By Ian Darney
Current transients in the conducting structure are a prolific source of interference in vehicles, aircraft, spacecraft and ships. A simple way of reducing the threat from these sources is to route a return conductor alongside each power supply conductor and connect it to the conducting structure at each end. Most of the steady-state current will flow along the structure, but the return conductor will carry the transient spikes.
An analysis was carried out on a representative assembly; a test rig. This simulates the response of a setup where a 10V step is applied to one end of a power line which delivers current to a 10 ohm load. The return conductor carries most of the return current during the first two micro-seconds. After about 20 micro-seconds the return current is shared equally between the return conductor and the ground. After 200 micro-seconds, the ground conductor carries most of the current.
Since the highest voltage spikes are due to the highest rate of change of current and since it is these spikes which affect the most sensitive circuitry, the use of dedicated return conductors will provide a significant reduction in the EMI suffered by the system.
Testing and Modelling
The test rig consists basically of a 15 mm water pipe fixed round one room of the house. A wooden batten assembled on the pipe provides a support for a cable to be routed along the length of the assembly. Various terminations can be connected at each end. Test equipment can then be used to monitor currents and voltages at one end of the line. It is possible to create a circuit model to simulate the response of any such assembly.
This rig is a fair representation of the wiring of a signal link or a of a power link assembled on a conducting structure. The copper pipe represents the structure, or ‘ground’. The cable represents any wiring harness assembled on the framework. The wooden batten provides spacing between wiring and structure. The terminations represent the interface circuitry of the equipment units installed at either end of the link.