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Seminar Report, 2010
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 center
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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
Figure 1 - Overcurrent protection circuit using Polyfuse
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 I2R heating.
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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.
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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
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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.
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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.
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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
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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).
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Typical Resistance Recovery after a Trip Event
Figure-7 shows typical behavior of a Polyfuse device that is tripped and then
Figure 7 – Typical resistance recovery after a trip event
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.
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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
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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
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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
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 KEYBOARD/MOUSE PROTECTION
FIGURE 11 – Protection of keyboard/mouse through Polyfuse Device
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.
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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.
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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
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