Poly Fuse
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#1
17-09-2009, 01:04 AM


Poly Fuse

Polyfuse is a new standard for circuit protection. It is resettable. Many manufacturers also call it polyswitch or multifuse. Polyfuses are not fuses but polymeric positive temperature co-efficient (PPTC) thermistors.

Current limiting can be accomplished by using resistors , fuses , switches or positive temperature co-efficient devices. Resistors are rarely an acceptable solution because the high power resistors that are usually required are expensive. One-shot fuses can be used, but they might fatigue, and they must be replaced after a fault event. Ceramic PTC devices tends to have high resistance and power dissipation characteristics.

The preferred solution is a PPTC device which has low resistance in normal operation and high resistance when exposed to a fault. Electrical shorts or electrically over-loaded circuits can cause over-current and over temperature damage.

Like traditional fuses , PPTC devices limit the flow of dangerously high current during fault conditions. Unlike traditional fuses, PPTC devices reset after the fault is cleared and the power to the circuit is removed.

THE BASICS

Technically, polyfuses are not fuses but polymeric positive temperature co-efficient (PPTC) thermistors. For thermistors characterized as positive temperature co-efficient , the device resistance increases with temperature. These comprise thin sheets of conductive plastic with electrodes attached to either side. The conductive plastic is basically a non-conductive crystalline polymer loaded with a highly conductive carbon to make it conductive. The electrodes ensure even distribution of power throughout the device.

Polyfuses are 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 20Ma to 100 amps.

OPERATING PARAMETERS FOR POLYFUSES

1) INITIAL RESISTANCE:- The resistance of the device as received from the factory of manufacturing.
2) OPERATING VOLTAGE:- The maximum voltage a device can withstand without damage at the rated current.
3) HOLDING CURRENT:- Safe current through the device.
4) TRIP CURRENT:- Where the device interrupts the current.
5) TIME TO TRIP:- The time it takes for the device to trip at a given temperature.
6) TRIPPED STATE:- Transition from the low resistance state to the high resistance state due to an overload.
7) LEAKAGE CURRENT:- A small value of stray current flowing through the device after it has switched to high resistance mode.
8) TRIP CYCLE:- The number of trip cycles (at rated voltage and current) the device sustains without failure.
9) TRIP ENDURANCE:- The duration of time the device sustains its maximum rated voltage in the tripped state without failure.
10) POWER DISSIPATION:- Power dissipated by the device in its tripped state.
11) THERMAL DURATION:- Influence of ambient temperature.
12) HYSTERESIS:- The period between the actual beginning of the signaling of the device to trip and the actual tripping of the device.
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MANOJ.MV607
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#2
08-07-2010, 03:44 PM

please send me full report of polyfuse.:-/
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projects wizhard
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#3
10-07-2010, 01:41 PM

A polymeric positive temperature coefficient device also known as polyfuse is a passive electronic component used to protect against overcurrent faults in electronic circuits.They come in many different commercial names like :
-PolySwitch
-OptiReset
-Everfuse
-Polyfuse
-Multifuse
Though non-linear thermistors they but after the current is removed, they come back to the conductive state. Thus they act like circuit breakers. Their major use is in computer power supplies, and also in aerospace/nuclear applications where replacement.

A PPTC device has a current rating. the PPTC device warms up above a threshold temperature when that limit is exceeded. The the electrical resistance of the PPTC device increases by several magnitudes making it like an open circuit thereby going to a tripped" state. The rated trip current can be anywhere from 20 mA to 100 A.

Composition:
It is made up of a a non-conductive crystalline organic polymer matrix , and also carbon black particles which makes it conductive. the polymer is in a crystalline state and carbon forced into the regions between crystals and carbon forced into the regions between crystals when the device is cool. In this state it pass a given current . the device will begin to heat when too much current exceeding the trip , the polymer will expand and the conducting state is reverted and it trips.

For more info refer these links:
en.wikipediawiki/Resettable_fuse
sharingmatrixfile/12036123/10-7.zip
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#4
12-10-2010, 02:49 PM


.doc   polyfuse.doc (Size: 326 KB / Downloads: 230)
POLYFUSE



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.

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 the circuit. 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 I2R heating.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.

