Technical Article

# Understanding Schematics

July 22, 2019 by Matthew Cook

## If you're looking to get a better handle on how to read schematics, this helpful guide will give you a head start.

If you're looking to get a better handle on how to read schematics, this helpful guide will give you a head start.

Every new electrical circuit board design starts as an idea. That idea is then defined, with words and diagrams, in a specification. Anyone can take an idea this far, but the next step requires a fundamental understanding of circuit schematics.

Circuit schematics are the bridge between conceptual electrical design and physical realization of a printed circuit board assembly, or PCBA.

##### Crowbar circuit

Schematics have two fundamental purposes. First, they communicate design intent. To someone skilled in the art of electrical design, schematics should clearly convey the intent of the design. And second, they exist to direct and drive the PCB layout.

To get a good start on understanding schematics you should understand some basic things: component symbols, reference designators (REFDES), nets, and outputs.

### Reference Designators (REFDES)

Reference designators are unique identifying labels for each physical component and they communicate a lot about the components to which they refer.

Proper REFDES use tells the schematic reader the type of component and the number of symbols per component. While there are standard symbols that represent various types of electrical components, which we will discuss next, not all schematics follow all of those standards.

In a case where every passive component is shown as a generic box with pins, reference designator prefixes can tell you a lot about the type of component that symbol represents. Reference designators also serve as the link to the bill of materials (BOM). The BOM has the part number of every component in your PCBA design, and it indicates which locations should have that part installed, by REFDES.

The industry-standard format for reference designators includes a letter code, indicating the type of component, followed by a unique number.

 BT = Battery J = Connector R = Resistor C = Capacitor K = Relay S or SW = Switch D = Diode L = Inductor T = Transformer F = Fuse P = Connector U = Integrated Circuit H = Hardware Q = Transistor Y = Crystal

We will indicate the REFDES for each component as we identify their symbols below.

### Component Symbols

Component symbols in a schematic represent physical components that will be soldered to the printed circuit board (PCB) during the assembly process. Sometimes they also might represent PCB structures, like vias or test points.

Component symbols are often an industry-standard shape or drawing that indicates what type of electrical components they are, though sometimes they are nothing but a rectangle with pins. Resistors, capacitors, inductors, diodes, and transistors all have standard symbols that we will cover briefly below.

Component symbols always have one or more pins to which electrical connections can be made. Each schematic symbol pin has a number that corresponds with the physical component drawing. One or more symbols may be used to define a single electrical component. Components with many pins are often represented by multiple schematic symbols simply to allow for readable schematics.

In the case of a part defined by multiple symbols, each partitioned symbol that refers to the same physical component shares the same reference designator.

#### Resistor

Resistors are extremely common electrical components. They are usually shown as a zigzag line in the US, though the international standard shows them as just a rectangle.

##### The US (top) and international (bottom) symbols for resistors

Resistors are identified in schematics with a reference designator (REFDES) starting with the letter “R”.

#### Capacitor

Capacitors are also very common. They are shown as two lines separated by a gap, conveying their fundamental construction of two charged plates separated by a dielectric. The two primary capacitor symbols are non-polarized and polarized.

Polarized capacitors are denoted by a curved line (to indicate a negative terminal) and/or a plus sign (to indicate a positive terminal).

##### Capacitor symbols. Shown are a non-polarized capacitor on the far left and three versions of a polarized capacitor.

Capacitors are identified in schematics with a reference designator (REFDES) starting with the letter “C”.

#### Inductor

Inductors, like resistors and capacitors, are fundamental passive components used in electric circuits. Inductors are shown as a series of curves representing their basic construction. Inductors are most simply constructed with a coil of wire around some core material.

##### Inductor symbol

Inductors are identified in schematics with a reference designator (REFDES) starting with the letter “L”.

#### Diode

Diodes are electrical components that only permit current to flow in one direction. There are a variety of diode types. Zener diodes, for example, don’t allow reverse current until the diode’s reverse voltage reaches a certain, defined level.

##### Diode symbol

A light-emitting diode (LED) produces light when current flows through it in the forward direction.  A Schottky diode is constructed so it operates similar to a simple diode, but switches faster and has a lower forward voltage drop.

##### Schottky diode symbol

Diodes are identified in schematics with a reference designator (REFDES) starting with the letter “D” or “Z” (for Zeners). “LED” is sometimes used for light-emitting diodes.

#### Transistor

Transistors are like electrical switches where a bias voltage or current in one area turns on the current flow across the main terminals.

There are two fundamental types of transistors: bipolar junction transistors (BJTs) and field-effect transistors (FETs).

Simplistically, BJTs are current-controlled devices where a current flowing into or out of the base pin turns on a larger current through the collector and emitter pins.

##### BJT symbols

Also simplistically, FETs are voltage-controlled devices, where a voltage on the gate pin turns on a current through the drain and source pins. There are a wide variety of drawings used for transistors, indicating different amounts of internal component details.

##### FET symbols

Transistors are identified in schematics with a reference designator (REFDES) starting with the letter “Q”. “M” is sometimes used for MOSFET devices. “T” is sometimes incorrectly used, and should be avoided.

For more detailed information on BJTs, FETs, IGBTs, and more, check out our article specifically on the schematic symbols for transistors.

#### Variable Resistors

Variable resistors, such as potentiometers and rheostats, are resistors that change resistance in accordance with a user’s adjustments. Two-terminal variable resistors are shown as a resistor with an arrow across it, while potentiometers (which have three terminals) add an arrow pointing at the side of the resistor symbol.

