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Updating Circuit Theory: Power Line Filter


By Ian Darney

A filter is described which absorbs transient energy in the supply line as well as minimising the effect of those transients on the load. Traditional filters simply reflect that energy back into the line where the only place for it to go is into the environment. This manifests itself as cross-coupled interference as well as radiated interference.

A model is created which simulates the interaction between the filter and the supply line. It is shown that the device performs its intended function in protecting the load and in preventing load transients from reaching the line. It also absorbs any transient energy, whatever the source. Any system which includes such filters will experience much reduced levels of EMI.

Basic Power Line

At least two conductors are involved in routing power from one location in a system to another; the send conductor and the return conductor. Figure 1 provides a schematic diagram of such a link. At the near end, the battery simulates a DC source of power, the 0.1 ohm resistor simulates the source impedance, and SW1 is initially open. The line itself is
15 m long. The load at the far end is a 10 ohm resistor in series with an isolating switch, SW2, which is initially closed.

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