Recreated polyfuse

SEMINAR REPORT ON POLYFUSE

INTRODUCTION

Current flow in a conductor always generates heat. Excess heat is

damaging to electrical components. Overcurrent protection devices are used to

protect conductors from excessive current flow. Thus protective devices are

designed to keep the flow of current in a circuit at a safe level to prevent the

circuit conductors from overheating.

A fuse is a one-time over-current protection device employing a fusible

link that melts (blows) after the current exceeds a certain level for a certain

length of time. Typically, a wire or chemical compound breaks the circuit when

the current exceeds the rated value. A fuse interrupts excessive current so that

further damage by overheating or fire is prevented. Wiring regulations often

define a maximum fuse current rating for particular circuits. Overcurrent

protection devices are essential in electrical systems to limit threats to human

life and property damage. Fuses are selected to allow passage of normal current

and of excessive current only for short periods.

Polyfuse is a resettable fuse that doesn’t need to be replaced like the

conventional fuse. Many manufacturers also call it PolySwitch or MultiFuse.

Polyfuse are designed and made 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, junction boxes and power distribution

centers.

SUBMITTED BY- MUMTAJ K. PAPPUWALE Page 1


OVERCURRENT PROTECTION

Polyfuse is a series element in a circuit. The PPTC device protects the

circuit by going from a low-resistance to a high-resistance state in response to

an overcurrent condition, as shown in Figure-1. This is referred to as "tripping"

the device. In normal operation the device has a resistance that is much lower

than the remainder of the circuit. In response to an overcurrent condition, the

device increases in resistance (trips), reducing the current in the circuit to a

value that can be safely carried by any of the circuit elements. This change is

the result of a rapid increase in the temperature of the device, caused by I 2R

heating.

Figure 1 - Overcurrent protection circuit using Polyfuse



PRINCIPLE OF OPERATION

Technically these are not fuses but Polymeric Positive Temperature

Coefficient (PPTC) Thermistors. Polyfuse device operation is based on an

overall energy balance. Under normal operating conditions, the heat generated

by the device and the heat lost by the device to the environment are in balance

at a relatively low temperature, as shown in Point 1of Figure-2. If the current

through the device is increased while the ambient temperature is kept constant,

the temperature of the device increases. Further increases in either current,

ambient temperature or both will cause the device to reach a temperature where

the resistance rapidly increases, as shown in Point 3 of Figure-2.

Figure 2 – Operating curve as resistance varies with temperature

Any further increase in current or ambient temperature will cause the device to generate heat at a rate greater than the rate at which heat can be dissipated, thus causing the device to heat up rapidly. At this stage, a very large increase in resistance occurs for a very small change in temperature, between points 3 and 4 of Figure-2. This is the normal operating region for a device in the tripped state. This large change in resistance causes a corresponding decrease in the current flowing in the circuit. This relation holds until the device resistance reaches the upper knee of the curve (Point 4 of Figure-2). As long as



the applied voltage remains at this level, the device will remain in the tripped state (that is, the device will remain latched in its protective state). Once the voltage is decreased and the power is removed the device will reset.

CONSTRUCTION & OPERATION

PPTC fuses are constructed with a non-conductive polymer plastic film

that exhibits two phases. The first phase is a crystalline or semi-crystalline state

where the molecules form long chains and arrange in a regular structure. As the

temperature increases the polymer maintains this structure but eventually

transitions to an amorphous phase where the molecules are aligned randomly,

and there is an increase in volume. The polymer is combined with highly

conductive carbon. In the crystalline phase the carbon particles are packed into

the crystalline boundaries and form many conductive paths, and the polymer-

carbon combination has a low resistance.

Figure 3 - Polymer film in semi crystalline phase and conducting chains of carbon molecules.

A current flowing through the device generates heat (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 the phase change a polymer-

carbon combination may have a resistance measured in milliohms or ohms, after

the phase change the same structure’s resistance may be measured in

megaohms. Current flow is reduced accordingly, but the small residual current

and associated I2R loss is enough to latch the polymer in this state, and the fuse

will stay open until power is removed.