##### Potentiometer symbol

Voltage-dependent resistors, or varistors, look similar to a variable resistor, but with a line across it instead of an arrow.

##### Varistor symbol

Special resistors are most often identified in schematics with a reference designator (REFDES) starting with the letter “R”, though “VR” (for variable resistors or potentiometers) or “RV” (for varistors) are sometimes used.

#### Integrated Circuit

Integrated circuits are entire electrical circuits created in semiconductor material in a single package. Integrated circuits are the processors, memory, operational amplifiers, and voltage regulators that look like squares or rectangles mounted on a printed circuit board.

Integrated circuits are shown as a box, or collection of boxes, with labeled pins for power, inputs, and outputs.

Integrated circuits are identified in schematics with a reference designator (REFDES) starting with the letter “U”, or sometimes the letters “IC”.

#### Crystal/Oscillator/Resonator

All three of these provide a known, steady frequency output when energized in a circuit. Crystals, oscillators, and resonators are not the same thing, having different characteristics and requiring different supporting circuitry, but their fundamental purposes are similar.

##### Crystal symbol

Crystals and oscillators are identified in schematics with a reference designator (REFDES) starting with the letter “Y”. “X” is occasionally used; this letter is also a catch-all for components not fitting another category.

#### Digital Logic Gates

There are many digital logic gates - more than can be described in detail in this overview. For a full explanation of digital logic and the many different types of logical gates, see the AAC textbook page on Digital Signals and Gates.

Logic gates are sold as integrated circuits, and therefore they are identified in schematics with a reference designator (REFDES) starting with the letter “U”, or sometimes “IC”, just like other integrated circuits.

#### Operational Amplifier

Operational amplifiers and comparators have a variety of useful functions in circuits, and they are shown as sideways triangles in schematics, with a (+) and a (-) input, and sometimes power and ground pins.

##### A dual-supply op-amp circuit (left) and a single-supply configuration (right) with power and ground pins indicated

Operational amplifiers and comparators are identified in schematics with reference designators (REFDES) starting with the letter “U”, or sometimes “IC”, just like other integrated circuits. Additionally, op-amps sometimes use a REFDES starting with “OP”.

Connectors and headers are where other circuits or cables are connected to the circuit described by the schematic. There are a wide variety of types and orientations of connectors, and they are likewise represented in schematics by a wide variety of symbols.

Sometimes the schematic symbols are just simple rectangles, and sometimes the schematic symbols are drawings made to look like the physical connectors they represent.

##### Connector symbols

Connectors and headers are most often identified in schematics with a reference designator (REFDES) starting with the letter “J” or the letter “P”.

#### Switch

Switches are usually shown with a schematic symbol that represents the type of switch and the number of poles/throws and pins.

##### Switch symbols

Switches are identified in schematics with a reference designator (REFDES) starting with the letters “SW”.

#### Battery

Batteries are shown with a schematic symbol that has a long line and a short line, together representing one battery cell. In practice, most battery schematic symbols are drawn as two cells regardless of how many cells the battery actually contains.

##### Battery symbol

Batteries are identified in schematics with a reference designator (REFDES) starting with the letter “B”.

#### Transformer

Transformers are usually shown with a schematic symbol that symbolically represents how a transformer works. It looks like two parallel inductor coils with something in between them, usually a line or two.

Transformers are identified in schematics with a reference designator (REFDES) starting with the letter “T”.

#### Fuse/PTC

Fuses or PTCs (positive temperature coefficient devices) are circuit protection devices that “blow” (burn out) or increase resistance dramatically in the case of too much current flowing through them.

Fuses are usually shown in schematics with a symbol that looks like a sideways letter “S”.

##### Fuse symbol

Fuses are identified in schematics with a reference designator (REFDES) starting with the letter “F”.

PTCs are usually shown as a box with a line through it diagonally; the same symbol is used for PTC thermistors

##### PTC symbols

PTCs are identified in schematics with a reference designator (REFDES) starting with the letter “R”, “VR”, or “PTC”.

#### Non-component Symbols

There are other symbols in schematics that do not represent physical components. Some symbols represent physical structures to be built into the PCB, itself, like test points or mounting holes.

##### Test point symbols

Other schematic symbols represent power or ground rails.

##### Ground symbol

Still other schematic symbols are used for interconnection between different pages of the schematic, with labels identifying what electrical net they are part of.

Non-component symbols often have no reference designators. Some will have reference designators (REFDES) starting with the letters “TP” (test points), “MH” (mounting holes), or “X” (generic catch-all for otherwise unspecified types).

For more in-depth information on several of the symbols discussed in this article, check out Robert Keim's treatment of schematic symbols for passive components.

### Nets

In schematic and printed circuit board parlance, nets are the PCB-wired electrical connections. Nets appear as lines connecting component symbol pins to other pins or nets.

It is a best practice when drawing schematics to label important nets so that they can be clearly identified when laid out on a PCB design. If two nets are not drawn as connected but have the same label, they will be treated as physically connected by the schematic capture software so that when the design is exported to a PCB layout tool they will be the same net.

##### Image of a schematic with two nets not drawn connected, but labeled the same so physically connected, in this case "STEPM_R_EN"

It is a schematic capture best practice to use special symbols to show net connections to other pages, or parts of the same page, when not drawn as connected. These are intrapage (within a page) or interpage (between pages) connection symbols.

##### Interpage connectors

For readability, good schematics avoid overlapping nets wherever possible—but this is not always possible. When two nets connect, most schematic drawing tools add a connection dot or circle. The absence of the connection dot means the two nets are not connected, just passing over each other. More advanced schematic drawing tools show a wire hop to make it even more clear that the two nets are not connected.

