Electronic component symbols
A symbol does not draw the component: it draws what it does. That is why a carbon-composition resistor and a 50 W wirewound one are represented the same way, and why the transistor symbol shows which way the current flows instead of showing a package.
01Two standards living side by side
There are two families of symbols and you run into both every day: European schematics and North American manuals. You do not have to pick one: you have to recognize both, because any project mixes datasheets from both origins.
| Component | IEC 60617 / IRAM | ANSI / IEEE 315 |
|---|---|---|
| Resistor | Empty rectangle | Zigzag |
| Inductor | Semicircles or filled rectangle | Turns (loops) |
| Logic gates | Rectangle with an internal symbol (&, ≥1, =1) | Distinctive shapes (D, shield, crescent) |
| Fuse | Rectangle with a line running through it | Curved stroke or zigzag |
| Earth / ground | Three decreasing strokes | Same, but with a distinction between signal ground, chassis and earth |
Even though IEC is the current standard, the distinctive shapes of gates—the D of the AND, the shield of the OR—are still used everywhere, even in Europe, because they can be told apart at a glance even when the drawing is small or poorly printed. Electronic CAD programs ship with both libraries. What matters is not mixing them within the same drawing.
02The symbols you should recognize without thinking
- Rectifier: triangle with a straight bar.
- Zener: the bar has two angled “tabs.” It operates reverse biased.
- Schottky: the bar is shaped like a capital S.
- LED: two outgoing arrows. The photodiode has the arrows incoming: one emits and the other receives.
- Varicap diode: bar with a capacitor symbol.
- NPN: the emitter arrow points out of the transistor (Not Pointing iN toward the base).
- PNP: the arrow points in.
- JFET: gate with an arrow, continuous channel.
- MOSFET: gate separated by a gap (the oxide); channel in three segments if it is enhancement-mode and continuous if it is depletion-mode.
- Almost all power MOSFETs are drawn with their body diode, which physically exists and is sometimes put to use.
In every symbol with an arrow—diodes, transistors, thyristors—the arrow indicates the conventional direction of current when the device conducts. With that one idea you can deduce the polarity of almost any component without memorizing anything.
03Reference designators
Each component in a schematic carries a unique label: R12, C3, U1. It is not a personal convention: it is standardized, and it is what links the schematic to the bill of materials, to the board and to the silkscreen printed on it.
| Letter | Component | Letter | Component |
|---|---|---|---|
| R | Resistor | Q | Transistor |
| C | Capacitor | U | Integrated circuit |
| L | Inductor | J | Female connector (jack) |
| D | Diode, LED | P | Male connector (plug) |
| T | Transformer | SW / S | Switch or pushbutton |
| F | Fuse | K | Relay or contactor |
| Y | Crystal or oscillator | M | Motor |
| RV / VR | Potentiometer or preset | TP | Test point |
- A designator is unique across the whole board: there cannot be two R7s.
- Number from top to bottom and from left to right, in the order they are read. Almost all programs do it on their own with a renumbering command.
- In large designs, number by blocks: 1xx the power supply, 2xx the input, 3xx the output.
- Never renumber once the board is manufactured and documented: the silkscreen, the bill of materials and the assembly instructions would fall out of sync.
04How values are written
Next to the designator goes the value, and there too there are conventions that prevent costly confusion.
| Written | Means | Why |
|---|---|---|
| 4k7 | 4.7 kΩ | The letter replaces the decimal point: if the drawing is photocopied badly, the point is lost and the value changes tenfold. |
| 2R2 | 2.2 Ω | The R acts as the decimal point for values below one kilohm. |
| 100n | 100 nF | For capacitors the unit is understood: always farads. |
| 1u o 1µ | 1 µF | The u is used when the keyboard or the program does not accept µ. |
| 10u/25V | 10 µF, 25 V | In electrolytics the voltage rating is as important as the capacitance. |
| 1N4007 | A type, not a value | Semiconductors and integrated circuits are identified by their manufacturer part number. |
The schematic says “R5 = 4k7.” On the bench you have to find the right resistor:
- With the color code: yellow-violet-red for 4700 Ω, plus the tolerance band.
- On SMD parts, the three digits 472 mean 47 followed by 2 zeros = 4700 Ω. In the four-digit code, 4701 is 470 with one zero.
- The prefixes and their equivalents are reviewed in the SI converter.
