Harnessing technology - Automotive electronics

Although the use of polymeric positive temperature coefficient (PPTC) devices in a decentralised approach to automotive harness protection has been a design option since the 1990s, its adoption has been slow among OEMs. In addition to the resistance to changing traditional design methods, the benefits of PPTCs may have been hampered by the thicker wires historically used in vehicles. In the past, mechanical strength dictated that the smallest wire used in the vehicle was 0.35mm^² (22AWG), which could carry currents from 8 to 10A. This limitation cancelled some of the benefits of using PPTCs for low current signal circuits – those carrying less than 8A. However, emerging technologies are enabling much smaller diameter wires capable of carrying more current, including wires as small as 0.13mm^² (26AWG) with a maximum 5A capability. Automotive engineers are now revisiting the technique of using PPTC overcurrent protection devices in a distributed architecture to improve weight savings, while helping to improve system reliability. An automotive industry study has demonstrated the significance of employing a decentralised architecture and Tyco Electronics' PolySwitch devices in mid to high range passenger vehicle. In addition to an estimated 50% savings in the weight of copper wires, design flexibility was improved significantly. Trailer tow light circuits To protect against possible short circuits and overloads, trailer tow circuits typically duplicate vehicle wiring with a separate fuse and power feed circuit. In this design, all lights are protected by a single fuse placed within a centrally located fuse block. In a decentralised architecture, PPTCs can be located in a lamp assembly, connector or splice block, effectively eliminating three fuses, a relay, three lengths of wire and the associated connectors. In a decentralised approach, individual PPTCs can be used at each corresponding junction node to help protect each light circuit, simplifying the design of brake, turn and hazard module and switches. LED CHML circuits The lower power consumption and design flexibility provided by leds make them increasingly popular in any lighting circuit, including centre high mount stop lamps (CHMSL). Using PPTCs and a decentralised architecture to protect LED CHMSLs offers increased design flexibility, reduces the number and weight of wires and enhances reliability. DC motors and actuators Motor circuits in a typical centralised configuration are protected by a large circuit breaker or fuse. In this design, heavy gauge wire and, therefore, larger interface pins and connectors are required. Since motors for rear door windows and locks and rear deck power antennas are not located near their control switches, their power feeds can be long and heavy. A decentralised architecture allows the designer to locate PPTCs strategically by mounting them on the switches, relays or electronic drive circuits that control the motors. PPTCs also limit the flow through the power feed circuit to the protected motor, allowing the power feed wire to be reduced in size significantly. A power window circuit, for instance, is typically fed power by a 3mm² wire protected by an upstream circuit breaker. By incorporating PPTCs in the motor control switches, the power feed wire can be reduced to 0.8mm² as voltage drop dictates. Reducing the size of the wire, in turn, permits the use of smaller terminals, interface connectors and switches. Additionally, micro controlled circuits can use less costly, lower power, non protected transistors in the drive circuits. Using small gauge wire decreases the size of the wiring assembly bundle and increases the wire's flexibility, improving the wiring assembly's dress and reducing the force needed to install the wire in the vehicle. Author profile: Guillemette Paour is with Tyco Electronics' circuit protection business unit.