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Voltage labels

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Guy Fraser edited this page Jul 29, 2017 · 2 revisions

What do the various voltage labels mean?

VCC vs. Vcc vs. Vcc?

source

The de facto convention used to be:

  • Upper case subscript suffix (eg. VCC) = DC (direct current)
  • Lower case subscript suffix (eg. Vcc) = AC (alternating current)

With circuits increasingly being defined/tested/documented in software or source code, subscript characters have become increasingly cumbersome to work with. At the same time, circuits have become increasingly digital in nature and thus DC voltages are increasingly prevalent.

As such, a new convention has emerged:

  • Lower case suffix (eg. Vcc) = DC

This is the convention we use.

What do the letters mean?

source:

They are derived from a more general naming convention.

If you have Va, Vb, or other voltages with a single subscript, then what is meant is the voltage of that node with respect to some reference node. That reference node is "ground" or "common" and is usually just indicated as GND or something similar. You are free to pick any node you want as your reference node, but usually one or two nodes are the most reasonable candidates.

If you have Vab or other voltages with two subscripts, then what is meant is the voltage of the first node with respect to the second node or:

Vab = Va - Vb

This notation is actually quite useful because it allows you to figure out an unknown voltage step by step.

For instance, let's say that I want to know Vac but I only know Vab and Vbc. I can use the subscripts to tell me that:

Vac = Vab + Vbc

This is true because:

Vac = Vab + Vbc

Vac = (Va - Vb) + (Vb - Vc) = Va + (-Vb + Vb) - Vc = Va - Vc

In an expression, I can combine to voltages provided the first subscript on one of them is the same as the second subscript on the other by dropping the common subscript and keeping the other two in their same position.

Vaf = Vab + Vbc + Vcd + Vdf

At this point, what would Vcc or Vee (or any other voltage with a repeated double subscript) be?

Vxx = Vx - Vx = 0V

Since any voltage with a repeated subscript will be 0V, the convention was adopted that a repeated subscript would indicate that the voltage was referring to a supply voltage, usually a DC supply. The letter would generally have some relevance to what the purpose of the supply was.

For BJT circuits, the three common supplies are:

  • Vcc = Collector voltage
  • Vee = Emitter voltage
  • Vbb = Base voltage

The Vbb is sometimes an actual supply, but more often it is the Thévenin equivalent voltage associated with the base bias network.

One fine point is that, by general convention, Vcc and Vee are associated with the collector and emitter sides of an NPN transistor. This is commonly, but not universally, the case even if the only transistors in the circuit are PNP transistors. A quick glance at the schematic will tell you if this is the case or not.

For FET and MOSFET circuits, the three common supplies are:

  • Vdd = Drain
  • Vss = Source
  • Vgg = Gate

The same remarks regarding polarity apply here.

The strict meaning of these has been lost among a growing number of people, and hence you will often see Vcc used as the positive supply in CMOS circuits and see Vdd in BJT circuits.

In many circuits, Vss and Vee are the same as the ground node, in which case it may or may not ever be noted as being Vss or Vee. In other circuits, there may be a more reasonable choice for the ground reference that is, generally, somewhere between the two values making Vss and Vee negative.

For more detailed info, see wikipedia.

3V3, VBATT, VUSB, etc?

  • 3V3 = 3.3V supply
  • GND = common, ground
  • VBATT = supply from a battery
  • VUSB = supply to/from USB; in our case it's always from USB
  • VREF = reference voltage

Energy harvesting

TBC

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