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  2. 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. 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 2222222222 2222222222 33
  4. 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. 4
  5. 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. 5
  6. 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. 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. 7
  8. 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. 8
  9. 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. 9
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  12. 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. 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. 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