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#5
10-03-2011, 02:21 PM

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.doc   SEMINAR+REPORT+ON+POLY+FUSE.doc (Size: 225 KB / Downloads: 162)
POLYFUSE
ABSTRACT

A 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.
Polyfuse is a new standard for circuit protection .It is resettable. Technically, Polyfuses are not fuses but polymeric positive temperature coefficient thermistors (PPTC). Re-settable fuses provide over current protection and automatic restoration.
1. INTRODUCTION
Polyfuses is a new standard for circuit protection .It is re-settable by itself. Many manufactures also call it as Polyswitch or Multifuse. Polyfuses are not fuses but Polymeric Positive temperature Coefficient Thermistors (PPTC).
We can use several circuit protection schemes in power supplies to provide protection against fault condition and the resultant over current and over temperature damage. Current can be accomplished by using resistors, fuses, switches, circuit breakers or positive temperature coefficient devices.
Resistors are rarely an acceptable solution because the high power resistors required are expensive .One shot fuses can be used but they might fatigue and they must be replaced after a fault event. Another good solution available is the resettable Ceramic Positive Temperature Coefficient (CPTC) device. This technology is not widely used because of its high resistance and power dissipation characteristics. These devices are also relatively large and vulnerable to cracking as result of shock and vibration.
The preferred solution is the PPTC device, which has a very low resistance in normal operation and high resistance when exposed to fault. Electrical shorts and electrically overloaded circuits can cause over current and over temperature damage.
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. THE BASICS
Technically Polyfuses are not fuses but Polymeric Positive Temperature Coefficient Thermistors. For thermistors characterized as positive temperature coefficient, the device resistance increases with temperature. The PPTC circuit protection devices are formed from thin sheets of conductive semi-crystalline plastic polymers with electrodes attached to either side. The conductive plastic is basically a non-conductive crystalline polymer loaded with a highly conductive carbon to make it conductive. The electrodes ensure the distribution of power through the circuit.
Polyfuses are usually packaged in radial, axial, surface mount, chip or washer form. These are available in voltage ratings of 30 to 250 volts and current ratings of 20 mA to 100A.
3. PRINCIPLE OF OPERATION
PPTC circuit protection devices are formed from a composite of semi-crystalline polymer and conductive carbon particles. At normal temperature the carbon chains form low resistance conductive network through the polymer. In case an excessive current flows through the device, the temperature of the conductive plastic material rises. When the temperature exceeds the device’s switching temperature, the crystallides in the polymer suddenly melts and become amorphous. The increase in volume during melting of the crystalline phase cause separation of the conductive particles and results in a large non-linear increase in the resistance of the device. The resistance typically increases by 3 or orders of magnitude.
The principle of operation and increase in resistance in shown in the Fig.1. The increase in resistance protects the equipment in the circuit by reducing the amount of current that can flow under the fault condition to a low steady state level. The device will remain in its latched (high resistance state)until the fault is cleared, providing continuous protection to the circuit. At this time the conductive polymer particles cool and recrystallises restoring the PPTC to a low resistance state within few seconds. The circuit and the affected equipment return to the normal operating condition.
Thus a polyfuse acts like a self-resetting solid-state circuit breaker, which makes it suitable for providing low cost over current protection. The resistance of polyfuse at room temperature is in the order of few ohms and increases rapidly above 110 C.
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#6
19-07-2011, 04:09 PM

A polyfuse as the name suggests, is not exactly a fuse but it is a one time programmable semiconductor component which is a polymeric positive temperature coefficient (PPTC) device which is used in standard CMOS process. It being a one time programmable switch, these polyfuses are commonly used to store permanent data like chip identification number or some other data that does not need to be changed in future, it can also be used as a circuit protection devices similar to a fuse. The polyfuses consist of siliced polysilicon which is a compound of silicon with an element like tungsten that is more electro-positive than silicon. These compounds being a positive temperature coefficient substance, they can be programmed by applying a high voltage across them. When a high voltage is applied, a high current flows through the polyfuse and this high current generates heat which alters the structure of the substance and results in an exponential increase of the resistance of the material used. Normally this ploy silicon line is a low resistive path. Since the resistance of the polysilicon line is increasing with increase in temperature this can also be considered as a thermistor characterized by positive thermal coefficient. The standard resistance of a polyfuse before programming is considered to be less than 100ohm and that after programming should be greater than 10K ohm. The schematic of a polyfuse is as shown in the figure.The programming of the polyfuse is done by firing up the NMOS transistor. When the gate of the nmos transistor is given a positive potential, a high current will begin to flow through the polyfuse. As high current flows through the material, heat is generated and this heat physically alters the structure of the polyfuse material and increases the resistance of the polyfuse. The major advantage of using a polyfuse in CMOS is its low programming time and high reliability.

These polyfuses have also been used as a circuit protection devices because it is a PPTC thermistor. While it is being used as a circuit protection device, it is connected in series with the supply and circuit. As current through the polysilicon increases due to some faults in the circuit, the resistance increases and limits the current through the circuit. Such polyfuses are available in various packages and with various voltage and current ratings. Since the polyfuse require less time to vary its resistance, it very useful in the protection of sensitive and expensive circuit components.
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#7
12-12-2011, 07:38 PM

Please can you help me with a ppt on poly fuse...
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#8
13-12-2011, 09:38 AM



to get information about the topic"Poly Fuse" refer the link bellow

topicideashow-to-poly-fuse

topicideashow-to-poly-fuse?pid=59808#pid59808
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