This relatively high-impedance path caused the chassis of the two boxes to be at different potentials.
What the product engineer didn't realize was that the signal return currents had to flow through the chassis, then the power cable, then through the building wiring, then through the power cable and chassis of the source box. The product designer explained that the signals were voltages referenced to the chassis ground of each box. When the EMC engineers examined the cable, they found that it had exactly 8 wires (one for each signal, but none for returning the signal current). The system employed an 8-bit communication bus that was routed on a cable between two boxes. Many years ago, EMC engineers at IBM were evaluating a product that had severe electromagnetic susceptibility problems. It is not uncommon to see the current path from the signal source to the load very carefully laid out while the path from the load back to the source is left to chance. sum of conduction and displacement current) flowing out one side of a device must equal the current flowing in the other.ĭigital circuit designers often neglect to consider where the currents in their designs will flow. However, net charge cannot be created or destroyed and the current (i.e. time-varying fields that result when the net charge on a conductor changes). Yes, there are displacement currents (i.e. The first rule to remember when identifying the path of a current is that all currents return to their source. Without understanding how and where the currents in each circuit flow, it can be difficult to anticipate problems in new designs or fix problems in existing designs. Current is primarily responsible for 3 of the 4 possible EMC coupling mechanisms described in the first chapter. One of the most important skills that an EMC engineer must develop is the ability to identify both the intentional and unintentional currents in an electronic system. one or more of the other pins on the integrated circuit.the ground plane of the printed circuit board.The ultimate destination of the signal current that flows out of a pin in an integrated circuit is,
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