# Is there a standard for schematic power symbols?

I notice that some schematics use a single, labelled, symbol for power that looks like an uppercase letter "T", while others use varying symbols (mostly types of arrow) to indicate various voltages; I consider the latter to be more readable and useful and I'd like to adopt it with my own schematics.

My question is, is there any standard or de-facto set of symbols for the various voltages? Is it reasonable to create my own scheme if I'm consistent within my own use?

My primary goal is to clearly indicate different voltage levels on the one schematic; I often work with a largely 5 V DC circuit that has one or more pieces using 3.3 V. Many of the voltage-specific schemes don't seem to differentiate between these voltages.

In any case, I will ensure that every power symbol on every schematic sheet is clearly labelled with the voltage; I completely understand that a symbol is not sufficient for describing power rails.

• "T" with the rail level (+3.3V, -12V, etc) written on top of it would probably be de-facto. If it's not a "T" but an arrow or even a circle with a plus/minus its still understandable. – Wesley Lee Dec 27 '17 at 1:04
• The horizontal of the "T" represents a portion of the power "rail". The markings above it indicate which rail. Usually a voltage is sufficient (+3.3 V, -12 V, etc.) but sometimes there may be independent similar voltages (on different fuses or, perhaps, an analog and separate digital 5 V supplies) which will be differentiated in some way. I like these because they are symbolic. – Transistor Dec 27 '17 at 1:20
• As other have stated the rail type drawing is pretty much the defacto-standard. You have to remember it's the name that counts. In all schematic packages every connection with the same name is the same net. As such the symbol itself is of secondary importance. In some packages there are actual part symbols for the rails so the software can treat the power nets differently, esp in the PCB design part. – Trevor_G Dec 27 '17 at 1:51

After a quick check of IEEE Std 315-1975 Graphic Symbols for Electrical and Electronics Diagrams (Including Reference Designation Letters), the following seems applicable:

3.9.3 Common connections
Conducting connections made to one another.
All like-designated points are connected.

NOTE — 3.9.3A: The asterisk is not part of the symbol.
Identifying values, letters, numbers, or marks shall
replace the asterisk. For the triangular symbol, this
identification shall be placed within the triangle
or, if essential for legibility, adjacent to the
triangle.


3.9.3.1 "Specific potential difference" seems directly applicable. Basically, you place a descriptive "designation" label at the end of the line segment that has a "specific potential difference with respect to a potential reference level."

---- *  (See note 3.9.3A)


For example,

[+5V_power_supply_circuit]---- +5V

+5V
|
[CIRCUIT_ELEMENT]


That's it. No "T", no "bubble", etc. And for what it's worth, I often see this method used by companies like Keysight Technologies, Tektronix, Fluke, etc. in the service manuals they publish for their test equipment products. For example, look up the 34401A digital multimeter on Keysight's website, and under the "Document Library" tab you'll find a link for the "Service Guide" manual for that DMM. The schematics are at the very end of that manual. (You might need to create a user account on Keysight's website to access their manuals. That's so annoying...) (n.b. Hewlett-Packard == Agilent == Keysight)

(n.b. Note 3.9.3A mentions "marks". That would permit the use of "T" symbols, bubbles, etc. at the end of the line segment.)

One final comment. The main thing here is clarity and consistency. Some EDA programs define a "T" symbol, or a bubble symbol, or some other symbol for use with power connections. Most electronics techs and electrical engineers clearly understand what these symbols mean if the symbol is accompanied with a meaningful label (e.g., +5V); so don't shy away from using these symbols if that's the symbology that's available to you or that you prefer. Just be consistent: prefer to use only the IEEE's style, or only "T" symbols, or only bubbles, etc. throughout the entire schematic; avoid using a confusing mix of different types of power distribution symbols on the schematic.

• +1 for research.. re "Some EDA programs define a "T" symbol" for those you really want to use those symbols. They are normally part of the way the software separates the power rails from other nets and is often integral to their PCB design suite where power nets are handled specially, eg planes etc – Trevor_G Dec 27 '17 at 2:23
• +1 So basically the standard is "nothing, or whatever you want"? That certainly explains the variety I see. If you use a basic "T" symbol for a supply rail, do you use an inverted-"T" for ground? I'd intended using arrows for supply rails because of the inverted-"T" symbol I'm used to for ground; if a downward-pointing triangle is more common as ground, I can see using a "T" for supply. – Calrion Dec 28 '17 at 1:44