuse may continue to conduct current

until current eventually so diminishes that plasma reverts to an insulating gas. Rated voltage

should be larger than the maximum voltage source it would have to disconnect. Rated voltage

remains same for any one fuse, even when similar fuses are connected in series. Connecting

fuses in series does not increase the rated voltage of the combination (nor of any one fuse).

Medium-voltage fuses rated for a few thousand volts are never used on low voltage circuits,

because of their cost and because they cannot properly clear the circuit when operating at very

low voltages.

VOLTAGE DROP

A voltage drop across the fuse is usually provided by its manufacturer. There is a direct

relationship between a fuse's cold resistance and its voltage drop value. Once current is applied,

resistance and voltage drop of a fuse will constantly grow with the rise of its operating

temperature until the fuse finally reaches thermal equilibrium or alternatively melts when

higher currents than its rated current are administered over sufficiently long periods of time.

This resulting voltage drop should be taken into account, particularly when using a fuse in low-

voltage applications. Voltage drop often is not significant in more traditional wire type fuses,

but can be significant in other technologies such as resettable fuse (PPTC) type fuses.

TEMPERATURE DERATING

Ambient temperature will change a fuse's operational parameters. A fuse rated for 1 A at 25

°C may conduct up to 10% or 20% more current at −40 °C and may open at 80% of its rated

value at 100 °C. Operating values will vary with each fuse family and are provided in

manufacturer data sheets.

PACKAGES AND MATERIALS

Fuses come in a vast array of sizes and styles to serve in many applications, manufactured in

standardized package layouts to make them easily interchangeable. Fuse bodies may be made

of ceramic, glass, plastic, fiberglass, molded mica laminates, or molded compressed fiber

depending on application and voltage class.

Cartridge (ferrule) fuses have a cylindrical body terminated with metal end caps. Some

cartridge fuses are manufactured with end caps of different sizes to prevent accidental insertion

of the wrong fuse rating in a holder, giving them a bottle shape.

Fuses for low voltage power circuits may have bolted blade or tag terminals which are secured

by screws to a fuse holder. Some blade-type terminals are held by spring clips. Blade type

fuses often require the use of a special purpose extractor tool to remove them from the fuse

holder.

Renewable fuses have replaceable fuse elements, allowing the fuse body and terminals to be

reused if not damaged after a fuse operation.

Fuses designed for soldering to a printed circuit board have radial or axial wire leads. Surface

mount fuses have solder pads instead of leads.

High-voltage fuses of the expulsion type have fiber or glass-reinforced plastic tubes and an

open end, and can have the fuse element replaced.

Semi-enclosed fuses are fuse wire carriers in which the fusible wire itself can be replaced. The

exact fusing current is not as well controlled as an enclosed fuse, and it is extremely important

to use the correct diameter and material when replacing the fuse wire, and for these reasons

these fuses are slowly falling from favor. (Current ratings from Table 53A of BS 7671: 1992)

DIMENSIONS

Fuses can be built with different sized enclosures to prevent interchange of types of fuse. For

example, bottle style fuses distinguish between ratings with different cap diameters.

Automotive glass fuses were made in different lengths, to prevent high-rated fuses being

installed in a circuit intended for a lower rating.

SPECIAL FEATURES

Glass cartridge and plug fuses allow direct inspection of the fusible element. Other fuses have

other indication methods including:

Indicating pin or striker pin — extends out of the fuse cap when the element is blown.

Indicating disc — a colored disc (flush mounted in the end cap of the fuse) falls out when the

element is blown.

Element window — a small window built into the fuse body to provide visual indication of a

blown element.

External trip indicator — similar function to striker pin, but can be externally attached (using

clips) to a compatible fuse.

Some fuses allow a special purpose micro switch or relay unit to be fixed to the fuse body.

When the fuse element blows, the indicating pin extends to activate the micro switch or relay,

which, in turn, triggers an event.

Some fuses for medium-voltage applications use two separate barrels and two fuse elements

in parallel

1.2.4 TYPES OF FUSES

A fuse unit essentially consists of a metal fuse element or link, a set of contacts between which it

is fixed and a body to support and isolate them. Many types of fuses also have some means for

extinguishing the arc which appears when the fuse element melts.

In general, there are two categories of fuses

Low voltage fuses

High voltage fuses

Usually isolating switches are provided in series with fuses where it is necessary to permit fuses

to be replaced or rewired with safety. In absence of such isolation means, the fuses must be so

shielded as to protect the user against accidental contact with the live metal when the fuse is being

inserted or removed.

