# V/I uppercase/lowercase convention?

Is there any standard in industry for using uppercase or lowercase for V and I in circuit diagrams? I ask this because my book seems to switch back and forth between the two without any rhyme or reason and I can't figure out any pattern for why it will choose one way over the other. It also switches back and forth for subscripts.

• Can you show an example? Jun 29 '15 at 5:49
• I was just wondering if there's a standard to go by, I don't have good lighting at the moment to take pictures to compare. If no one answers by morning I will though. Jun 29 '15 at 6:06

I agree with you that it is important to know the meaning of the different ways for using such symbols. And the same applies also to the voltage-to-current ratios (resitances, impedances). For my opinion the standard is (or should be) as follows:

• Uppercase (V,I) for DC and rms values
• Uppercase for ohmic resistors R=V/I
• Lowercase (v,i) for signals as a function of time: v(t), i(t)
• Lowercase (v,i) for (small) differential signals available for a certain DC bias point only.
• Lowercase (r) for differential (dynamic) small-signal resistances r=v/i.

As a negative example, in small-signal equivalent circuits, sometimes the inverse transconductance gm of a BJT is used as Re=1/gm. This is very confusing because this does not represent any (ohmic) resistor and, more than that, can be mixed-up with an external emitter resistor RE.

EDIT/UPDATE: Regarding impedances:

For reactive elements (L, C) the voltage-to-current ratios are called "impedance". Because this applies to rms values of sinusoidal signals only the symbol for impedances also is written in uppercase letters Z=V/I.

• +1 @LvW, I'd like to request OP to take this up as an answer. The answer provided by me is not so specific and rather a link-only kind, while this one is right to the point. Jun 29 '15 at 6:34
• Didn't see it at first, but this is exactly the info I needed. Thanks! Jun 29 '15 at 6:35

quantities obtained applying various kind of transforms to the time domain signal should be uppercase, specifically:

• Phasors, i.e. complex representation of sinusoidal signals.

• Frequency-domain signals, i.e. Fourier transforms of time-domain signals. For example the frequency response of a system and related input and output signals or the like:

• $Y(f) = H(f) \cdot X(f)$
• $V_o(\omega) = G(\omega) \cdot V_i(\omega)$
• s-domain signals, i.e. Laplace transforms of time-domain signals. For example the transfer function of a system and related input and output signals or the like:

• $Y(s) = H(s) \cdot X(s)$
• $V_o(s) = G(s) \cdot V_i(s)$
• z-domain signals, i.e. Z-transformed signals corresponding to discrete-time signals (e.g. as used in digital signal processing). For example the transfer function of a digital system and related input and output signals or the like:

• $Y(z) = H(z) \cdot X(z)$