27-12-2012, 01:52 PM
PolyFuse - A New Standard For Circuit Protection
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INTRODUCTION
Afuse is a one-time over-current protection device employing a fusible link that melts (blows) after the current exceeds a certain level for a cer-
PolyFuse tain length of time. Typically, a wire or chemical compound breaks the circuit when the current exceeds the rated value. PolyFuse is a resettable fuse that doesn’t need to be replaced like the conventional fuse. Many manufacturers also call it PolySwitch or MultiFuse. (PolySwitch and MultiFuse are registered trademarks of Raychem Corp. and Bourns Inc., respectively.) Resettable fuses provide over-current protection and automatic restoration. Usually packaged in radial,
axial, surface-mount, chip, disk or washer form, these are available in voltage ratings of 30 to 250 volts and current ratings of 20 mA to 100 amps.
The basics
Technically, PolyFuses are not fuses but polymeric positive-temperature coefficient thermistors. For thermistors characterised as positive temperature coefficient, the device resistance increases with temperature. These comprise thin sheets of conductive plastic with electrodes attached to either side.
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. Figs 2, 3 and 4 show the use of a PolyFuse for transformer, speaker and battery protection, respectively. 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. Nowadays speakers are designed
and sold independently of amplifiers. Therefore, 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. Fuses protect the speaker, but a blown fuse is always a source of
frustration. Using a PolyFuse in series with the speaker 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. However, a PolyFuse cannot protect against mechanical overload due to inadequate cabinet design.