REWIREABLE FUSES

The most commonly used fuse in 'house wiring' and small current circuit is the semi-enclosed

or rewire able fuse. (Also sometime known as KIT-KAT type fuse). It consist of a porcelain

base carrying the fixed contacts to which the incoming and outgoing live or phase wires are

connected and a porcelain fuse carrier holding the fuse element, consisting of one or more

strands of fuse wire, stretched between its terminals. The fuse carrier is separate part and can

be taken out or inserted in the base without risk, even without opening the main switch. If fuse

holder or carrier gets damaged during use, it may be replaced without replacing the complete

unit. The fuse wire may be of lead, tinned copper, aluminum or an alloy of tin-lead. The actual

fusing current will be about twice the rated current. When two or more fuse wire are used, the

wires should be kept apart and ad e rating factor of 0.7 to 0.8 should be employed to arrive at

the total fuse rating. The specification for rewire able fuses are covered by IS: 2086-

1963.Standard ratings are 6, 16, 32, 63, and 100A. A fuse wire of any rating not exceeding the

rating of the fuse may be used in it that is a 80 A fuse wire can be used in a 100 A fuse, but not

in the 63 A fuse. On occurrence of a fault, the fuse element blows off and the circuit is

interrupted. The fuse carrier is pulled out, the blown out fuse element is replaced by new one

and the supply can is resorted by re-inserting the fuse carrier in the base. Though such fuses

have the advantage of easy removal or replacement without any danger of coming into the

contact with a lie part and negligible replacement cost but suffers from following

Disadvantages

Unreliable Operations

Lack of Discrimination

Small time lag

Low rupturing capacity

No current limiting feature

Slow speed of operations

1.2.4.1 TOTALLY ENCLOSED OR CARTIDGES TYPE FUSE.

The fuse element is enclosed in a totally enclosed container and is provided with metal contacts

on both sides. These fuses are further classified as

D-type

Link type

Link type cartridges are again of two type’s viz. Knife blade or bolted type.

D- Type Cartridges Fuses

It is a non-interchangeable fuse comprising s fuse base, adapter ring, cartridge and a fuse cap.

The cartridge is pushed in the fuse cap and the cap is screwed on the fuse base. On complete

screwing the cartridge tip touches the conductor and circuit between the two terminals is

completed through the fuse link. The standard ratings are 6, 16, 32, and 63 amperes. The

breaking or rupturing capacity is of the order of 4k A for 2 and 4ampere fuses the 16k A for

63 A fuses. D-type cartridge fuse have none of the drawbacks of the rewire able fuses. Their

operation is reliable. Coordination and discrimination to a reasonable extent and achieved with

them.

Link type Cartridge or High Rupturing Capacity (HRC)

Where large number of concentrations of powers are concerned, as in the modern distribution

system, it is essential that fuses should have a definite known breaking capacity and also this

breaking capacity should have a high value. High rupturing capacity cartridge fuse, commonly

called HRC cartridge fuses, have been designed and developed after intensive research by

manufactures and supply engineers in his direction. The usual fusing factor for the link fuses

is 1.45. The fuses for special applications may have as low as a fusing factor as 1.2.The

specifications for medium voltage HRC link fuses are covered under IS: 2202-1962

KNIFE BLAD TYPE HRC FUSE

It can be replaced on a live circuit at no load with the help of a special insulated fuse puller.

BOLTED TYPE HRC LINK FUSE

It has two conducting plates on either ends. These are bolted on the plates of the fuse base.

Such a fuse needs an additional switch so that the fuse can be taken out without getting a shock.

Preferred ratings of HRC fuses are 2, 4, 6, 10, 16, 25, 30, 50, 63, 80, 100,125, 160, 200, 250,

320, 400, 500, 630,800, 1000 and 1,250 amperes.

1.3 WHAT IS A POLYFUSE?

Polyfuses is a new standard for circuit protection. It is re-settable by itself. Many manufactures

also call it as Polyswitch or Multifuse. Polyfuses are not fuses but Polymeric Positive temperature

Coefficient Thermistors (PPTC).

We can use several circuit protection schemes in power supplies to provide protection against fault

condition and the resultant over current and over temperature damage. Current can be

accomplished by using resistors, fuses, switches, circuit breakers or positive temperature

coefficient devices.

Resistors are rarely an acceptable solution because the high power resistors required are expensive.

One shot fuses can be used but they might fatigue and they must be replaced after a fault event.

Another good solution available is the resettable Ceramic Positive Temperature Coefficient

(CPTC) device. This technology is not widely used because of its high resistance and power

dissipation characteristics. These devices are also relatively large and vulnerable to cracking as

result of shock and vibration.

The preferred solution is the PPTC device, which has a very low resistance in normal operation

and high resistance when exposed to fault. Electrical shorts and electrically overloaded circuits

can cause over current and over temperature damage.

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. [4] [1]

1.3.1 THE BASICS

Technically Polyfuses are not fuses but Polymeric Positive Temperature Coefficient Thermistors.

