PRINCIPLE OF OPERATION

Technically these are not fuses but

Polymeric Positive Temperature

Coefficient (PPTC) Thermistors.

Polyfuse device operation is based on

an overall energy balance. Under

normal operating conditions, the heat

generated by the device and the heat

lost by the device to the environment

are in balance at a relatively low

temperature, as shown in Point A of

Figure. Point A is that point which

shows that polyfuse works in normal

working conditions i.e. normal current

flows through the circuit .If the current

through the device is increased while

the ambient temperature is kept

constant, the temperature of the device

increases. Further increases in either

current, ambient temperature or both

will cause the device to reach a

temperature where the resistance

rapidly increases as shown in fig.

Any further increase in current or

ambient temperature will cause the

device to generate heat at a rate greater

than the rate at which heat can be

dissipated, thus causing the device to

heat up rapidly. At this stage, a very

large increase in resistance occurs for

a very small change in temperature,

between points B and C of

Figure2.1.Point C is that point at which

transition from low resistance state to

high resistance state takes place. This

is the normal operating region for a

device in the tripped state. This large

change in resistance causes a

corresponding decrease in the current

flowing in the circuit. This relation

holds until the device resistance

reaches the upper knee of the curve

(Point C of Figure2.1). At this point

maximum resistance of the device can

be obtained. As long as the applied

voltage remains at this level, the device

will remain in the tripped state (that is,

the device will remain latched in its

protective state). Once the voltage is

decreased and the power is removed

the device will reset.

Fig: Temperature versus Resistance

Characteristics of Polyfuse

Any further increase in current or

ambient temperature will cause the

device to generate heat at a rate greater

than the rate at which heat can be

dissipated, thus causing the device to

heat up rapidly. At this stage, a very

large increase in resistance occurs for

a very small change in temperature,

between points B and C of

Figure2.1.Point C is that point at which

transition from low resistance state to

high resistance state takes place. This

is the normal operating region for a

device in the tripped state. This large

change in resistance causes a

corresponding decrease in the current

flowing in the circuit. This relation

holds until the device resistance

reaches the upper knee of the curve

(Point C of Figure2.1). At this point

maximum resistance of the device can

be obtained. As long as the applied

voltage remains at this level, the device

will remain in the tripped state (that is,

the device will remain latched in its

protective state). Once the voltage is

decreased and the power is removed

the device will reset.

PPTC Principle of Operation

Fig 1: PPTC devices protect the circuit

by going from a low-resistance state to

a high-resistance state in response to

an overcurrent or over temperature

condition.

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. At normal

temperature, the conductive particles

form low-resistance networks in the

polymer (Fig 2).

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.