Circuit symbols
Presented by Mick Maytum
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Summary
All protection engineers read and interpret circuit
diagrams. This operation is often a bilingual one
as U.S. symbols are often mixed with
international ones. Further, manufacturers often
use unique symbols of their own. This
discussion covers the origins of various circuit
symbols, how they are drawn (correctly) and the
major differences among them.
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IEEE Graphical Symbols
ANSI/IEEE Std 315-1975 - IEEE Graphic Symbols for Electrical and
Electronics Diagrams (ANSI Y32) is often referenced for component
symbols. ANSI/IEEE Std 315A-1986, a supplement extends the range of
symbols and is based on IEC 617 (Now the online database standard IEC
60617)
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IEC Graphical Symbols
The IEC 60617 is a subscription database of graphical symbols for use in
electrotechnical diagrams.
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Symbol Presentation
Example PNP Transistor
IEC 60617 Shareware Visio
IEEE 315 JEDEC
Visio Thick-Base
Centre join
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Symbol Drawing
IEC 60617 - Introduction
The relative proportions of the symbols should be retained when they are
reduced or enlarged.
Symbols may be turned or mirror-imaged if their meaning will not be changed
thereby.
If the symbol for a particular device or design cannot be found in the
standard it should be possible to produce it by an appropriate combination of
the published symbols (primitives).
For clarity, symbols are usually shown with connecting lines.
If the symbol for a particular device or design cannot be found in the
standard it should be possible to produce it by an appropriate combination of
the published symbols e.g. adding “hooked” line for non-linearity.
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Symbol Drawing
IEC 60617 - computer-aided draughting - 1
To facilitate the use of the IEC 60617 symbols with a computer-aided system:
The symbols have been designed so that they can be used on a grid system with a
modulus M. The modulus used in this standard, 2.5 mm, is not mandatory;
Connecting lines to a symbol coincide with the grid lines and end at grid line
intersections;
The sides of rectangles and the diameter of circles have been made multiples of 2
M.
For smaller symbols also dimensions equal to 1.5 M, 1 M and 0.5 M have been
used;
A distance of at least 2 M (5 mm) has been provided between connecting lines to
meet the requirements of ISO-Standard 3098/1-1974: Technical Drawings –
Lettering, for a minimum character height of 2.5 mm.
It is an additional requirement of computer-aided systems that each symbol
should have a reference point located at a grid intersection (shown as a star).
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Symbol Drawing
IEC 60617 - computer-aided draughting - 2
Stars are reference points
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Symbol Drawing
IEC 60617 - computer-aided draughting – 3: PNP Transistor
Drawing on a grid maintains the connectivity and relative dimensions no matter
what the final size.
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Symbol Drawing
Other stuff
ISO 3098/1 Technical drawings - Lettering
Preferred heights h of Capital letters are in root 2 ratios:
2.5 mm - 3.5 mm - 5 mm - 7 mm - 10 mm - 14 mm - 20 mm
ISO 3461-2 General principle of creating graphic symbols
Preferred module M size with rate of root 2 is:
2.5 mm - 3.5 mm - 5 mm - 7 mm - 10 mm - 14 mm - 20 mm
ISO 128 Line thickness
0.25 mm - 0.35 mm - 0.5 mm - 0.7 mm - 1 mm - 1.4 mm - 2 mm
Ratio of thick to thin shall not be less than 2:1
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Pre-drawn Symbols
The quickest and most convenience way of producing circuit diagrams is to use
software with an electronic symbol library.
General purpose drawing programs e.g. Visio
CAD programs e.g. AutoCAD
Dedicated electronic circuit drawing programs
Spice or other modelling programs
Points to check
Are the available symbols compliant to symbol drawing standards?
If needed, can you make symbols from primitive shapes?
How can the drawings be exported to other programs?
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Manufactures don’t help
Here are the symbols used for a bidirectional thyristor overvoltage protectors
from various manufactures.
4 1 3 1
MANUFACTURERS MANUFACTURER MANUFACTURERS MANUFACTURER
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IEEE SPDC
has started making pre-drawn symbol sets
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IEEE SPDC
pre-drawn symbol set example
Varistor Avalanche Breakdown Inductor
Diode
Switch SPST Capacitor Resistor
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