For thermistors characterized as positive temperature coefficient, the device resistance increases

with temperature. The PPTC circuit protection devices are formed from thin sheets of conductive

semi-crystalline plastic polymers with electrodes attached to either side. The conductive plastic is

basically a non-conductive crystalline polymer loaded with a highly conductive carbon to make it

conductive. The electrodes ensure the distribution of power through the circuit.

Polyfuses are usually packaged in radial, axial, surface mount, chip or washer form. These are

available in voltage ratings of 30 to 250 volts and current ratings of 20 mA to 100A.

Polyfuses are usually packaged in radial, axial, surface-mount, chip, disk, or washer form. The

conductive plastic is basically a non-conductive crystalline polymer loaded with a highly

conductive carbon to make it conductive. PPTC devices limit the flow of dangerously high current

during fault conditions. PPTC devices reset after the fault is cleared and the power to the circuit is

removed. [1]

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

Fig. 1.2 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. [3]

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.

Fig 1.3 PPTC (Polymeric Positive Temperature Coefficient)

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.

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

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. [3]

1.6 RESISTANCE TEMPERATURE CHARACTERISTICS

The resistance/temperature characteristics of the two types are shown in Fig 1.4. 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. Fig 1.4 shows the voltage-current characteristic of an NTC thermistor.

Fig.1.4: Resistance &Temperature Characteristics of NTC and PTC 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. 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

Although sometimes referred to as "resettable fuses," PPTC devices are non-linear thermistors

used to limit current. PPTC circuit protection devices are made from a composite of semi-

crystalline polymer and conductive particles.

Fig 2.1 Operating curve as resistance varies with temperature

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 1 of Fig 2.1.

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 3 of Figure 2.1.

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 2.1.

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 3). 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.

However, if the temperature rises above the device's switching temperature (TSw) either from high

current through the part or from an increase in the ambient temperature, the crystallites in the

polymer become amorphous. The increase in volume during this phase separates the conductive

particles, resulting in a large non-linear increase in the resistance of the device.

In this case, the device resistance typically increases by three or more orders of magnitude. This

increased resistance helps protect the equipment in the circuit by reducing the amount of current

that can flow under the fault condition to a low, steady-state level. The device remains in its latched

(high-resistance) position until the fault is cleared and power to the circuit is cycled; at which time

the conductive composite cools and re-crystallizes, restoring the PPTC to a low- resistance state

in the circuit and the affected equipment to normal operating conditions.

Because PPTC devices transition to their high-impedance state based on the influence of

temperature, they help provide protection for two fault conditions: overcurrent and over

temperature. Overcurrent protection is provided when the PPTC device is heated internally due to

I2R power dissipated within the device. High current levels through the PPTC device heat it

internally to its switching temperature, causing it to "trip" and go into a high impedance state.

The PPTC device can also be made to trip by thermally linking it to a component or equipment--

such as a motor--that needs to be protected against damage caused by over temperature conditions.

If the equipment temperature reaches the PPTC device's switching temperature, the PPTC device

will transition to its high-impedance state, regardless of the current flowing through it. In this way,

the PPTC device can be used either to reduce the current to the equipment to very low levels, or

as an indicator to the control system that the equipment is overheating. The control system can

then determine what action is appropriate to protect equipment and personnel.

PPTC devices are employed as series elements in a circuit. Their small form factor helps conserve

valuable board space and, in contrast to traditional fuses that require user-accessibility, their

resettable functionality allows for placement in inaccessible locations. Because they are solid-state

devices, they are also able to withstand mechanical shock and vibration. [4][6]

2.1 VOLTAGE-TEMPERATURE CHARACTERISTICS

Fig 2.2 Voltage versus Temperature Characteristics of Polyfuse

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.

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. [6]

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 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

Fig 3.3 Transition of Molecules from Semi crystalline 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.

[5]

3.1 RESISTANCE RECOVERY AFTER A TRIP EVENT

Typical Resistance Recovery after a Trip Event Fig 3.3 shows typical behavior of a PolySwitch

device that is tripped and then allowed to cool.

Fig 3.4 Resistance Recovery after a Trip Event

This figure illustrates how, even after a number of hours, the device resistance is still greater than

the initial resistance. 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 PolySwitch

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. [1]

3.2 OPERATING CHARACTERISTICS OF POLYMERIC PTC

Operating Characteristics of Polymeric PTC Figure 5 shows a typical pair of operating curves for

a PolySwitch 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

75oC the heat input from the environment is substantially greater than it is at 0oC, so the additional

I2R 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). [1]

Fig 3.5 Example of Operating Characteristics of PPTC

3.3 DEVICE RESET TIME

Fig 3.6 Resistance Recovery after a Trip Event

Returning to Figure 3.6, we note that after a trip event, the resistance recovery to a quasi-stable

value is very rapid, with most of the recovery occurring within the first one-to-two minutes. Figure

3.7 shows the resistance recovery curve for a number of other leaded PolySwitch devices. The

power dissipation values were also measured to provide the user with a sense of the thermal

environment the device was placed in for the measurement. [1]

Fig 3.6 Typical Resistance Recovery after a Trip Event

As with other electrical properties, the resistance recovery time will depend upon both the design

of the device and the thermal environment. Since resistance recovery is related to the cooling of

the device, the greater the heat transfer, the more rapid the recovery (see Figure 6 for miniSMD075

devices on boards with traces of 0.010 inch and 0.060 inch).

