Some other good answers, but here is some history:
In the old days of valve ( tube ) electronics, there were two main supplies.
The heater supply for the cathodes of the tubes came from the first
battery - the A battery. So it got called Va.
And the plate supply - the accelerating voltage - was supplied by the second
battery, naturally the B battery, so Vb.
Sometimes to generate other low voltages in the circuit, eg for grid bias
supplies, there were other batteries, so C, D, E batteries. ( this was way
before the days of self-bias circuits, and it was just as easy to stick in
another battery than add expensive complicated things like resistors )
Then transistors came along, and because the voltages were all low, but not
heater supplies, Vc came to be used for the supply voltage. Of course the c
came to be understood to be the Collector as well, which was easy.
Then they realised it was necessary to differentiate between the supply and
the voltage actually at the node, so the doubled nomenclature was invented,
so Vcc for the supply, Vc at the terminal.
The extension to FETS with their drain and source is obvious.
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Regards,
Adrian Jansen
J & K MicroSystems
Microcomputer solutions for industrial control