### Important Outputs: Netlist and BOM

#### Netlist

The most important output of a schematic is the netlist. This file, or set of files, is the primary input to the PCB layout software, and it is used by layout designers to guide placing and routing for all of the circuits on a board.

Netlist formats vary, but generally they specify in a pretty simple form every component or symbol in the schematic, and every connection (net) between them. If you have named your nets in your schematic, those net names will appear in the netlist as the connection point between parts. If you did not name a net, the netlist output tool will generate a name for it.

Typically, a netlist will contain multiple tables: one listing the parts and their names, one listing the net names and their connections, etc. Netlists can also be used to include additional information necessary for SPICE circuit simulations. See some simple example netlist outputs here.

#### BOM (Bill of Materials)

The other important output of a schematic is a bill of materials or BOM. The BOM output is a spreadsheet or database that matches every REFDES in the schematic with a physical component and a part number.

There are a variety of formats for the BOM output, depending on how sophisticated your schematic and parts database are, and what kind of output you want. At the simplest end of the spectrum, you may have a list of reference designators, each with a manufacturer’s part number.

##### Screenshot of an OrCAD BOM output

More complex BOMs will include your company’s internal part numbers, quantities of a part used in multiple places, multiple vendor part numbers that can be used for a given part, etc. The BOM contains the information necessary to take a schematic and actually build it into an assembly.

There is much more to schematics than just these key things. Entire industries and careers are built around schematic design and PCB assembly. But understanding these five things will put you well on your way to understanding the most important fundamentals of schematics.

Are you looking at a schematic and need help with something not covered in this article? Tell us about it in the comments and we may put together an article to help!

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Mandeep321 July 26, 2019

Simple, informative article with clear information. Good work. Thanks, Matthew.

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LarryWN8P July 26, 2019

Matthew, You are putting out a tremendous amount of misinformation. Suggest you read over: ANSI/ASME Y14.44 concerning reference designations; IEEE 315, Clause 22.4 for the list of class letters to be used in forming a REF DES; IEC 60617 for the proper graphic symbol to use (most of these are found in IEEE 315A); and NIST Special Publication 811 on proper SI presentation. All people doing schematic capture or reading an electrical/electronic schematic diagram should have access to these “standards”.
—Regards, Larry

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rseriani July 26, 2019
Larry, Since i retired from my day job last year, I no longer have access to the documents you mention. Furthermore, I have no intention of spending the \$800+ it would cost to obtain them. I don't believe i am the only reader of All About Circuits who is in this situation. Would you be more specific about the "tremendous amount of misinformation" you stated the author has put out? Richard
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• Mark Schuurman 1 July 27, 2019
Larry, please clarify your statement of violating any standard with one or more examples.
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• Vincent Himpe July 27, 2019

LarryWN8P is right.

J and P are not ‘connectors’. J is Jack and P is Plug
Jack is defined as ‘the least movable part in a connector assembly’. SO the part on the board is J while the cable end
is P.

The zener symbol is wrong.

BJT’s are NOT current controlled ! that is a huge misconception. The base-emitter current is a side effect. Look up Gummel-Poon model.

Net labels are just that . Labels. they should NEVER be used to infer connectivity. Connectivity is made using wires and busses in a schematic. Just throwing some parts with netlabels stuck to their pins on a sheet is NOT a schematic ! Nets leaving a sheet do so using ports. ports are wired higher up in the hierarchy.
Do not use decorators on netnames ( like R\S\T\ ) as cad tools use different methods. some tools need only a single backslash to form an overstrike while others require it per letter. It would not be the first time a flying probe tester barfs on importing the netlist.

Wire hops are not to be used. The golden rule is:
- crossing wires NEVER connect.
- junctions are always t-shaped. There are no 4-way junctions. only 3-way junctions.

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• Raster 1 July 27, 2019
Please clarify BJT’s are NOT current controlled. Many words you post was wrong.
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• Heath Raftery July 28, 2019
Heh, goes to show the EE world is vast, with unique pockets of lore. The article was fit for purpose. I run an electronic design team like and am a stickler for standards. None of these things concerned me. Your preferences would be considered not wrong, but obsolete and substandard in my world. I would prefer seeing the authors' used in practice.
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LarryWN8P July 28, 2019

Hello,
I just lost two days of work replying. When I clicked on “Submit” a screen came up with “the form has expired”. I clicked on something that said go back. I highlighted all that I had typed and saved it. I then bailed out of AAC to clear the connection and logged back in. When I went to the Comments window and did a paste nothing showed up. I had about a dozen pages of comments. If I feel better about it tomorrow I will start writing again and do it piece by piece.
—Larry