3.3 OPERATING PARAMETERS

There are few operating parameters of the Polyfuse which are described below:

LEAKAGE CURRENT: A PTC is said to have “tripped” when it has transitioned from its

low resistance state to a high resistance state due to overload current. Protection is

accomplished by limiting the current fl ow to a low leakage level. Leakage current can range

from less than a hundred milliamps at rated voltage up to a few hundred milliamps at lower

voltages. The fuse on the other hand completely interrupts the current fl ow and this open

circuit results in no leakage current after it has been subjected to an overload current.

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.

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.

FAULT CURRENT: The PTC is rated for a maximum short circuit current at rated voltage.

This fault current level is the maximum current that the device can safely limit keeping in mind

that the PTC will not actually interrupt the current flow (see LEAKAGE CURRENT above).

The typical short circuit rating of a board-mounted PTC is 40A; for battery strap PTCs, this

value can reach 100A. Fuses do in fact interrupt the current fl ow in response to the overload

and the range of interrupting ratings vary from tens of amperes up to 10,000 amperes at rated

voltage.

OPERATING VOLTAGE RATING: General use PTCs are not rated above 60V while fuses

are rated up to 600V.

HOLD CURRENT RATING The hold (operating) current rating for PTCs can be up to 14A

while the maximum level for fuses can exceed 30A.

TEMPERATURE DERATING: The useful upper limit for a PTC is generally 85°C while

the maximum operating temperature for fuses is 125°C. The following temperature derating

curves (see chart at bottom of page) that compare PTCs to fuses illustrate that more derating

is required for a PTC at a given temperature. Additional operating characteristics can be

reviewed by the circuit designer in making the decision to choose a PTC or a fuse for

overcurrent protection.

AGENCY APPROVALS: PTCs are recognized under the Component Program of

Underwriters Laboratories to UL Standard 1434 for Thermistors. The devices have also been

approved for use in Canada by Underwriters Laboratories. Approvals for fuses include

Recognition under the Component Program of Underwriters Laboratories and the CSA

Component Acceptance Program. In addition, many fuses are listed in accordance with

UL/CSA/ANCE (Mexico) 248-14, Supplemental Fuses.

RESISTANCE: Reviewing product specifications indicates that similarly-rated PTCs have

about twice (sometimes more) the resistance of fuses. [3][5]

CHAPTER 4

DESIGN CONSIDERATIONS FOR PPTC DEVICES

Some of the critical parameters to consider when designing PPTC devices into a circuit include

device hold current and trip current, the effect of ambient conditions on device performance;

device reset time, leakage current in the tripped state and the automatic or manual reset conditions.

4.1 HOLD AND TRIP CURRENT

Fig. 4.1: Hold and Trip Current

Region A shows the combination of current and temperature at which the Region A

describes the combinations of current and temperature at which the Poly Switch 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 Poly Switch

device will allow for normal operation of the circuit.

Region C it is possible for the device to either trip or remain in the low-resistance state.

4.2 EFFECT OF AMBIENT CONDITIONS ON DEVICE PERFORMANCE

The heat transfer environment of the device can significantly affect the device performance. In

general, by increasing the heat transfer of the device, there is a corresponding increase in power

dissipation, time to trip and hold current. The opposite occurs if the heat transfer from the device

is decreased. Furthermore, changing the thermal mass around the device changes the time to trip

of the device.

If the heat generated is greater than the heat lost to the environment, the device will increase in

temperature resulting in a trip event. The rate of temperature rise and the total energy required to

make a device trip depends on the fault current and heat transfer environment. Under normal

operating conditions the heat generated by the device and the heat lost to the environment are in

balance.

Increases in current or ambient temperature or increase in both, cause the device to reach a

temperature at which the resistance rapidly increases. This large change in resistance causes a

corresponding decrease in the current flowing through the circuit, protecting the circuit from

damage.

4.3 TIME TO TRIP

The time to trip of a PPTC device is defined as the time needed from the onset of a fault current to

trip the device. Time to trip depends upon the size of the fault current and the ambient temperature.

4.4 DESIGN CRITERIA

To select the best device for a specific application, circuit designers should consider the following

design criteria:-

4.4.1 CHOOSE THE APPROPRIATE FORM FACTOR

Select from radial- leaded, surface-mount, or chip parts. For mounting on circuit boards, a radial-

leaded or surface- mount configuration is preferred. Radial-leaded parts are typically wave

soldered to the board. Chip parts are designed to be held in clips, usually in an electric motor.