Figure 4 - Polymer film in amorphous phase and broken carbon chains

The process is almost reversible, in that when the temperature falls, the

polymer returns to its crystalline structure, the volume decreases, and the carbon

particles touch and form conductive paths. However, the exact same conductive

paths never form so that the resistance after reset is slightly different from

before. The resistances of a PPTC fuse may triple or quadruple after the first

reset, but thereafter changes are relatively unimportant.



OPERATING PARAMETERS

Initial Resistance: It is the resistance of the device as received from the factory of manufacturing.

Operating Voltage: The maximum voltage a device can withstand without damage at the rated current.

Holding Current: Safe current passing through the device under normal operating conditions.

Trip Current: It is the value of current at which the device interrupts the current.

Time to Trip: The time it takes for the device to trip at a given temperature.

Tripped State: Transition from the low resistance state to the high resistance state due to an overload.

Leakage Current: A small value of stray current flowing through the device after it has switched to high resistance mode.

Trip Cycle: The number of trip cycles (at rated voltage and current) the device sustains without failure.

Trip Endurance: The duration of time the device sustains its maximum rated voltage in the tripped state without failure.

Power Dissipation: Power dissipated by the device in its tripped state.

Thermal Duration: Influence of ambient temperature.

Hysteresis: The period between the actual beginning of the signaling of the device to trip and the actual tripping of the device.



HOLD AND TRIP CURRENT AS A FUNCTION OF TEMPERATURE

Figure 5 illustrates the hold- and trip-current behavior of Polyfuse devices

as a function of temperature. One such curve can be defined for each available

device. Region A describes the combinations of current and temperature at

which the Polyfuse device will trip (go into the high-resistance state) and

protect the circuit. Region B describes the combinations of current and

temperature at which the Polyfuse device will allow for normal operation of the

circuit. In Region C, it is possible for the device to either trip or remains in the

low-resistance state (depending on individual device resistance).

Figure 5 – Hold current & Trip current variation with temperature



OPERATING CHARACTERISTICS

Figure 6 – Operating characteristics of polyfuse as current increases with time

Figure-6 shows a typical pair of operating curves for a PPTC device in

still air at 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).



Typical Resistance Recovery after a Trip Event

Figure-7 shows typical behavior of a Polyfuse device that is tripped and

then allowed to cool. Over an extended period of time, device resistance will

continue to fall and will eventually approach initial resistance. However, since

this time can be days, months, or years, it is not practical to expect that the

device resistance will reach the original value for operation purposes. Therefore,

when Polyfuse devices are chosen R1MAX should be taken into consideration

when determining hold current. R1MAX is the resistance of the device one hour

after the thermal event.



Figure 7 – Typical resistance recovery after a trip event

ADVANTAGES OVER TRADITIONAL FUSES

Conventional thermal fuses are not resettable and are therefore limited in

their ability to match the low temperature protection of PPTC devices. The

selection of a low fusing temperature in conventional thermal fuses is limited by

the need to avoid nuisance tripping in temporary high ambient temperature

environments, such as car dashboards on a hot day or high storage temperatures.

Even thermal fuses with 94°C or higher fusing temperatures often nuisance trip

during normal operation or pack assembly.

Figure 8 – Table showing a comparison between a PPTC polyfuse and types of fuses

Hence, the major benefits of polyfuse are as-

Low base resistance

Latching (non-cycling) operation

Automatic reset ability

Short time to trip

No arcing during faulty situations

Small dimensions and compact design



Internationally standardized and approved

No accidental hot plugging

Withstand mechanical shocks and vibrations and comply with the safety norms

APPLICATIONS

PolyFuses are used in automobiles, batteries, computers and peripherals,

industrial controls, consumer electronics, medical electronics, lighting, security

and fire alarm systems, telecommunication equipment and a host of other

applications where circuit protection is required.

