TE Circuit Protection, a pioneer of Polymeric Positive Temperature Coefficient (PTC) Resettable devices, has developed Overcurrent and Overtemperature Protection Devices to help protect power electronics circuits in automotive, battery power supply circuits, telecommunications and networking systems. PolySwitch PPTC (Polymeric Positive Temperature Coefficient) devices 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 PolySwitch device, however, resets after the fault is cleared and power to the circuit is removed, thereby helping to reduce warranty, service and repair costs.

Reasons for Overcurrent (also Overtemperature) in Electronic Circuits

An overcurrent is an abnormally high current that has the potential to cause failure in an electrical circuit. An out-of-range condition in the power source or a decrease in load impedance can cause an overcurrent. Source-generated overcurrents usually arise from overvoltages caused by the abnormal operation of a powersupply,or as a consequence of overvoltages on a powerline. Source-generated overcurrents may also arise from voltage sags. Powerline overvoltages may arise from power crosses, surges, transients, or swells.

  • A power cross occurs when a high-voltage circuit is inadvertently connected to a low-voltage circuit, for example, when a power line falls onto a telephone line during a storm. Surges are short-duration increases in system voltage due to external events, such as lightning.
  • Transients are short-duration increases in system voltage due to the emptying of a circuit energy storage element, such as an inductor or capacitor. Swells are relatively long-duration increases in system voltage, generally caused by a failure in the system, for example, loss of the neutral connection at the transformer supplying a house.
  • Higher than normal voltages result in higher than normal currents in linear circuits. In nonlinear circuits, lower than normal voltages may lead to higher than normal currents, which is why voltage sags can cause an overcurrent problem. A common light bulb is an example of a nonlinear device that draws more current as the voltage is lowered.
  • A partial or total failure of a circuit load can cause load- generated overcurrents. The failure lowers the total resistance in the circuit, allowing more current to flow. An example is a stalled motor, which gets hot because of excessive power draw, resulting in the insulation on the motor windings being destroyed, thus allowing adjacent windings to touch (short-circuit).

Overcurrent and Overtemperature Protection using a TE Polymeric PTC Device

A polymeric positive temperature coefficient (PPTC) overcurrent protection device 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. This is called “tripping” the device. The nonlinear PTC effect is when materials undergoing a phase change may exhibit a resistance that increases very sharply over a narrow temperature range. Certain types of conductive polymers exhibit this effect. These conductive polymers are useful for making overcurrent protection devices, generally called polymeric PTC overcurrent limiters, circuit protection devices, or resettable thermistor type devices.

PolySwitch circuit protection devices are made from a composite of semi-crystalline polymer and conductive particles. At normal temperature, the conductive particles form low-resistance networks in the polymer. However, if the temperature rises above the device’s switching temperature (TSw) either from high current through the part or from an increase in the ambient temperature, the crystallites in the polymer melt and become amorphous. The increase in volume during melting of the crystalline phase separates the conductive particles resulting in a large non-linear increase in the resistance of the device.

Reference: TE Circuit Protection.