4.4.2 CHOOSE A VOLTAGE RATING

The voltage rating of a PPTC device should equal or exceed the source voltage in a particular

circuit. Also the expected fault voltage should not be later than the PPTC voltage device. When a

PPTC device trips, the majority of circuit voltage appears across the device because it is the highest

resistance element present in the circuit.

4.4.3 CHOOSE A HOLD CURRENT RATING

(At the proper ambient operating temperature). Hold current is defined as the greatest steady state

current the PPTC device can carry without tripping into a high resistance state. Designers must

choose a PPTC device with a hold current at maximum ambient temperature equal to or greater

than the steady state operating current.

4.4.4 CHECK TRIP TIME

Designers should determine what fault currents may occur and how quickly the most sensitive

system components could be damaged at these currents. A PPTC device should be selected that

trips before these sensitive components would be damaged. Many applications experience a start-

up surge current from a capacitance or motor. Normally, this in-rush current does not contain

enough energy to trip the PPTC device, but the designers should confirm performance in their

application over the range of expected ambient conditions.

4.4.5 CHECK MAXIMUM INTERRUPT CURRENT

A PPTC device normally has a maximum interrupt current rating, i.e., the maximum fault current

that the device consistently interrupts while remaining functional. [3]

CHAPTER 5

TYPES OF POLYFUSES

5.1 SURFACE MOUNT RESETTABLE FUSES

Fig 5.1 Surface Mount Resettable Fuses

This surface mount polyfuse family of polymer of polymer based resettable fuses provides

reliable over current protection for a wide range of products such as computer motherboards,

USB hubs and ports, CD/DVD drives , digital cameras and battery packs. Each of these

polyfuse series features low voltage drops and fast trip times while offering full resettability.

This makes each an ideal choice for protection in datacom and battery powered applications

where momentary surges may occur during interchange of batteries or plug and play

operations.

The SMD0805 with the industry’s smallest footprint, measuring only 2.2mm by 1.5mm,

features four hold current ratings from 100mA to 500mA with a current interruption capability

of 40A at rated voltage. Both the SMD1206 and SMD1210 series are optimized for protection

of computer peripherals, PC cards and various port types.

5.2 RADIAL-LEADED RESETTABLE FUSES

Due to the automatic resetting of the polyfuse, these components are ideal for applications, where

temporary fault conditions (e.g.: during hot plugging) can occur. The radial-leaded RLD-USB-

series 709 is specifically designed for universal serial bus (USB) applications with lower

resistance, faster trip times and lower voltage drops.

Fig 5.2 Radial-Leaded Resettable Fuses

5.3 BATTERY STRAP RESETTABLE FUSES

Fig 5.3 Battery Strap Resettable Fuses

This type profile strap type polyfuse family of resettable fuses provides thermal and over

charge protection for rechargeable battery packs commonly used in portable electronics such

as mobile phones, notebook computers and camcorders.

Both Li-Ion and NiMH pack designs are enhanced with 0.8mm high form factor on the VTD-

719 series. The LTD-717 series is optimized for prismatic packs and exhibits faster trip times-

down to 2.9 sec at five times the fuse’s hold current rating. [5][6][3]

CHAPTER 6

TECHNOLOGY COMPARISON

6.1 TECHNOLOGY COMPARISON - CPTC DEVICES

Ceramic PTC (CPTC) devices can be used to help provide resettable protection. However, their

application is limited due to their relatively high operating temperature, high resistance and large

size. The composition of the CPTC device tends to be brittle, which makes it vulnerable to damage

from shock, vibration, as well as the thermal stress of heating and cooling found in many motor

and transformer applications.

Figure 6.1 and Figure 6.2 show the results of testing, comparing CPTC and PPTC devices,

performed by Tyco Electronics. The PolySwitch™ PPTC devices were compared to CPTC devices

as primary protection elements using two identical transformers. The PPTC and the CPTC devices

were selected to have the same hold current. In this test, a fault was created with a secondary short,

while current, coil temperature and time-to-trip were measured. As shown in Figure 2, the PPTC

device reacted more quickly, and at a lower temperature.

Fig 6.1 Time-to-trip comparison of CPTC device versus PPTC device in secondary short on 120VAC transformer

Fig 6.2 Comparison of maximum surface temperatures of CPTC device and PPTC device in tripped state.

Compared to the CPTC device, which reached a surface temperature of about 75°C to 185°C

during test, the PPTC device exhibited a lower surface temperature of about 100C to 120°C in

the tripped state. The PPTC device also had lower resistance in the circuit, was lower in

capacitance and was less frequency-dependent.

In Figure 2, thermal images illustrate the difference in surface temperatures of the CPTC and

PPTC devices. In this comparison of a 220VAC trip, the CPTC device reached a maximum

temperature of 184.5°C, whereas the PPTC device reached a maximum temperature of 118.9°C.

