Polyfuses are resettable fuses made of a conductive polymer mixed with carbon. They act as a thermistor, where the resistance increases with temperature. During an overcurrent event, the temperature rise causes the polymer to change structure from crystalline to amorphous, increasing resistance and stopping current flow. Once it cools, the polymer returns to its original crystalline state and conductivity is restored, resetting the fuse. Polyfuses provide overcurrent protection and automatic restoration, coming in various packages for different applications such as equipment, speakers, and batteries.

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2. Polyfuses are resettable fuse.
These are generally positive-temperature coefficient
thermistors.
The device resistance rises with temperature.
These comprise thin sheets of conductive plastic with
electrodes attached to either side.
These fuses provide over-current protection and
automatic restoration.
Voltage rating-----3o to 250 volts.
Current rating-----20mA to 100A. 2

3. 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. 3333333322
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4. • PPTC fuses are constructed with a non-conductive polymer plastic film
that exhibits two phases.
• The first phase is a crystalline or semi-crystalline statewhere 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.
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5. • 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.
• When the temperature falls, the polymer returns to its crystalline
structure, the volume decreases, and the carbon particles touch and
form conductive paths.
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6. At room temperature numerous carbon
chains form conductive paths through the
plastic material.
In case an excessive current flows through the
device, the temperature of the conductive
plastic material rises.
When the temperature exceeds its phase-
transformation temperature, the crystalline
structure suddenly changes into an expanded
amorphous state.
Now the conductive particles move apart from
each other.
 A dramatic increase in resistance stops the
flow of current through the device.
The critical current level at which this
happens is known as the trip current. 6

7. 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 against overload.
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8. Nowadays, in speakers, there are possibilities of damage due to mismatches; for
example, high-power amplifiers coupled with low-power speakers or a speaker
coil driven with a high volume. The protection choices for loudspeaker systems
are limited. Using a PolyFuse in series with the speaker will protect it fromover-
current/over-heating damage. Choosing a correct trip -current rated PolyFuse is
important to match the power level of the speaker.
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9. 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 can play a vital role in the
charging and discharging cycles of batteries.
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12. Iprog Ilinear: Linear resistor
mA Imax
characteristics
Iheat: Temp. is raising
Imelt Imelt: Tungsten Silicide is
Imin Ialloy
Iheat melting
Ilinear Iosc Imax: Maximum current of
minimum resistance
tprog 0µs 1µs 2µs 3µs Imin: Local current min.
Vprog Iosc: Oscillation because of break
V
Ialloy: No autonomous current
pinch off

13.  Surface Mount Resettable Fuses
This surface mount polyfuse family of polymer of polymer basedresettable 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 anideal choice for protection in data com and battery powered applications
where momentary surges may occur during interchange of batteries or plug and
play operations.
Radial-Leaded Resettable Fuses
Due to the automatic resetting of the polyfuse, these components are idealfor
applications, where temporary fault conditions (eg: during hot plugging)
canoccur. The radial-leaded RLD-USB-series 709 is specifically designed for
universalserial bus (USB) applications with lower resistance, faster trip times
and lower voltage drop. 13

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15. Low Current Programmed Poly Fuse
- High energy is forcing the - Relatively high ohmic
Tungsten seperation resistor
- Break before Tungsten completely - Lifetime drift to lower
removed resistor values possible
Tungsten Plug Field Oxide Tungsten Plug
Tungsten
HALO
Tungsten Silicide Tungsten Silicide
Poly Silicon Poly Silicon
Tungsten Field Oxide
Substrate
Substrate

16. THANKING EVERYONE FOR YOUR KIND PERUSAL
THIS PRESENTATION IS PREPARED BY:
ASANSOL ENGINEERING COLLEGE
ELECTRICAL DEPERTMENT
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