PRINCIPLE OF OPERATION
ADVANTAGES OVER CONVENTIONAL FUSES
Polyfuse :A New Standard For Circuit Protection.
Fuses work between life and death in circuit choosing the right fuse is
important for prolonged protection of the circuitry from over-current and
voltage spikes. Conventional fuse is either ‘ok’ or ‘blown,’ but PolyFuse is a
resettable fuse by itself.
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 in thin chip form It is placed in series to protect a circuit.
It made of PPTC Material.
Polyfuse provide over-current protection and automatic restoration.
Polymeric positive temperature coefficient
device (PPTC, commonly known as a
resettable fuse) its provide protection against
overcurrent faults in electronic circuits.
They are actually acting more like circuit
breakers, allowing the circuit to function
without opening the chassis or replacing
anything after fault condition.
Technically Polyfuses are not fuses but Polymeric Positive Temperature
Coefficient Thermistors. For 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 electrodes ensure the distribution of power through the
Polyfuses are usually packaged in radial, axial, surface mount, chip
or washer form.
These are available in voltage ratings of 30 to 250 volt current
rating of20mA to 100A
PRINCIPLE OF OPERATION
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 Fig point 1.
If the current through the device is increased while the ambient temperature is
kept constant,so the temperature of the device increases.
Further increases in current, ambient temperature or both will cause the device to
reach a temperature where the resistance rapidly increases.
Any further increase in current or ambient temperature will cause the device
to generate heat the rate of heat generation is greater than the rate of heat
dissipation, 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.This large change
in resistance causes a corresponding decrease in the current flowing in the
As long as the applied voltage remains at this level, the device will remain in
the tripped state (the device will remain in its protective state). Once the
voltage is decreased and the power is removed the device will reset.
TIYPES OF POLYFUSE
PPTC fuses are constructed with a non-conductive polymer plastic film that
exhibits two phases.
To select the best device for a specific application,circuit
designers should consider the following design criteria.
1. Choose the appropriate form factor
2. Choose a voltage rating
3. Choose a current rating.
4. Check trip time.
5. Check maximum interrupt current.
Advantages over conventional fuses
1. Over current protection.
2. Automatic resettability.
3. Short time to trip.
4. No arching during faulty situations.
5. Small dimensions and compact designs.
6. Withstand mechanical shocks and vibrations.
7. Life time- up to 10 times longer.
PolyFuses are used in automobiles.
industrial controls, consumer electronics.
medical electronics, lighting.
security and fire alarm systems.
Largest application electrical engineering discuss below.
The equipment powered by a transformer get overheated due to excessive current or short-
circuit. A PolyFuse on the secondary side of the transformer will protect the equipment
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.
PPTC resettable fuses compete with another common overcurrent protection device,
namely positive temperature coefficient (PTC) ceramic thermistors. However, PPTC fuses
offer several advantages.
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.