S A F E T Y

CMYK

ELECTRONICS FOR YOU SEPTEMBER 2004

S A F E T Y

T.A. BABU

Fuses work between life and death of a circuitry.Choosing the right fuse is important for prolongedprotection of the circuitry from over-current andvoltage spikes. Conventional fuse is either ‘ok’ or‘blown,’ but PolyFuse is a resettable fuse by itself

Afuse is a one-time over-current pro-tection device employing a fusiblelink that melts (blows) after the

current exceeds a certain level for a cer-

PolyFuse

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 registeredtrademarks of Raychem Corp.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 basicsTechnically, PolyFuses are

not 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 plasticwith electrodes attached to either side.

The conductive plastic is basicallya non-conductive crystalline polymerloaded with a highly conductive carbonto make it conductive. The electrodes

A New Standard For Circuit Protection

Fig. 1: Effects of ambient temperature onthe resistance of a PolyFuse

Edge Over ConventionalFuses1. Over-current protection2. Low base resistance3. Latching (non-cycling) operation4. Automatic resettability5. Short time to trip6. 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

Radial-leaded metal-oxide varistor devices from Raychemcan be combined with PolySwitch devices to help provideprotection for electric motors, telecom equipment andvarious other systems

S A F E T Y

CMYK

ELECTRONICS FOR YOU SEPTEMBER 2004

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.

Fig. 2: Transformer protection by PolyFuse

Fig. 3: Speaker protection by PolyFuse

Fig. 4: Battery protection by PolyFuse

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.

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ensure even distribution of power through-out the device.

Operation. At room temperature, nu-

Bourns 600V PPTC resettable MultiFuse

Operating Parameters forPolyFuses1. Initial resistance. The resistance of the

device as received from the factory2. Operating voltage. The maximum

voltage 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 straycurrent 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 dissipatedby the device in its tripped state

11. Thermal duration. Influence of ambi-ent temperature

12. Hysteresis. The period between theactual beginning of the signaling ofthe device to trip and the actual trip-ping of the device