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LarryWN8P July 29, 2019

Hello, This is my 1st installment of piece by piece posting.
To alleviate the pinch on the pocket book I will tell you I have been able to use the resource of the Internet to find and download the documents that I talked about, in PDF at no charge (as in free), as follows (Just remember the old saying about don’t believe anything you hear—and only half of what you read.):
1. ANSI/ASME Y14.44 “Reference Designations for Electrical and Electronics Parts and Equipment”: I have a personal copy of this standard but I did run across a website that had this posted. I think the extension was .su so it was out of Russia.
2. IEEE 315 “Graphic Symbols for Electrical and Electronics Diagrams (Including Reference Designation Letters)”. I may have found this under its alternate nomenclature of ANSI Y32.2. You will use Clause 22.4 “Class Designation Letters: Alphabetical List” the most but you should understand all of Clause 22 “Class Designation Letters”.
3. IEC 60617 “Graphic Symbols”: The IEC has a lock on this standard. It is an online database, but in order to access it you have to pay big bucks (or Swiss Francs) as an annual fee. I have found bits and pieces of this standard on the Internet, enough for my needs. As I recall I found one site out of Romania.
4. IEEE 315A “Supplement to Graphic Symbols for Electrical and Electronics Diagrams”: As the title states this is a supplement and not a revision. IEEE 315A implements the newer, at the time, IEC graphic symbols, which today are in IEC 60617. I may have found this standard under its alternative nomenclature of ANSI Y32.2 Supplement.
5. NIST Special Publication 811 “The NIST Guide for the use of the International System of Units”: This is a no charge (as in free) PDF download from the NIST website. Go to <www.nist.gov> and search for Special Publication 811 or use your favorite Internet browser. The information in this document is the same as in ANSI/IEEE/ASTM SI 10 or IEC 80000-1, however, you would have to pay for those. The information on the proper use of SI (System International, the Modern Metric System) comes from the BIPM SI Brochure, which the 9th Edition was published 2019-05-20.
6. ANSI/ASME Y14.34 “Associated Lists”: I found a PDF of the 2008 edition, the latest edition is 2013. This standard defines a Parts List (PL) and states after Clause 3.20.2 “NOTE: Other terms previously used to describe a parts list are list of materials, bill of materials, stock list, and item list”. In other words the term “bill of materials” is deprecated and should not be used. Also from Clause 3.15 “Find Number or Item Number” there is text that states “NOTE: Reference designations for electrical and electronic parts and equipment, in accordance with ASME Y14.44, may be used as find numbers or item numbers.”

It takes three items to properly define an electrical/electronic assembly: A schematic diagram, a parts list (PL), and an assembly drawing (for a printed circuit board assembly this would be a parts placement diagram). The documents/standards that I have called out should provide you with all the information you need to produce a coherent set of documentation. And remember that the thread that runs through and holds these documents together is the reference designator (REF DES).

If you need help in finding any of the documents I have listed send me an e-mail at <lawrence underscore joy at yahoo dot com> and I’ll try to help you out.
—Larry

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LarryWN8P July 30, 2019

Hello, This is installment Nr 2 of my postings.
I have a series of articles about reference designators hosted by Dr. Peter Dalmaris’ website at <https://techexplorations.com/reference-designations-for-electrical-and-electronics-parts-and-equipment>. There is a section with questions and answers that you might want to read over. And if there is a scenario that doesn’t cover your situation send me an e-mail at <lawrence underscore joy at yahoo dot com> and I’ll try to answer.
—Regards, Larry

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LarryWN8P July 31, 2019

1. The “8V” and “0V” should be “8 V” and “0 V” respectively. There is ALWAYS a space between the value (the number) and the following SI letter symbol. [See NIST SP 811 at Clause 7.2]  If this is text this can be done. If this is a voltage/power net, and not allowed to have spaces, either use an underscore or have a note on the schematic to the effect that:
0V means 0 V DC
8V means 8 V DC or that
3V3 means 3.3 V DC
that is if you think an explanation is warranted.
2. Concerning the small circles at 8V and 0V. These are terminal symbols [see IEEE 315, Clause 5.1.1] and if they are actual components they would carry a REF DES using the class letter “E” [see IEEE 315, Clause 22.4 for E]. If the “8V” is a voltage “net” then use the “Specific potential difference” symbol (which is just a straight line) [see IEEE 315, Clause 3.9.3.1]. I overlay the direction of flow symbol (an open arrow) [see IEEE 315A, Clause 1.7.1 , the added IEC symbol, which comes from IEC 60617] pointing in the direction of current flow.
3.1 Component F1: Congratulations, this is the correct IEC 60617 symbol for a fuse [you will also see this in IEEE 315A, Clause 9.1.1, the top symbol marked as an IEC symbol].
3.2 The rating of the fuse indicated as “0.25A” should be “0.25 A” or “250 mA” .
4.1 Concerning the C1 and C2 capacitor symbols. The capacitor symbol uses only straight parallel lines, no thickened lines, no curved lines, no other type of line [see IEEE 315, Clause 2.2.1, Style 1, which is the only way depicted in IEC 60617].
4.2 “100nF” should be “100 nF” and “47nF” should be “47 nF”.
5.1 Congratulations this is the proper symbol for a Zener diode [see IEEE 315A, Clause 8.5.6.1, the added IEC symbol, which is from IEC 60617].
5.2 “ZD” is not the proper class letter for a Zener diode. Use D or VR [see IEEE 315, Clause 22.4 under D or VR]. I personally use D as this means diode, and is only one letter, while VR means voltage regulator, which I reserve for identifying an LM117 and such.
5.3 “9.1V” should be “9.1 V”
6. R1 value of “1K” should be “1 kΩ”. If your schematic capture program can not handle the Greek letters “Ω” or” “µ” or the degree symbol “°” your program needs an upgrade. I use KiCad (KEE-CAD) which accepts UTF-8 symbols.
7. Correct Q1 symbol for a thyristor [see IEEE 315A, Clause 8.6.12.1, the added IEC symbol, which comes from IEC 60617].
8.1 For SD1: Congratulations again, this is the newest symbol for a diode [see IEEE 315A, Clause 8.5.1, the added IEC symbol, which comes from IEC 60617].
8.2 The class letter for a diode is “D” or “CR” [see IEEE 315, Clause 22.4 under “D or CR”]. Personally I use D as it is one letter instead of two and I think CR originally meant crystal rectifier.