Some of its applications in protecting various equipments are discussed

as below-

TRANSFORMERS PROTECTION

Figure 9 – Transformer protection by Polyfuse

The equipment powered by a transformer gets overheated due to

excessive current or short-circuit. A Polyfuse on the secondary side of the

transformer will protect the equipment against overload as shown in Figure-9.

SPEAKER PROTECTION



Figure 10 – Speaker protection by Polyfuse

Nowadays speakers are designed and sold independently of amplifiers.

Therefore, there are possibilities of damage due to mismatches. The protection

choices for loudspeaker systems are limited. Fuses protect the speaker, but a

blown fuse is always a source of frustration. Using a Polyfuse in series with the

speaker as shown in Figure-10 will protect it from over-current/over-heating

damage. Choosing a correct trip-current rated Polyfuse is important to match the

power level of the speaker.

BATTERY PROTECTION

Figure 11 – Battery protection circuit for Li-ion batteries

The Figure-11 below shows a schematic of a typical single-cell Li-ion

battery pack for cellular phone applications, using a Polyfuse. Batteries are

constantly charged and discharged over their life-cycle. Over-charge results in

an increase in the temperature of the electrolyte. This could cause either a fire or

an explosion. Polyfuse play a vital role in the charging and discharging cycles

of batteries. The Polyfuse low resistance overcomes the additional series

resistance introduced by the MOSFETs and the low trip temperature can

provide protection against thermal runaway in the case of an abusive

overcharge.



1. KEYBOARD/MOUSE PROTECTION

FIGURE 11 – Protection of keyboard/mouse through Polyfuse Devic

The operating current of keyboard/ mouse is usually from 200 to 500 mA,

but in a short circuit the current will increase many times. Using PPTC in series

between the connector and host power supply will limit the current cut the

keyboard/ mouse port to the specified maximum.



CONCLUSION

PPTC resettable fuses are designed for today’s demanding electronic and

electrical industries. The concept of a self-resetting fuse of course predates this

technology. Bimetal fuses, for example are widely used in appliances such as

hairdryers, but these are generally large current devices. PPTC resettable fuses

compete with another common overcurrent protection device, namely positive

temperature coefficient (PTC) ceramic thermistors. However, PPTC fuses offer

several advantages. First, they have lower resistance and therefore lower 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 PPTC fuses, the change may be 6–7

orders of magnitude. However, ceramic PTC fuses don’t exhibit the increase in

resistance after a reset.

The vast majority PPTC fuses on the market have trip times in the range

1–10 seconds, but there are PPTC fuses with trip times of a few milliseconds.

Generally speaking, however, these devices are considered slow-trip fuses. The

blow time depends on the overcurrent, so that a fuse that may open within a few

milliseconds with a severe overload, may take tens of seconds for a light

overload. They are ideal for all low voltage DC and AC application.



REFERENCES

Electronics For You, Edition- September, 2004

Raychem circuit protection products- Tyco Electronics

http://www.circuitprotection.com

http://www.wikipedia.com

http://www.inter-technical.com



ABSTRACT

A traditional fuse is a one time over current protection device employing

fusible link that melts after the current exceeds a certain level for a certain

length of time. Typically, a wire or chemical compound breaks the circuit

when the current exceeds the rated value. Like traditional fuses, polyfuse

limit the flow of dangerously high current during fault condition. Unlike

traditional fuses, it reset after the fault is cleared and the power to the circuit

is removed. It is the main advantage of polyfuse over other circuit protection

devices. Polyfuse is a new standard for circuit protection. They provide both

over current protection and automatic restoration.



INDEX

Sr.

No.

Topics Page No.

01 Introduction 01

02 Over current production 02

03 Principle of Operation 03

04 Construction And Operation 04

05 Operating Parameters 06

06 Hold And Trip Current As A Function Of Temperature

07

07 Operating Characteractics 08

08 Typical Resistance Recovery after a Trip Event

09

09 ADVANTAGES OVER TRADITIONAL FUSES 10

10 Applications 11

11 Conclusion 14



12 reference 15


Business

Recreated polyfuse