06-05-2013, 04:59 PM
Seminar Report on PolyFuses
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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 of PPTC 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.
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 3 of Figure-2.
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.
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).
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 I2R 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.