[4]

6.2 TECHNOLOGY COMPARISON-BIMETAL CIRCUIT BREAKERS

Bimetal circuit breakers, although widely used to help protect electric motors, do not latch and

require additional action to interrupt their on-off cycle. The bimetal strip is constructed of two

different metals bonded together. When the bimetal's current rating is exceeded, heat generated by

the excessive current causes the bimetal strip to bend and open a set of contacts to stop current

flow. With no current flowing, the device returns to its normal shape, closing the contacts so

current flow may resume. In the case of a stall, the bimetal circuit breaker continues to cycle until

power is removed.

The cycling nature of a bimetal circuit breaker has several disadvantages. Among those are

material fatigue and a tendency to burn contacts, spark or to weld shut. If the device "fails closed"

Fig 6.3 Icemaker motor (rotor locked) test results with bimetal device protection

Fig 6.4 Icemaker motor (rotor locked) test results with PPTC device protection

Damage to the motor as well as sensitive follow-on electronics can occur as a result of an

overcurrent event. Potential noise or "chatter" and electro-magnetic interference (EMI) can also

make bimetal circuit breakers incompatible with advanced electronic control systems.

Recent testing by Tyco Electronics compared the thermal and electrical characteristics of a popular

bimetal thermal protector and the PolySwitch LVR device, each installed on an icemaker motor.

The protection devices were coupled to the motor winding and the motor shaft was locked during

the test period. The voltage, current, temperatures of winding/core and the temperature of the

PPTC device and the bimetal protector were recorded during the test.

Figure 6.3 and Figure 6.4 illustrate the results of the two tests. In the test using a bimetal circuit

breaker, the motor winding reached a temperature of approximately 129°C at 60 minutes. This was

significantly higher than the test that used a PPTC protection device, where the motor winding

reached a temperature of 44°C within the same time frame. [4]

6.3 POLYSWITCH RESETTABLE DEVICES PRODUCT SELECTION

GUIDE

Table 6.1 Thermal Derating

Table 6.2 PolySwitch Characteristic

6.3.1 SELECTION STEPS FOR POLYFUSES

STEP 1. DETERMINE YOUR CIRCUIT'S PARAMETERS

You will need to determine the following parameters of your circuit: Maximum ambient operating

temperature Normal operating current Maximum operating voltage Maximum interrupt

current

STEP 2. SELECT A POLYSWITCH DEVICE THAT WILL ACCOMMODATE THE

CIRCUIT'S MAXIMUM AMBIENT TEMPERATURE AND NORMAL OPERATING

CURRENT

Use the Thermal Derating [hold Current (A) at Ambient Temperature (oC)] table and choose the

temperature that most closely matches the circuit's maximum ambient temperature. Look down

that column to find the value equal to or greater than the circuit's normal operating current. Now

look to the far left of that row to find the part family or part for the PolySwitch device that will

best accommodate the circuit.

STEP 3. COMPARE THE SELECTED DEVICE'S MAXIMUM ELECTRICAL RATINGS

WITH THE CIRCUIT'S MAXIMUM OPERATING VOLTAGE AND INTERRUPT

CURRENT

Use the Electrical Characteristics table to verify the part you selected in Step 2 will handle your

circuit's maximum operating voltage and interrupt current. Find the device's maximum operating

voltage (Vmax) and maximum interrupt current (Imax). Ensure that Vmax and Imax are greater

than or equal to the circuit's maximum operating voltage and maximum interrupt current.

STEP 4. DETERMINE TIME-TO-TRIP

Time-to-trip is the amount of time it takes for a device to switch to a high-resistance state once a

fault current has been applied across the device. Identifying the PolySwitch device's time-to-trip

is important in order to provide the desired protection capabilities. If the device you choose trips

too fast, undesired or nuisance tripping will occur. If the device trips too slowly, the components

being protected may be damaged before the device switches to a high-resistance state.

Use the Typical Time-to-trip Curves at 20oC to determine if the PolySwitch device's time-to-trip

is too fast or too slow for the circuit. If it is go back to Step 2 and choose an alternate device.

STEP 5. VERIFY AMBIENT OPERATING TEMPERATURE

Ensure that your application's minimum and maximum ambient temperatures are within the

operating temperature of the PolySwitch device. Most PolySwitch devices have an operating

temperature range from -40oC to 85oC with some exception to 125oC.

Step 6. Verify the PolySwitch device dimensions Use the Dimensions table to compare the

dimensions of the PolySwitch device you selected with the application's space considerations.

