Standards for representing circuits
A schematic is not a drawing of the circuit: it is a map of its connections. Symbols are standardized precisely so that the topology can be read at a glance.
01A schematic is not a drawing of the circuit
A photo of a board shows where each component is; the schematic shows how they are connected. They are two different documents and both are needed, but to understand what a circuit does the second one is what helps.
| Document | What it shows | What it is used for |
|---|---|---|
| Functional diagram | Logical connections, without real positions | Understanding, analyzing, repairing |
| Multi-line diagram | Each conductor, one by one | Detailed wiring |
| Single-line diagram | A single line represents several conductors | Electrical distribution, panels |
| Layout drawing | Physical location of the components | Assembly and maintenance |
| Wiring diagram | Which terminal goes to which terminal | Building the panel |
That is why the symbols are standardized: what you want to read at a glance is the topology, and for that it helps if a resistor always looks the same.
02Two families of symbols
Two main conventions coexist, and you have to be able to read both because datasheets come mixed:
| Family | Standard | Distinguishing feature |
|---|---|---|
| European / international | IEC 60617, IRAM | Rectangular shapes: the resistor is a rectangle |
| American | ANSI Y32.2, IEEE 315 | Distinctive shapes: the resistor is a zigzag |
Most schematic capture programs come with both libraries and many designers mix them: a zigzag resistor—out of habit—with rectangular gates. It is not ideal, but you see it every day. What you cannot do is change convention within the same drawing.
Choose a component and compare how each standard draws it. The reference designator is the letter used to name it on the drawing.
03Reference designators
Every component carries a letter and a number that identify it throughout the whole set of documents: in the schematic, in the board silkscreen and in the bill of materials. That way you know that the R7 on the drawing is the same R7 in the parts list.
| Letter | Component | Letter | Component |
|---|---|---|---|
| R | Resistor | Q | Transistor |
| C | Capacitor | U or IC | Integrated circuit |
| L | Inductor | J or P | Connector |
| D | Diode or LED | SW or S | Switch or pushbutton |
| T | Transformer | K | Relay or contactor |
| F | Fuse | M | Motor |
| X | Terminal block | Y | Valve or solenoid |
In electrical installations IEC 81346 also applies, which adds prefixes for location and function. In board-level electronics the letter and number are enough, numbering from left to right and from top to bottom.
04Single-line and multi-line diagrams
In an electrical installation drawing it makes no sense to draw the three conductors of a three-phase circuit plus the neutral and the earth: a single line is drawn with a mark that indicates how many conductors it carries and of what cross-section. That is the single-line diagram, and it is the one submitted to get an installation approved.
- Single-line: one line per circuit, with the number of conductors indicated. You see the distribution of a panel at a glance.
- Multi-line: one line per conductor. It is the one used by whoever builds the panel to do the wiring.
- Functional or control diagram: shows the control logic—contacts and coils—without the power circuit.
The documentation of an installation usually includes all three, plus the layout drawing on top of the architectural plan.
05Drawing rules
- Signal flows from left to right and power from top to bottom: input on the left, output on the right, positive at the top, ground at the bottom.
- A connection has a dot, a crossing has none. A four-conductor crossing is never made with a dot: it is drawn as two offset T junctions, so that there is no ambiguity if the dot is lost when photocopying.
- Horizontal and vertical lines, with right angles, and as few crossings as possible.
- Ground and power symbols instead of long wires that cross the whole drawing.
- Net labels to connect distant blocks without drawing the wire.
- Values next to the symbol, aligned, and in a standard format: 4k7 instead of 4.7 kΩ prevents the decimal point from being lost in a copy.
06In the lab
Take a simple board—a regulated power supply with an LM317, for example—and reverse-engineer its schematic by following the traces with the multimeter in continuity mode. Compare it with the original schematic.
Draw the same circuit with IEC and with ANSI symbols. Discuss which one reads faster and why.
Survey the workshop panel and draw its single-line diagram with the AEA symbols: service entrance, main switch, residual-current device, circuit breakers and branch circuits, with cross-sections and protective devices.
07Common mistakes
- Mixing standards within the same drawing.
- Four-conductor crossings with a dot, which become ambiguous when copied.
- Repeating a reference designator: there cannot be two R5.
- Drawing the schematic following the physical layout of the board: the result is unreadable.
- Omitting the values or placing them far from the component.
- Forgetting the power pins of the integrated circuits, or not declaring them anywhere in the drawing.
08Self-assessment
What is the difference between the IEC and the ANSI symbol for a resistor?
IEC draws it as a rectangle; ANSI, as a zigzag.
Which letter designates an integrated circuit?
U, and IC in older drawings.
When is a single-line diagram used?
In electrical distribution and panels, where the structure of the circuit matters and not each conductor.
Why is a four-conductor crossing not drawn with a dot?
Because if the dot is lost in a copy, the drawing changes meaning. Two offset T junctions are drawn instead.
What does writing 4k7 mean?
4.7 kΩ. The letter takes the place of the decimal point precisely so that it is not lost when printing.
In which direction should a schematic be laid out?
Signal from left to right, power at the top and ground at the bottom.
09Further reading
- International Electrotechnical Commission. IEC 60617 · Graphical symbols for diagrams. IEC. The official symbol database; IRAM adopts this criterion.
- Paul Horowitz and Winfield Hill. The Art of Electronics. 3rd ed., Cambridge University Press, 2015. The appendix on good schematic practice is brief and excellent.
- Asociación Electrotécnica Argentina (Argentine Electrotechnical Association). AEA 90364 regulations. AEA. Symbols and documentation required for electrical installations in Argentina.