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Updating Circuit Theory: Power Line Transients


By Ian Darney

Transient spikes due to load switching are a prolific source of interference in any electronic system. Circuit models can be created to simulate the interaction between equipment and cable. 

Power Line filters are an essential part of the design of any unit of electronic equipment because they form a barrier between the electromagnetic interference (EMI) which has been picked up by the supply line and the equipment to which the power is being delivered. But they do this by reflecting that EMI back down the line. This doubles the level of unwanted power carried by the cable and doubles the EMI created by that line. Moreover, when the equipment unit is switched off, the transient power stored in the filter has only one place to go; out into the environment.

This article analyses the effect of an inductive load and of a capacitive load on the cable. It concludes that a filter with reactive components will interact with the supply cable to create high frequency resonance, and this resonance will be a rich source of EMI.

Capacitive Load

If a switch at the input end of a power line is closed, as illustrated by Figure 1, then a step current will start to flow along a path defined by the routing of the cable. This step will propagate at a velocity similar to that of the speed of light in a vacuum. If it is assumed that the line is lossless, then the amplitude of the step arriving at the far end of the line will be the same as that delivered to the near end.

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