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.
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.
Any further increase in current or ambient temperature will cause the device to generate heat at a rate greater than the rate at which heat can be dissipated, thus causing the device to heat up rapidly. At this stage, a very large increase in resistance occurs for a very small change in temperature, between points 3 and 4 of Figure-2. This is the normal operating region for a device in the tripped state. This large change in resistance causes a corresponding decrease in the current flowing in the circuit. This relation holds until the device resistance reaches the upper knee of the curve (Point 4 of Figure-2). As long as the applied voltage remains at this level, the device will remain in the tripped state (that is, the device will remain latched in its protective state). Once the voltage is decreased and the power is removed the device will reset.
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.
A current flowing through the device generates heat (I2R losses). As long as the temperature increase does not cause a phase change, nothing happens. However, if the current increases enough so that corresponding temperature rise causes a phase change, the polymer’s crystalline structure disappears, the volume expands, and the conducting carbon chains are broken. The result is a dramatic increase in resistance. Whereas before the phase change a polymer-carbon combination may have a resistance measured in milliohms or ohms, after the phase change the same structure’s resistance may be measured in megaohms. Current flow is reduced accordingly, but the small residual current and associated I2R loss is enough to latch the polymer in this state, and the fuse will stay open until power is removed.
The process is almost reversible, in that when the temperature falls, the polymer returns to its crystalline structure, the volume decreases, and the carbon particles touch and form conductive paths. However, the exact same conductive paths never form so that the resistance after reset is slightly different from before. The resistances of a PPTC fuse may triple or quadruple after the first reset, but thereafter changes are relatively unimportant.
· Initial Resistance: It is the resistance of the device as received from the factory of manufacturing.
· Operating Voltage: The maximum voltage a device can withstand without damage at the rated current.
· Holding Current: Safe current passing through the device under normal operating conditions.
· Trip Current: It is the value of current at which the device interrupts the current.
· Time to Trip: The time it takes for the device to trip at a given temperature.
· Tripped State: Transition from the low resistance state to the high resistance state due to an overload.
· Leakage Current: A small value of stray current flowing through the device after it has switched to high resistance mode.
· Trip Cycle: The number of trip cycles (at rated voltage and current) the device sustains without failure.
· Trip Endurance: The duration of time the device sustains its maximum rated voltage in the tripped state without failure.
· Power Dissipation: Power dissipated by the device in its tripped state.
· Thermal Duration: Influence of ambient temperature.
· Hysteresis: The period between the actual beginning of the signaling of the device to trip and the actual tripping of the device.
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).
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.
The vast majority PPTC fuses on the market have trip times in the range 1–10 seconds, but there are PPTC fuses with trip times of a few milliseconds. Generally speaking, however, these devices are considered slow-trip fuses. The blow time depends on the overcurrent, so that a fuse that may open within a few milliseconds with a severe overload, may take tens of seconds for a light overload. They are ideal for all low voltage DC and AC application.