Electronic Proximity Examples (NPN - PNP)

For example, here are two schematic diagrams showing two different kinds of electronic proximity switch. The first switch sinks current in from the LED through its output terminal, through its transistor, and down to ground. The second switch sources current from the positive supply terminal through its transistor and out to the LED through its output terminal (note the direction of the thick arrow near the output screw terminal in each circuit :

These terms simply make no sense when viewed from the perspective of electron flow notation. If you were to actually trace the directions of the electrons, you would find that a device “sourcing” current has electrons flowing into its connection terminal, while a device “sinking” current sends electrons out to another device where they travel (up) to a point of more positive potential. In fact, the association between conventional flow notation and sourcing/sinking descriptions is so firm that I have yet to see a professionally published textbook on digital circuits that uses electron flow. This is true even for textbooks written for technicians and not engineers.

Once again, though, it should be understood that either convention of current notation is adequate for circuit analysis. I dearly wish this horrible state of affairs would come to an end, but the plain fact is it will not. Electron flow notation may have the advantage of greater correspondence to the actual state of affairs (in the vast majority of circuits), but conventional flow has the weight of over a hundred years of precedent, cultural inertia, and convenience. No matter which way you choose to think, at some point you will be faced with the opposing view. Pick the notation you like best, and may you live long and prosper.