That’s all for today. In the coming days I will go over the article section by section.
—Regards, Larry

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tmbarton August 02, 2019
Yawn. And yet, for all it's a terrible travesty of poor adherence to somebody's standard, I had absolutely no problem determining the intent, function and detail of the circuit as it was presented. It is perfectly clear as is. Standards are great as a starting point and as guidance. Rigid adherence to them is often the fallback position of folk who like to talk the talk but have difficulty walking the walk. I would suggest that if you're going to be so critical of an article clearly aimed at beginners to lead them towards a basic understanding of circuit diagrams you might consider asking for your subscription money back. Oh wait ...
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LarryWN8P August 02, 2019

Hello, 4th installment covering Reference Designators (REFDES):
1. 2nd paragraph, 1st sentence “...and the number of symbols per component.” I think your reference is to a “multiple-element part”. [See ASME Y14.44 at Clause 2.1.4 that discusses suffix letters.]
2. 3rd paragraph, 1st line, at the end “reference designator prefixes.” A REF DES prefix is the A1 in A1R1 and the A2A1 of A2A1C3. What you are referring to is the class designation letter or just class letter. [See the Wikipedia article that you have a link to.]
3. 3rd paragraph, 3rd line “...bill of materials (BOM)”. [See ASME Y14.34 “Associated lists”.] After Y14.34-2008, Clause 3.20.2 is “NOTE: Other terms previously used to describe a parts list are list of materials, bill of materials, stock list, and item list”. The term “bill of materials” is deprecated and should not be used.
4. 3rd paragraph, 3rd line, at the end “PCBA”. Presumably this means printed circuit board assembly. The IPC defines this as a PBA—printed board assembly. Whether you call the assembly a printed wiring board or printed circuit board it’s a PBA. The military calls them a circuit card assembly (CCA).
5. For the short table of class letters I have the following comments:
5.1 For “J” add that this is the class letter for the most fixed connector of a mating pair, irregardless of gender.
5.2 For “P” add that this is the class letter for the most movable connector of a mating pair, irregardless of gender.
5.3 Add the class letter “X” as it is the class letter for a socket, a fuse holder, a lamp holder, and by extension you could use it for a battery holder. Further on down this article it is stated that the class letter “X” is a catchall class letter—this is not so. Examples of use of the class letter “X”: If you have a fuse with REF DES F1 then the fuse holder would be XF1, for a DIP op-amp with REF DES AR2 the DIP socket would have a REF DES of XAR2. [See Figure 2, Figure 3, and Figure 4 in ASME Y14.44 for examples using the class letter “X”.]
5.4 For the switch, “S” is the class letter to use. Delete “SW” (it has a connotation of software and in my day it meant shortwave). Why use two letters when one will do?
—Until next time, Larry

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LarryWN8P August 03, 2019

Hello, this is my 5th installment. This concerns the Component Symbols:
4th paragraph “...each partitioned symbol that refers to the same physical component shares the same reference designator”. [See ASME Y14.44, Clause 2.1.4 “Suffix Letter”.] This clause details the assignment of a suffix letter for multi-element parts.

Commonly used schematic symbols table:
1. Diode: Change the symbol to that of the Crowbar circuit SD1. This is the symbol of IEEE 315A, Clause 8.5.1, the added IEC symbol, which is from IEC 60617.
2. Capacitor: Congratulations the capacitor symbol is perfect.
3. Inductor: Loops are NOT used in an inductor (or transformer) symbol. See IEEE 315A, Clause 6.2.1, the IEC labeled symbol, which is half circles. Also see IEEE 315A, Clause 6.2.1A “Choke, Reactor”, which is the only way shown in IEC 60617.
4. Resistor: See IEEE 315, Clause 2.1.1. The top zig zag symbol is the “American” symbol, which is the one I stick to. The bottom symbol that is a rectangle with a 3:1 length to width ratio is the only symbol shown in IEC 60617.
5. DC voltage source: This is the symbol for a single battery cell [see IEEE 315, Clause 2.5.2] and a generalized direct-current source [see IEEE 315, Clause 2.5.3]. For an ideal voltage source see IEEE 315A, Clause 2.18.2 and mark it as a DC source.
6. AC voltage source: This symbol is from IEEE 315, Clause 2.7 “Oscillator” and is not an IEC symbol. For an ideal voltage source use the symbol of IEEE 315A, Clause 2.18.1 and mark it as an AC source.
7. All of the digital (binary or 2-state) device symbols are okay.
—Larry

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LarryWN8P August 04, 2019

Hello, This is 6th Installment:
Resistor
2nd paragraph, change wording to “Resistors are identified in schematics with a REF DES using the class letter “R”.”

Capacitor
1. 2nd paragraph, change wording to “Polarized capacitors are denoted by a plus sign “+” to indicate a positive terminal.” Comment and note: Curved lines are no longer used.
2. For the Capacitor symbols figure delete the 2nd and 4th symbols and change the wording of the caption to “Shown are a non-polarized capacitor on the left and a polarized capacitor on the right.”
3. 3rd paragraph, change wording to “Capacitors are identified in schematics with a REF DES using the class letter “C”.”
—Larry

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LarryWN8P August 05, 2019

Hello, 7th Installment
Inductor
1. Inductor symbol figure: Loops are not used to indicate a coil. Use half circles only [see IEEE 315A, Clause 6.2.1, the symbol marked as an IEC symbol]. Also see IEEE 315A, Clause 6.2.1A “Choke, Reactor” symbol. This symbol is from IEC 60617.
2. 2nd paragraph, revise the wording to be “Inductors are identified in schematics with a REF DES using the class letter “L”.”

