Overview

Co-tech. Ele. Circuit Protection’s PTC(  Polymeric Positive. Temperature Coefficient) resettable fuse are used to help protect against harmful overcurrent surges and overtemperature faults.Like traditional fuses, these devices limit the flow of dangerously high current during fault conditions. The PPTC Resettable fuses, however, resets after the fault is cleared and power to the circuit is removed, thereby reducing warranty, service and repair costs. This is achieved by using a polymeric PTC material, which is a matrix of a crystalline organic polymer containing dispersed conductive particles, usually carbon black. The sharp increase in resistance, as shown in Figure 1, is due to a phase change in the material. In its cool state the material is mostly crystalline, with the conductive particles being forced into the amorphous regions between the crystallites.

Overcurrent Protection using a PPTC Resettable fuses

A PPTC( PolySwitch Polymeric Positive. Temperature Coefficient) resettable fuse is a series element in a circuit. The PPTC resettable fuse protects the circuit by going from a low-resistance to a highresistance state in response to an overcurrent. This is called “tripping” the device. Figure 2 shows a typical application. Generally the device has a resistance that is much less than the remainder of the circuit and has little or no influence on the normal performance of the circuit. But 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 the generation of heat within the device by I2R heating.

Principles of operation

The operation of polymeric PTC resettable fuse 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, for example, Point 1 in Figure 3. 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, such as Point 3 in Figure 3.

Principles of operation

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 in Figure 3. 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 in Figure 3). For a device that has tripped, as long as the applied voltage is high enough 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.

Example of Hold and Trip Current as a Function of Temperature

Figure 4 illustrates the hold- and trip-current behavior of PTC resettable fuse 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 PTC resettable fuse will trip (go into the high-resistance state) and protect the circuit. Region B describes the combinations of current and temperature at which the PTC resettable fuse will allow for normal operation of the circuit. In Region C, it is possible for the device to either trip or, remain in the lowresistance state (this will depend on the individual device resistance).

Operating Characteristics of Polymeric PTC

Figure 5 shows a typical pair of operating curves for a polymeric PTC resettable fuse in still air at 0°C, 20°C and 60°C. The curves are different because the heat required to trip the device comes both from electrical I2R heating and from the device environment. At 60°C the heat input from the environment is substantially greater than it is at 0°C, so the additional I2R needed to trip the fuse is correspondingly less, resulting in a lower trip current at a given trip time (or a faster trip at given trip current).

Typical Resistance Recovery after a Trip Event

Figure 6 shows typical behavior for a PTC resettable fuse that is tripped and then allowed to cool. In this figure, we can clearly see that even after a number of hours the device resistance is still greater than the initial resistance. Over an extended period of time, the resistance will continue to fall and will eventually approach the initial resistance.