05Digital symbols
They already appeared in combinational logic; here they matter as a drawing convention, and there are three details that change the meaning:
A small circle on an input or output means negation. It turns the AND into a NAND and, on an input, indicates that the signal is active-low.
RESET with a bar over it—or written /RESET or RESET#—indicates
that the signal acts when it is at zero. Mistaking it leaves a circuit
permanently in reset.
On a clock input it indicates that it acts on an edge. If it also has a circle, on the falling edge. On an output it indicates three-state or open collector.
In a schematic, a microcontroller or a dual op amp is almost never drawn with its pins in physical order. They are split into functional blocks—U1A and U1B for the two halves of a dual op amp, or a separate power block with all the VCC and GND pins—and each pin is placed wherever the schematic is clearest. The pin number is written next to it, and it is the only thing that matters for routing. Anyone looking in the schematic for “pin 1 at the top left” is reading the wrong drawing.
06In the lab
Put together a tray with twenty varied real components. Each student identifies them, draws their standard symbol, and writes down the designator that would correspond to each and its value. Everyone corrects together. It is the lab that fixes the vocabulary of the topic.
With a scrap circuit board and its schematic (or that of an equivalent circuit), locate on the board five components marked in the schematic, using the silkscreen. Then measure them in circuit and verify that they match the indicated value.
Take a schematic in ANSI symbols from a datasheet and redraw it in IEC, or the other way around. Check that the circuit is exactly the same. It also serves to train the reading of imported documentation.
In the electronic CAD program you will use, create a new symbol from scratch—for example a sensor that is not in the library—with its pins correctly numbered and its default designator. It is the first step of the next topic.
07Common mistakes
| Symptom | Usual cause |
|---|---|
| The transistor was mounted with the emitter and collector swapped | The symbol was read without looking at the arrow, or it was assumed that the order of the pins in the symbol is that of the package. It never is: check the datasheet. |
| The electrolytic capacitor blew up | Reversed polarity: in the symbol, the straight bar is the positive and the curved (or hatched) one the negative. |
| The circuit stays in reset all the time | An active-low signal treated as active-high. The bar over the name or the circle on the pin indicated it. |
| The Zener does not regulate | Mounted forward biased. The Zener operates reverse biased, and its symbol is recognized by the tabs on the bar. |
| Two components with the same designator | Copying and pasting a block without renumbering. The program detects it in the electrical rules check. |
| The bill of materials does not match the board | The designators were renumbered after the silkscreen was generated. |
| A capacitor of the correct capacitance was bought and it does not fit or fails | The working voltage and the type (ceramic, polyester, electrolytic) were not checked: the symbol does not tell them apart, the written value does. |
08Self-assessment
How do you tell an NPN from a PNP in the symbol?
By the emitter arrow: in the NPN it points out of the transistor and in the PNP it points in. The arrow marks the conventional direction of current.
What does the small circle on a gate output mean?
Negation. An AND with a circle on the output is a NAND. On an input, it indicates that the signal is active-low.
What component is “C7 100n/63V”?
The seventh capacitor in the circuit, of 100 nF and 63 V working voltage. From the value and the voltage, almost certainly polyester or ceramic.
Why is it written 4k7 and not 4.7k?
Because the decimal point is easily lost when photocopying, scanning or printing at low quality, and 4.7 kΩ would end up being read as 47 kΩ. The prefix letter takes the place of the point.
What is the difference between the LED symbol and the photodiode symbol?
The direction of the arrows: on the LED they point out (it emits light) and on the photodiode they point in (it receives it).
What does a small triangle on the clock input of a flip-flop indicate?
That the input acts on an edge and not on a level. If there is also a circle, the edge is falling.
Can you rely on the symbol following the pin order of the package?
No, and it almost never does. The symbol is drawn so that the schematic can be understood; the correspondence with the package is given by the pin number and, in the board design, by the footprint associated with the component.
Which designator letter corresponds to a relay, a crystal and a female connector?
K for the relay, Y for the crystal and J for the female connector.
A schematic mixes zigzag and rectangle resistors. Is that wrong?
It is wrong as a document: it is not electrically incorrect, but a drawing must use a single standard consistently. The mix usually indicates that blocks were copied from different sources without reviewing them.
What does an SMD part marked 472 mean?
47 followed by 2 zeros: 4700 Ω, that is 4k7. It is the same system as the color code, but written in digits.