Diode
1. Diode symbol figure: Change the symbol to the newer diode symbol that is with a line through the triangle from the anode to the cathode, like that of the Crowbar circuit SD1. [See IEEE 315A, Clause 8.5.1, the added IEC symbol, which is from IEC 60617.]
2. Zener diode symbol figure: This figure needs changing to the symbol that is in the Crowbar circuit ZD1. [See IEEE 315A, Clause 8.3.1 “Breakdown” symbol to be used as the cathode symbol, which is from IEC 60617.]
3. Schottky diode symbol figure: The additional line from the anode to the cathode through the triangle needs to be added. [See IEEE 315A, Clause 8.3.5 “Schottky effect” symbol that is to be used as the cathode symbol of the diode symbol, which is from IEC 60617.]
4. 3rd paragraph, change wording to “Diodes are identified in schematics with a REF DES using class letter “D” or “CR”, Zeners use class letter “D” or “VR”, and an LED uses the class letter(s) “DS”.” Note that “Z” or “ZD” is not used for a Zener diode. [See IEEE 315, Clause 22.4 listing for “D or CR”, “D or VR”, “DS”, and “Z”.
—Larry

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LarryWN8P August 06, 2019

Hello, 8th Installment.
Transistor
1. BJT symbols figure:
1.1 See IEEE 315A, Clause 8.6.2A for the IEC 60617 NPN symbol. Delete the envelope symbol (the circle encompassing the NPN symbol) and, thus, the connection dot so the symbol is like that for the PNP symbol. Note that the arrow is open, not filled.
1.2 See IEEE 315A, Clause 8.6.1, the added IEC symbol, for the IEC 60617 PNP symbol. Again, note that the arrow is open, not filled.
2. FET symbols figure:
2.1 For the NMOS (n-channel insulated-gate, depletion-type) see IEEE 315A, Clause 8.6.10.2, the added IEC symbol, which is from IEC 60617. Note that the gate terminal is always drawn in line with the source terminal.
2.2 For the PMOS (P-channel insulated-gate, depletion-type) see IEEE 315A, Clause 8.6.11.2, the added IEC symbol, which is from IEC 60617. Again, note that the gate terminal is always drawn in line with the source terminal.
3. 5th paragraph (paragraph after FET symbols figure), change the wording to “Transistors are identified in schematics with a REF DES using the class letter “Q”.” Note: Do not use “M” or “T” as letters for a transistor as “M” is the class letter for a meter and “T” is the class letter for a transformer.
—Regards, Larry

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LarryWN8P August 08, 2019

Hello, 9th Installment
Variable Resistors
1. Varistor symbol figure: Change the symbol to that of IEEE 315, Clause 2.1.5, which is for a nonlinear resistor (intrinsic). You also might want to show IEEE 315, Clause 2.1.6 symbol, which is for a symmetrical varistor (intrinsic) [this symbol is the same as for the nonlinear resistor with a “V” placed by it to indicate it is a voltage dependent, non-linear resistor].
2. 3rd/last paragraph: Change wording to “The REF DES for a resistor uses the class letter “R”, whether the resistor is fixed, variable, stepped, or multi-element (commonly referred to as a network and being in a DIP, SIP, or SMT package). The REF DES for a symmetrical varistor or voltage-sensitive resistor uses the class letter(s) “RV”. And for an asymmetrical varistor the class letter to use is “D or CR”.”

Do not use “VR” as the class letter for a variable resistor or potentiometer as this class letter is defined to be a voltage regulator (such as an LM117/317). [See IEEE 315, Clause 22.4, class letter “VR”.]
—Regards, Larry

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LarryWN8P August 09, 2019

Hello, 10th Installment
Integrated Circuit
3rd/last paragraph: Change wording to “Integrated circuits are identified in schematics with a REF DES using class letter “U”. Op-amps use class letter “AR” and voltage regulators use class letter(s) “VR”.” [See IEEE 315, Clause 22.2.4 “Specific versus general” that staates “The letters A and U (for assembly) shall not be used if more specific class letters are listed in Clause 22.4 for a particular item.” And refer to Clause 22.4 list at “AR”, “U”, and “VR”.]

Do not use “IC” as a class letter(s) as this is reserved to mean “initial conditions”.
—Regards, Larry

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Analog Ground August 09, 2019

There is a thread in these comments about the material in the article violating various industry standards. I have no doubt this is all true. However, with the advent of the maker community and extremely low cost of PCB fabrication and assembly, schematic style is diverting in lots of different directions. For example, look up “Fritizing”! Now THAT is extreme! Electronics has become democratized. The old standards will inevitably be pushed aside.

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LarryWN8P August 12, 2019

Hello, 11th Installment.
Crystal/Oscillator/Resonator
2nd paragraph, change wording to “Crystals and crystal oscillators are identified in schematics with a REF DES using the class letter “Y”.” Note that the statement “X” is occasionally used; this letter is also a catch-all for components not fitting another category.” is INCORRECT. The class letter “X” is used for a socket, fuse holder, lamp holder, and by extension could be used for a battery holder. [See IEEE 315, Clause 22.4 for class letters “X” and “Y”.] Examples of using the class letter “X” are as follows:
Fuse “F1”. The fuse holder/socket would be “XF1”.
A THT DIP op-amp “AR2”. The DIP socket would be “XAR2”.
an IC “U3”. The socket would be “XU3”.
A display “DS4”. The socket would be “XDS4”.
A plug-in board “A1”. The board socket would be “XA1”.
[See ASME Y14.44, Figure 2, Figure 3, and Figure 4.]
—Regards, Larry