DEFINITIONS OF TERMS

lH the maximum steady state current at 20oC that can be passed through a PolySwitch device

without causing the device to trip

IT the minimum current that will cause the PolySwitch device to trip at 20oC

Vmax the maximum voltage that can safely be dropped across a PolySwitch device in its tripped

state also called: Maximum Device Voltage, Maximum Voltage, Vmax, and Max Interrupt Voltage

Imax the maximum fault current that can safely be used to trip a PolySwitch device PD the power

(in watts) dissipated by a PolySwitch device in its tripped state

Rmax the maximum resistance prior to the trip of PolySwitch device

Rmin the minimum resistance prior to the trip of PolySwitch device

R1 max the maximum resistance of a PolySwitch device at 20oC 1 hour after being tripped or after

reflow soldering. Also called: Maximum Resistance

RTripped TYP the typical resistance of PolySwitch 1 hour after the initial trip and reset [1]

CHAPTER 7

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: [4]

PPTC CPTC Bi-metal Conventional

Resettable Yes Yes Yes No

Size Small Medium Large Large

Resistance Low High Low Low

power loss Low High Low Low

Cost Low Medium High High

Table 7.1 Advantages over Conventional Fuses

7.2 EDGES OVER CONVENTIONAL FUSES

Over current protection

Low base resistance

Latching operation

Automatic resettability

Short time to trip

No arching during faulty situations

Small dimensions and compact designs

Internationally standardized and approved

No accidental hot plugging

Withstand mechanical shocks and vibrations

Life time- up to 10 times longer

7.3 TO FUSE OR NOT TO FUSE

Despite the advantages of resettable devices, there are circumstances where a fuse may be the

preferred form of circuit protection. Under conditions where restoration of normal operation poses

a potential safety hazard, or where service on the equipment should be performed after a fault

condition has occurred, a fuse or circuit breaker is appropriate. For example, a single use fuse is

recommended on a garbage disposal because the blades could cause serious harm to the user if the

motor were to suddenly resume operation.

When selecting an overcurrent protection device, designers must also consider reset conditions,

restoration time, and ambient conditions that can affect the performance of the device. Ultimately,

designers must decide what level of protection is required for their applications and only a system

test can determine whether or not a specific protection device is appropriate.

Although "resettable fuse" is a common marketing term used in describing PPTC devices, they are

not fuses at all. They are, in fact, non-linear thermistors that limit current. Because PPTC devices

go into a high resistance state under a fault condition, normal operation can still result in hazardous

voltage being present in parts of the circuit. It is important that the circuit designer recognize

critical differences between the two devices.

Fuses are current interruption devices and, once a fuse "blows," the electrical circuit is broken.

There is no longer current flowing through the fuse. This electrical interruption, or open circuit, is

a permanent condition. However, once a PPTC device trips, there is a small amount of current

flowing through the device. PPTC devices require a low joule heating leakage current or external

heat source in order to maintain their tripped condition. Once the fault condition is removed, this

heat source is eliminated. The device can then return to a low resistance status and the circuit is

restored to normal operating conditions. [4]

CHAPTER 8

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 equipment’s are discussed as below:

8.1 INTERMITTENT OPERATION MOTOR PROTECTION

Fig 8.1 Typical PPTC device application in motor circuit

Intermittent operation motors are usually designed to operate for a limited time. In general,

operating these products for longer than the designed maximum limit usually results in stalling,

overheating and, ultimately, failure. Fault conditions arise when the power is held on, either

because of contact failure or customer misuse.

To prevent overheating, the circuit protection device used must "trip" quickly, but not sooner than

intended, to avoid creating a nuisance condition for the user. However, developing a protection

scheme that effectively protects the motor without nuisance tripping presents a design challenge.

Nuisance tripping is often caused by inrush currents associated with certain electrical components

found on motorized equipment. The major advantage of using a PPTC device is that it can be

specified with a trip current substantially below the normal operating current of the motor, but

with a time-to-trip that is several times longer than a full system operating cycle, to avoid nuisance

tripping.

Figure 8.1 shows how a PPTC device can be installed in a motor circuit to help protect against

damage from overcurrent or over temperature events. When the device is enclosed within the

motor housing it reacts to the current flowing in the motor, as well as any temperature rise that

may occur during a fault condition. [4]

8.2 CONTINUOUS-OPERATION MOTOR PROTECTION

Continuous-operation motors are typically designed to optimize size and cost. Since they are often

used to drive fans, some airflow can be diverted through the motor to allow operation under more

stress than would otherwise be possible. As a result, the stall current of fan motors is usually only

two times the run current, compared to a ratio of three to four times run current that is common in

other applications. This complicates the task of finding and sizing a fuse that will open reliably if

the fan becomes blocked; yet not blow from an inrush when the motor is first switched on.

As noted in the discussion on intermittent operation motors, PPTC devices offer advantages in

motor protection schemes. By altering their characteristics, as the motor's vulnerability changes

with temperature, they can provide a slower response when appropriate.

