If you're pumping multiple amperes, the current distribution pattern through a circuit board's artwork pads should not be taken for granted.
The discussion came up one day during a power supply project as to whether circuit board artwork pads would carry any current on the far side of a pad from which current was arriving via a circuit board trace. If not, maybe some copper could be saved and maybe a little bit of circuit board space might become available.
Since a lot of amperage was involved, this was not a trivial issue. In short, would there be any current flow through the copper that was directly to the right of the center line in Figure 1 below?
Figure 1 The artwork pad In question.
It would seem that some of the current coming in from the left would have to have reversed direction and that seemed a little counter intuitive.
I made a very simplistic circuit model of the pad as follows (Figure 2):
Figure 2 Artwork pad model.
Let’s let each resistance in this model be some element called ‘R’ and let’s make the assumption that somehow we caused one volt to appear at the node circled in red. Then, with a series of algebraic manipulations, we can find the various currents and voltages in this network that would be needed for the sake of that one volt being what we’d assumed it would be.
It looks like a slew of results, but it all pans out. If some current entering the 9.3333..volt node is flowing from left to right, we do indeed get the 1/R current flowing from right to left. We do indeed get a reversal of current direction.
In the copper in itself, things might look a little more like the following (Figure 3).
Figure 3 Artwork pad – a closer look.
Whatever current is flowing in the copper from the left will do an end run around the thru hole and will indeed loop back around. Therefore, copper on the right hand side of the center line in Figure 1 does not go to waste; it does carry current and it should not be omitted.
This article was originally published on EDN.
John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).