Moreover, by placing protection devices closer to the connectors, the trace length can be reduced and the overall junction box can be downsized. Alternatively, the junction boxes can be divided into smaller units and relocated around the vehicle without considering user accessibility. In these cases, the PPTC devices help designers achieve an electrical architecture that more closely reflects the optimized tree structure and its attendant benefits.

PPTC devices are available in a wide array of form factors, facilitating a variety of interface options with the junction box or electronic module. Through-hole and surface-mount devices lend themselves to installation in fuse boxes or modules using printed circuit boards. Strap devices can also be used in metal fret boxes. A new generation of bladed devices can also be inserted like a bladed fuse or bi-metal breaker in the junction box. Even though these devices are resettable and do not need to be user accessible, the bladed form factor allows designers to replace a fuse or a bi-metal device without waiting for the next redesign of the junction box.

Conclusion

Employing a decentralized architecture combined with PPTC overcurrent protection devices can significantly reduce weight in automotive designs. Although a decentralized approach has been understood for many years, the recent availability of thinner wires that can carry higher current, as well as new industry incentives, makes this approach clearly superior to conventional fusing techniques. Using PPTC devices in a decentralized harness protection scheme offers many important design benefits. The resettable functionality, low-resistance characteristics, and a wide array of current ratings provided by these devices can help automotive designers reduce wire length and weight while facilitating design flexibility and system reliability.

PPTC Device Principle of Operation

The PolySwitch PPTC device is made of a composite containing a conductive filler, such as carbon black, that provides conductive chains throughout the device. This device exhibits low-resistance characteristics under normal operating conditions, but when excessive current flows through it, its temperature increases and the crystalline polymer changes to an amorphous state.

As illustrated below, this transition causes the polymer to expand, breaking the conductive paths inside. During a fault event, the device resistance typically increases by three or more orders of magnitude. This increased resistance helps protect 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 remains in its latched (high resistance) position until the fault is cleared and power to the circuit is cycled; at which time the conductive composite cools and re-crystallizes, restoring the PPTC device to a low-resistance state in the circuit and the affected equipment to normal operating conditions.

PolySwitch PPTC devices help protect the circuit by going from a low-resistance state to a high-resistance state in response to an overcurrent or overtemperature condition Select figure to enlarge.