F E T S A A F E Y T Y PolyFuse A New Standard For Circuit Protection Fuses work between life and death of a circuitr y. Choosing the right fuse is important for prolong ed protection of the circuitry from over-current and voltage spikes. Conventional fuse is either ‘ok’ or ‘blown,’ but PolyFuse is a resettable fuse by i tself tain length of time. Typi- cally, 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 conven- tional fuse. Many manufac- turers also call it PolySwitch or MultiFuse. (PolySwitch and MultiFuse are registered trademarksofRaychemCorp. Radial-leaded metal-oxide varistor devices from Raychem can be combined with PolySwitch devices to help provide protection for ele ctric motors, tele com equipment and various other syst ems and Bourns Inc., respec- tively.) 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 rat- ings of 30 to 250 volts and current ratings of 20 mA to 100 amps. The basics
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A
F E TSAA F EYT Y
PolyFuseA New Standard For Circuit Protection
Fuses work between life and death of a circuitry.
Choosing the right fuse is important for prolonged
protection of the circuitry from over-current and
voltage spikes. Conventional fuse is either ‘ok’ or
‘blown,’ but PolyFuse is a resettable fuse by itself
tain length of time. Typi-cally, a wire or chemicalcompound breaks the circuitwhen the current exceedsthe rated value.
PolyFuse is a resettablefuse that doesn’t need to bereplaced like the conven-tional fuse. Many manufac-turers also call it PolySwitchor MultiFuse. (PolySwitchand MultiFuse are registeredtrademarksofRaychemCorp.
Radial-leaded metal-oxide varistor devices from Raych
emcan be combined with PolySwitch devices to help provideprotection for electric motors, telecom equipment andvarious other systems
and Bourns Inc., respec-tively.)
Resettable fuses provideover-current protection andautomatic restoration.Usually packaged in radial,axial, surface-mount, chip,disk or washer form, theseare available in voltage rat-ings of 30 to 250 volts andcurrent ratings of 20 mA to100 amps.
The basics
Fig. 1: Effects of ambient temperature onthe resistance of a PolyFuse
The conductive plastic is basicallya non-conductive crystalline polymerloaded with a highly conductive carbonto make it conductive. The electrodes
Edge Over ConventionalFuses1. Over-current protection2. Low base resistance3. Latching (non-cycling) operation4. Automatic resettability5. Short time to trip
T.A. BABU
fuse is a one-time over-current pro-
tection device employing a fusible
link that melts (blows) after thecurrent exceeds a certain level for
a cer-
ELECTRONICS FOR YOU SEPTEMBER
2004
Technically, PolyFuses arenot fuses but polymeric positive-tempera-ture coefficient thermistors. Forthermistors characterised as positive tem-perature coefficient, the device resistanceincreases with temperature. These com-prise thin sheets of conductive plastic
with electrodes attached to either side. 6. No arcing during faulty situations7. Small dimensions and compact design8. Internationally standardised and ap-
proved9. No accidental hot plugging10. Withstand mechanical shocks and vi-
brations and comply with the safetynorms
11. Lifetime up to ten times longer
SAFETY
Bourns 600V PPTC resettable MultiFuse
ensure even distribution of power through-out the device.
Operation. At room temperature, nu-
Operating Parameters forPolyFuses1. Initial resistance. The resistance of
thedevice as received from the factory
2. Operating voltage. The maximumvoltage a device can withstand with-out damage at the rated current
3. Holding current. Safe current throughthe device
4. Trip current. Where the device inter-rupts the current
5. Time to trip. The time it takes for thedevice to trip at a given temperatureand current
6. Tripped state. Transition from the low-resistance state to the high-resistancestate due to an overload
7. Leakage current. A small value of stray
current flowing through the device af-ter it has switched to high-resistancemode
8. Trip cycle. The number of trip cycles(at rated voltage and current) the de-vice sustains without failure
9. Trip endurance. The duration of timethe device sustains its maximum ratedvoltage in the tripped state withoutfailure
10. Power dissipation. Power dissipated
by the device in its tripped state11. Thermal duration. Influence of
ambi-ent temperature
12. Hysteresis. The period between the
actual beginning of the signaling of
the device to trip and the actual trip-
ping of the device
ELECTRONICS FOR YOU SEPTEMBER
2004
Fig. 2: Transformer protection by PolyFuse
Fig. 3: Speaker protection by PolyFuse
Fig. 4: Battery protection by PolyFuse
merous carbon chains form conductivepaths through the plastic material. In casean excessive current flows through thedevice, the temperature of the conductiveplastic material rises. When the tempera-ture exceeds its phase-transformationtemperature, the crystalline structuresuddenly changes into an expandedamorphous state. Now the conductiveparticles move apart from each other. Adramatic increase in resistance stops theflow of current through the device. Thecritical current level at which thishappens is known as the trip current.However, a negligible leakage currentcontinues to flow. The device remainslatched until the fault is cleared, provid-ing continuous protection.
Once the applied voltage disappears,the polymer particles rapidly returnto their original structure. The resistancedrops again and automaticallyresets within a few seconds. Thus aPolyFuse acts like a self-resetting solidstatecircuit breaker, which makesit suitable for providing low-cost over-current protection.
The resistance of PolyFuse (expressedon log scale) at room temperature is afew ohms and rapidly increases above110°C.
ApplicationsPolyFuses are used in automobiles, bat-teries, computers and peripherals,industrial controls, consumer electronics,medical electronics, lighting, securityand fire alarm systems, telecommunica-tion equipment and a host of otherapplications where circuit protection isrequired.
Figs 2, 3 and 4 show the use of aPolyFuse for transformer, speaker and bat-tery protection, respectively.
The equipment powered by a trans-former get overheated due to excessivecurrent or short-circuit. A PolyFuse on thesecondary side of the transformer will pro-tect the equipment against overload.
Nowadays speakers are designedand sold independently of amplifiers.Therefore, there are possibilities ofdamage due to mismatches; for example,high-power amplifiers coupled withlow-power speakers or a speaker coildriven with a high volume. The protec-tion choices for loudspeaker systemsare limited. Fuses protect the speaker,but a blown fuse is always a source offrustration.
Using a PolyFuse in series withthe speaker will protect it fromover-current/over-heating damage. Choos-ing a correct trip-current rated PolyFuseis important to match the powerlevel of the speaker. However, a PolyFusecannot protect against mechanicaloverload due to inadequate cabinetdesign.
Batteries are constantly charged anddischarged over their life-cycle.Over-charge results in an increase in thetemperature of the electrolyte. Thiscould cause either a fire or an explosion.PolyFuse can play a vital role inthe charging and discharging cycles ofbatteries.