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LarryWN8P August 13, 2019

Hello, 12th Installment.
Digital Logic Gates
2nd/last paragraph, change to read “Logic gates are sold as integrated circuits, and therefore they are identified in schematics with a REF DES using the class letter “U”.” Do not use “IC” as a class letter as this is used to mean initial conditions. [See IEEE 315, Clause 22.4, class letter “U”.
—Regards, Larry

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LarryWN8P August 14, 2019

Hello, 13th Installment.
Operational Amplifier
1. In the 2nd figure that starts “A dual-supply op-amp…” The figure on the right of “...a single-supply configuration…” shows an earth ground symbol. The ground symbol should be that of IEEE 315, Clause 3.9.2 “Chassis or frame connection; equivalent chassis connections (of printed-wiring boards [PCBs])”. This is what I call the pitch fork symbol.
2. 2nd/last paragraph, change wording to “Operational amplifiers are identified in schematics with a REF DES using class letter(s) “AR”.” [See IEEE 315, Clause 22.4, the listing for “AR”.] Do not use as class letter(s) “IC” or “OP”. The class letter for a comparator could be “U” or, because we know that a comparator is a special op-amp, you could use “AR”.
—Regards, Larry

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LarryWN8P August 15, 2019

Hello, 14th Installment.
1. 2nd paragraph, “...sometimes the schematic symbols are drawings made to look like the physical connectors they represent.” Comment: The schematic diagram should NOT show the physical/mechanical representation of a connector, this is the realm of the associated assembly drawing. After all you don’t show on a schematic diagram the physical packaging of transistors or other components.
2. Connector symbols figure.
2A. Replace the figure on the left with that of IEEE 315A, Clause 5.3.4.1, the middle symbol. This is the only way the present day IEC 60617 depicts connectors.
2B. For the figure on the right replace the male (arrowhead) symbol with that of IEEE 315A, Clause 5.3.2 “Male contact”, the added IEC symbol. It’s a thickened line and is from IEC 60617.
2C. For the figure on the right replace the female symbol with that of IEEE 315A, Clause 5.3.1 “Female contact”, the added IEC symbol. It is a half circle and is from IEC 60617.
3. 3rd/last paragraph, change the wording to “Connectors and headers are identified in schematics with a REF DES using the class letters “J”, “P”, or “X” (depending).
3A. The class letter “J” is assigned based on if the connector of a mating pair is the most fixed.
3B. The class letter “P” is assigned based on if the connector of a mating pair is the most movable.
3C. And the class letter “X” is assigned if the connector is a socket, most fixed, of a mating connector.”
3D. If you have two cables that connect together with mating connectors, each of the connectors use a class letter of “P”. This means you can have a “P” connecting to a “P” but you would never have a “J” connecting to a “J”.”
Note that the assignment of “J”, “P”, or “X” is not based on gender/sex. This answers the question of connectors that have no gender or may have both types of contacts.
—Regards, Larry

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LarryWN8P August 16, 2019

Hello, 15th Installment.
Switch
1. See IEEE 315A, all of Clause 4 “Graphic Symbols for Contacts, Switches, Contactors, and Relays”, which symbols are all from IEC 60617.
2. 2nd/last paragraph, change wording to “Switches are identified in schematics with a REF DES using class letter “S”.” [See IEEE 315, Clause 22.4, the listing for “S”.] Do not use class letter(s) “SW” as this has a connotation of software and in my day it meant shortwave. And why use two letters when one will do?
—Regards, Larry

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LarryWN8P August 18, 2019

Hello, 16th Installment.
Battery
2nd/last paragraph, change wording to “Batteries are identified in schematics with a REF DES using the class letter(s) “BT”.” Do not use the class letter “B” for a battery as this is the class letter for an electric motor and a (squirrel cage) blower/fan. [See IEEE 315, clause 22.4 for the class letters “B” and “BT”.]
—Regards, Larry

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LarryWN8P August 19, 2019

Hello, 17th Installment.
Transformer
1. For the transformer symbol shown. See IEEE 315, clause 6.4.1 “Transformer…” Note that loops are deprecated and only semicircles are used to show the windings. And from IEEE 315A, clause 6.1.2, the added IEC symbol, which is a single line to indicate a magnetic core.
2. 2nd/last paragraph should read “Transformers are identified in schematics with a REF DES using the class letter “T”.”
—Regards, Larry

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LarryWN8P August 20, 2019

Hello, 18th Installment.
Fuse/PTC
1. Comment: Fuses and PTCs are two different animals. A fuse is a one-time thermal cutout while those components with trade names such as “Polyfuse” or “Polyswitch” are a PTC (positive temperature coefficient) device and are NOT fuses.
2. The IEC 60617 symbol for a fuse can be found in IEEE 315A at clause 9.1.1, the top symbol identified with “IEC”. Or go online to <https://symbols.radicasoftware.com> and look for fuses.

3. In the paragraph under the Fuse symbol figure change the wording to “Fuses are identified in schematics with a REF DES using the class letter “F”.”
4.A The symbol for a general thermistor is found in IEEE 315, clause 2.1.12.1, which is the symbol for a resistor with a “t°” by it to show temperature dependence. For IEC 60617 the “t°” symbol can be replaced with a “ϴ” (Theta) symbol.
4.B The symbol for a linear thermistor is in IEEE 315 at clause 2.1.12.1.1, which is the general symbol for a thermistor with a diagonal line placed across the symbol.
4.C For a nonlinear thermistor see IEEE 315, clause 2.1.12.1.2, which is the linear symbol with a kink in the slash.
4.D Finally for a positive temperature coefficient thermistor see IEEE 315, clause 2.1.12.1.3, which is the nonlinear symbol with a “+” sign in front of the temperature dependence symbol. Or go to <https://symbols.radicasoftware.com> and look for “Thermistors”.

