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Recreated polyfuse

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  • 1. SEMINAR REPORT ON POLYFUSE SUBMITTED BY- MUMTAJ K. PAPPUWALE Page 1 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 ofPPTC 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 centers.
  • 2. SEMINAR REPORT ON POLYFUSE SUBMITTED BY- MUMTAJ K. PAPPUWALE Page 2 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 device. In normal operation the device has a resistance that is much lower than the remainder of thecircuit. 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 I2 R heating. Figure 1- Overcurrent protection circuit using Polyfuse
  • 3. SEMINAR REPORT ON POLYFUSE SUBMITTED BY- MUMTAJ K. PAPPUWALE Page 3 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 3of 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.
  • 4. SEMINAR REPORT ON POLYFUSE SUBMITTED BY- MUMTAJ K. PAPPUWALE Page 4 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 (I2 R 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
  • 5. SEMINAR REPORT ON POLYFUSE SUBMITTED BY- MUMTAJ K. PAPPUWALE Page 5 the phase change the same structure’s resistance may be measured in megaohms. Current flow is reduced accordingly, but the small residual current and associated I2 R 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.
  • 6. SEMINAR REPORT ON POLYFUSE SUBMITTED BY- MUMTAJ K. PAPPUWALE Page 6 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.
  • 7. SEMINAR REPORT ON POLYFUSE SUBMITTED BY- MUMTAJ K. PAPPUWALE Page 7 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
  • 8. SEMINAR REPORT ON POLYFUSE SUBMITTED BY- MUMTAJ K. PAPPUWALE Page 8 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 0o C and 75o C. The curves are different because the heat required to trip the device comes both from electrical I2 R heating and from the device environment. At 75o C the heat input from the environment is substantially greater than it is at 0o C, so the additional I2 R 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).
  • 9. SEMINAR REPORT ON POLYFUSE SUBMITTED BY- MUMTAJ K. PAPPUWALE Page 9 Typical Resistance Recovery after a Trip Event Figure-7 shows typical behavior of a Polyfuse device that is tripped and then 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. Figure 7 – Typical resistance recovery after a trip event
  • 10. SEMINAR REPORT ON POLYFUSE SUBMITTED BY- MUMTAJ K. PAPPUWALE Page 10 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
  • 11. SEMINAR REPORT ON POLYFUSE SUBMITTED BY- MUMTAJ K. PAPPUWALE Page 11 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 systems are limited. Fuses protect the speaker, but a blown fuse is always a source of frustration. Using a Polyfuse in series with the
  • 12. SEMINAR REPORT ON POLYFUSE SUBMITTED BY- MUMTAJ K. PAPPUWALE Page 12 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.
  • 13. SEMINAR REPORT ON POLYFUSE SUBMITTED BY- MUMTAJ K. PAPPUWALE Page 13 1. KEYBOARD/MOUSE PROTECTION FIGURE 11 – Protection of keyboard/mouse through PolyfuseDevic 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.
  • 14. SEMINAR REPORT ON POLYFUSE SUBMITTED BY- MUMTAJ K. PAPPUWALE Page 14 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 I2 R 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.
  • 15. SEMINAR REPORT ON POLYFUSE SUBMITTED BY- MUMTAJ K. PAPPUWALE Page 15 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
  • 16. SEMINAR REPORT ON POLYFUSE SUBMITTED BY- MUMTAJ K. PAPPUWALE Page 16 ABSTRACT A traditional fuse is a one time over current protection device employing fusible link that melts 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. Like traditional fuses, polyfuse limit the flow of dangerously high current during fault condition. Unlike traditional fuses, it reset after the fault is cleared and the power to the circuit is removed. It is the main advantage of polyfuse over other circuit protection devices. Polyfuse is a new standard for circuit protection. They provide both over current protection and automatic restoration.
  • 17. SEMINAR REPORT ON POLYFUSE SUBMITTED BY- MUMTAJ K. PAPPUWALE Page 17 INDEX Sr. No. Topics Page No. 01 Introduction 01 02 Over current production 02 03 Principle of Operation 03 04 Construction And Operation 04 05 Operating Parameters 06 06 Hold And Trip Current As A Function Of Temperature 07 07 Operating Characteractics 08 08 Typical Resistance Recovery after a Trip Event 09 09 ADVANTAGES OVER TRADITIONAL FUSES 10 10 Applications 11 11 Conclusion 14 12 reference 15
  • 18. SEMINAR REPORT ON POLYFUSE SUBMITTED BY- MUMTAJ K. PAPPUWALE Page 18