In applications where a fan is motor-driven, both the PPTC device and the motor can benefit from

being placed in the air stream. With this method, the trip current of the PPTC device will be greatly

increased because the airflow tends to prevent it from reaching its trip temperature. However, if

the fan stalls for any reason, the cooling effect of the airflow ceases, causing the overrated motor

to heat up quickly. This condition causes the PPTC device to trip and limit current flowing to the

motor.

Unlike a single-use fuse, the PPTC device helps prevent damage where faults may cause a rise in

temperature with only a slight increase in current draw “providing both overcurrent and over

temperature protection with a single, installed component. [4]

8.3 INDUSTRIAL CONTROLLER PROTECTION

Traditionally, single-use fuse technology has been used to protect electronic circuits from damage

caused by overcurrent events. With this approach, the fuse blows when a wiring fault or part failure

Fig 8.2 PPTC devices help protect the interfaces between controllers and remote devices as well as power inputs

Creates a condition in which excessive currents can flow, therefore breaking the electrical

connection and helping prevent the potential for more widespread damage or fire hazards.

The problem with this technology is that a failure in one system component can disable other

components downstream and throughout the system. When this happens, the fuse must be accessed

and replaced on all the affected components before the system can be made operational again.

In comparison, controllers and remote devices that utilize resettable fault protection technology

can help minimize the impact that failure has on the system, reduce the number of system

components affected, and shorten repair time. PPTC devices offer a practical alternative to fuse

technology and help protect valuable electronic systems, reduce warranty and service costs, and

improve user satisfaction.

In many industrial controller applications, replacing single-use fuses with PPTC devices allows

designers to maintain the same level of overcurrent protection on the critical interfaces, while

generally eliminating the need for fuse replacement or service when an external fault condition

causes high current conditions in the system.

In addition to controllers, any remote sensor, indicator, or actuator that requires a power, analog,

or communications bus interface can benefit from the use of PPTC devices. These system

Components are subject to damage caused by miswiring, power cross, or loose neutral connections

on AC mains inputs (Figure 8.2). [4]

8.4 IN TRANSFORMER PROTECTION

Fig 8.3 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 8.1. [1]

8.5 IN BATTERY PROTECTION

Fig 8.4 Battery Protection by 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. [1]

8.6 IN SPEAKER PROTECTION

Fig 8.5 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 will protect it from over-current/overheating.

[1]

8.7 IN MOTORS, FANS AND BLOWERS

Fig 8.6 Application of Polyfuse in motor protection

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. [3]

8.8 IN COMPUTERS

8.8.1 KEYBOARD/ MOUSE:

Fig 8.7 Use of PPTC Device in 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. [3]

8.8.2 HARD DISK DRIVER

Hard disk driver is an 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. [3]

Fig 8.8 Application of Polyfuses in Hard Disk Driver

8.9 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. [4][6]

Fig 8.9 Polyfuses in rechargeable Battery Packs

8.10 IN AUTOMOTIVE SECTORS

8.10.1 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 8.10 Polyfuses in Automotive Circuits for the Solution of Wire Harness

8.10.2 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 equipment’s for over current protection. The following figure

shows that a Polyfuse is connected in automotive electronics equipment’s to protect the circuit.

Fig 8.11 Use of Polyfuse in Automotive Electronics

8.11 IN TELECOM SECTORS

8.11.1 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 equipment’s from

damage.

Fig 8.12 Polyfuses used for Network Equipment in Telecom Sectors

CHAPTER 9

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.

CHAPTER 10

REFERENCES

White Papers

[1] T. A. Babu, “Polyfuse A New Standard For Circuit Protection”, ELECTRONICS FOR YOU

Sept 2004

[2] M. Alavi et al., “A PROM element based on salicide agglomeration of Polyfuses in a CMOS

logic process,” in 1997

[3] J. Fellner, P. Boesmueller, and H. Reiter, “Lifetime study for a poly fuse in a 0.35 μm polycide

CMOS process,” IRPS, 2005.

[4] John Halpin, “Design consideration for Implementing Circuit Protection Devices”

[5] Lisa Jones, Karina Kinsman ,“PPTC Design consideration for Automotive Circuits”

Compliance Engineering magazine ,May 2004

[6] Bourns, “Multifuse PPTC Thermistors for Power over Ethernet Protection” IEEE Application

Notes, Sep 2003

Websites

[1]http://www.te.com/content/dam/te/global/english/products/Circuit-protection/knowledge-

center/documents/circuit-protection-psw-fundamentals.pdf

[2] http://elinux.org/Polyfuses_explained

[3] http://www.slideshare.net/khanpin2/polyfuse

[4] http://www.eetimes.com/document.asp?doc_id=1272524

[5] http://www.scribd.com/doc/27177923/POLYFUSE-Seminar-Report

[6] http://www.scribd.com/doc/49071314/SEMINAR-REPORT-ON-POLY-FUSE