The “+” temperature symbol should be added.
5. Change the last paragraph to “PTCs are identified in schematics with a REF DES using the class letter(s) “RT”.” Do not use “R”, “VR”, or “PTC”—”VR” means voltage regulator.
6. To complicate things a little there is a reading of IEEE 315, clause 22.2.1 “Actual versus intended function”, which states “If a part serves a purpose other than its generally intended one, the function actually performed shall be represented by the graphic symbol used on the schematic diagram; the class letter shall be chosen from the list in clause 22.4 and shall be indicative of its physical characteristics. For example, a semiconductor diode used as a fuse would be represented by the graphic symbol for a fuse (actual function), but the class letter would be D…” Thus a PTC acting as a circuit breaker (actual function) would use the graphic symbol for a circuit breaker, but the class letter(s) to use would be “RT”. The IEC 60617 symbol for a circuit breaker is as follows:

And the class letter(s) to use would be “RT” to indicate it is a thermistor.
—Regards, Larry

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LarryWN8P August 21, 2019

Hello, 19th Installment.
Non-component Symbols
1. Test point symbols figure. These are NOT the symbols for a test point. The “Test-Point Recognition Symbol” is shown in IEEE 315, clause 1.5 (see all the subclauses 1.51. through 1.5.4). It is a small filled circle either by itself or with a leader.
1.A The non-class letter for a test point is “TP”. [See IEEE 315, clause 22.4, the listing for “TP” and the footnote that goes with it that states “Not a class letter, but commonly used to designate test points for maintenance purposes.”]
1.B What this means is if you have a component that you attach a test probe to, the component itself would carry a class letter of “E” (for miscellaneous electrical part or terminal) that would be on a parts list (PL) and might also have a TP# for the maintenance manual.
2. Ground symbol figure should be changed to read “Earth ground symbol”. [See IEEE 315, clause 3.9 “Circuit Return”. See especially clause 3.9.1, 3.9.2, and 3.9.3.2.]
3. 4th paragraph should read “Non-component symbols often have no REF DES. Some will have REF DES using the non-class letter(s) “TP” (test points), “MH” (mounting holes), “PTH” (plated-thru holes), “NPH” (non-plated holes), or “FD” (for fiducials).
3.A Note: DO NOT USE “X” as a generic catch-all for otherwise unspecified types. “X” is the class letter for a socket [see IEEE 315, clause 22.4 for the “X” listing]. Otherwise you could use the class letter “Z” (meaning general network (where specific class letters do not fit)). [See IEEE 315, clause 22.4 for the listing of the “Z” class letter.]
—Regards, Larry

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LarryWN8P August 22, 2019

Hello, 20th Installment.
Nets
1. For the figure (picture) titled “Image of a schematic…”:
1A. The earth ground symbol shown at the lower left corner of J5 needs to change to the chassis or PCB equivalent ground (the pitchfork) symbol.
1B. For R9 the value of “10kΩ” should be “10 kΩ”.
2. For the “Interpage connectors” figure (picture):
2A. The value of the capacitor “0.1uF” should change to “0.1 µF” or “100 nF”.
2B. The capacitor symbol needs to change to two parallel lines, no curved lines are used for a capacitor symbol.
3. For the “Connected nets” figure (picture):
3A. “+5V” should be “+5 V”, but it is understood that modern day ECAD programs don’t like spaces.
3B. For R27 the value “18kΩ” should be “18 kΩ”.
3C. For R26 the value “2kΩ” should be “2 kΩ”.
3D. For C34 the value “0.1uF” should be “0.1 µF” or “100 nF”.
3E. The C34 capacitor symbol should be two parallel lines, no curved lines are used for a capacitor.
3F. The earth ground symbol needs to change to the chassis or PCB equivalent ground (the pitchfork) symbol.
4. For the “Non-connected nets (with wire hop)” figure (picture):
4A. The C3 value of “6.8pF” should be “6.8 pF”.
4B. I don’t know of any ECAD program that uses the wire hop. That is why the IPC recommendation is to offset.
—Regards, Larry

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LarryWN8P August 23, 2019

Hello, 21st (and last) Installment.
Important Outputs: Netlist and BOM
Netlist
All okay
BOM (Bill of Materials)
Comment: I use the term “parts list” with the attendant abbreviation “PL” in accordance with ANSI/ASME Y14.34 “Associated Lists”. The latest version is 2013 (reaffirmed 2018), but the 2008 version is available online in PDF. After subclause 3.20.2 is a note that states “Other terms previously used to describe a parts list are list of materials, bill of materials, stock list, and item list.”

This standard also has in clause 3.15 “Find Number or Item Number” a note that states “Reference designations for electrical and electronic parts and equipment in accordance with ASME Y14.44, may be used as find numbers or item numbers.”
—Regards, Larry

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LarryWN8P August 24, 2019

Hello,
For those of you who have read all of these postings and feel that the standards are out of date or are of no consequence, consider the following: If you were driving a car and came across a traffic sign that had ALTO or HALT on it you probably wouldn’t know what it meant. But if these words were on a characteristic octagon shaped sign it wouldn’t matter if the word was ALTO, HALT, or STOP, you would know what it meant.
—Regards, Larry

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