Digital Circuit Boards: Mach 1 GHz

4.1 Introduction

It is interesting to consider the way energy is supplied to a switched electrical load in a building. For our discussion, pick a moment in time when the voltage between the “hot” lead and the grounded power conductor is at its peak of 170 V. Note that this peak of voltage appears at all points in the building at the same time.

The events we will consider will last no longer than 1 ms. At t = 0, a switch closes placing a 17-ohm resistive load across the line. Assume that the power wiring looks like 100-ohm transmission lines. At the moment of switch closure, the voltage drops to 24.7 V. This means that a step wave of −145 V starts to travel down the power wiring. When this wave reaches a branch point in the wiring, the wave divides. A part of the wave reflects and travels back to the load. At the load this wave divides again and a fraction of the wave adds voltage to the load. The reflected part of this wave then travels back to the first branch point. A part of the first wave that reached the first branch point continued outward toward other branch points. In a short period of time, waves will have traveled and reflected all over the facility. Each time one of these reflected waves returns to the load the voltage is increased.

As the waves travel and reflect, the voltage at each branch point first drops and then begins to recover. What is happening is the energy stored in the electric field in the building wiring is being moved around to supply energy to the load ...