What Is a Diode Symbol? A/K, Types, and PCB Mapping

The diode symbol represents a generic family of diode components and the connection of two terminals. The long straight edge of the symbol serves as the cathode (identified as K), and the opposite end of the symbol represents the anode (identified as A). The identity of these two terminals does not change, however, even if the orientation of the symbol’s elements is changed.

The graphic does not represent a physical package, nor does it establish a PCB footprint for the diode. The graphic does not provide information about voltage, current, power rating, speed specifications, or thermal limits. You should reference the symbol before verifying that you are using the correct part (i.e., seeing the correct pin assignment), verify the pads and CAD package information, and look up the datasheet for additional information about the diode.

What Does the Bar Mean on a Diode Symbol?

The long straight edge of the diode symbol represents the cathode (terminal K), and the terminal opposite (labeled A) is referred to as the anode. Therefore, the long edge indicates the terminal and not the actual physical size or outline of the diode.

The arrow-shaped part of the diode symbol indicates the direction toward the cathode terminal and is useful as a mnemonic for conventional current direction under forward-bias conditions (although it does not necessarily mean that current is flowing). A change in orientation will change the screen direction of the symbol, however, and will not change the designation of K and A for a diode.

IEC identifies the generic semiconductor diode as symbol S00641 in the public preview for its IEC 60617 database, and the current database is IEC 60617:2026 DB. This identifier establishes a standard reference, but the actual model and object represented will be determined by the EDA library used in the project.

Older drawings produced in North America may have been done under the conventions established in IEEE/ANSI 315-1975. IEEE has declared IEEE/ANSI 315-1975 Inactive-Reserved and it can therefore be used as reference material when reading legacy schematic diagrams; it is not sufficient evidence of the current status of a new library.

There is also a diode-shaped symbol on the multimeter which indicates that the multimeter is in diode-test mode. In a schematic diagram, this symbol is used to represent an electronic circuit component. The two contexts share similar visual language, although they perform different functions.

How Do Anode and Cathode Stay Identifiable After Rotation?

To identify the anode and cathode ends of a diode after it has been rotated, first identify the cathode bar and give this terminal the identifier K, then assign the opposite terminal the identifier A. This will work consistently whether the diode is rotated at 0, 90, 180, or 270 degrees. Left, right, top, and bottom represent positions on the screen, not the electrical identities of the terminals.

Voltage determines the bias condition. According to Toshiba, forward bias occurs when VA > VK, while reverse bias occurs when VA < VK. A silicon pn-junction diode has a forward-conduction reference of approximately 0.7 V, but the actual value will vary depending on the diode, the amount of current flowing through it, and the temperature of the diode (“How do diodes work?“).

The cathode of the diode does not automatically connect to circuit ground or the negative supply voltage. Either the anode terminal or the cathode terminal can be connected to a switching node, to another component, or to a power rail in the circuit. The type of electrical circuit and the netlist will determine which terminals connect to these points; the symbol does not change the meaning of the terminal connections, it just shows you how to identify them.

Currently, KiCad’s generic D object is an example of a concrete library object, where the cathode is designated as pin 1 (K) and the anode is designated as pin 2 (A). The footprint and datasheet fields are empty. Therefore, treat K=1 and A=2 as part of that library object, but not as a rule for every EDA library, vendor symbol, or package.

How Symbol Modifiers Separate Common Diode Families

A visible indicator helps identify the diode family, not the specific component part number that may be ordered.

All generic ordinary, Zener, Schottky, LED, photodiode, and variable-capacitance diodes have their own unique symbols within the official KiCad Device library (KiCad Device library).

Photodiodes have arrows pointing inwards because incident light reaches the device, and LEDs have arrows pointing outwards because the device emits light (Diode curves; High Impedance Sensors). For more information regarding the specific polarity of LEDs and how it relates to PCB preparation, see “LED Symbol and PCB Mapping“.

For instance, Diodes Incorporated lists the 1N4001 to 1N4007 as 1 A rectifiers with repetitive peak reverse-voltage ratings of 50 V to 1000 V, while the 1N5408G uses the same ordinary rectifier symbol but provides a maximum reverse voltage of 1000 V with a rating of 3 A; thus, the graphic representation of the family does not designate the reverse-voltage or current rating of the components.

onsemi lists the 1N4733A as having a rating of 5.1 V, 1 W, 5%; these values are based on actual part specifications and therefore cannot be inferred just from the bent cathode line.

Diodes Incorporated describes the ordinary 1N4001 rectifier as having a forward voltage of 1.0 V at 1 A. STMicroelectronics lists the same-current-class 1N5817 Schottky as having a forward voltage drop of 0.45 V at 1 A—less than half of the ordinary rectifier drop—but a 0.5 mA reverse leakage current at rated reverse voltage, as opposed to approximately 5 µA for the 1N4001, which is about a hundredfold more leakage. The Schottky modifier will show you which type of trade-off family you are reviewing; however, it will not allow you to determine whether the leakage rate for the component in question fits within your standby-current budget.

By referring to Infineon’s BAR64-02V datasheet, its typical forward resistance is around 2.1 Ω at 10 mA, with a capacitance of approximately 0.2 pF at zero bias, making the PIN diode useful as an RF switching element up to several GHz. Conversely, another PIN part with a wider intrinsic region may trade that low capacitance for a higher reverse-voltage rating. Thus, the PIN symbol modifier simply confirms the presence of a PIN structure and does not indicate which side of that trade-off the individual component occupies.

How Is the Diode Symbol Used in Common Circuits?

Trace A/K Through the Library, Footprint, and Package

Identifying A and K provides only the first step in checking a PCB design.

There are five checks you need to perform before you finalize a PCB design.

1. Ensure that you have both the actual pin names/numbers and the A/K identifiers.
2. Check that the footprint pads assigned in your software correspond to the same A/K identifiers.
3. Open the manufacturer’s datasheet for the selected component and find the pin view, package view, and marking that is unique to that component.
4. Make sure that the pin view you found matches the placement rotation, silkscreen artwork, and assembly drawings.
5. Reconcile the BOM part number with the CAD library revision and the manufacturing output.

In KiCad, a generic symbol can represent many components and footprints, with the final assignment made later (Generic and fully specified symbols), which is why it is essential to confirm that the A/K labels were used to identify the parts correctly. Using a generic symbol for the component means that correct A/K labels alone still may not prove that the final package has been mapped correctly.

For example, the datasheet provided by Vishay for the 1N4148W identifies a cathode bar for the SOD-123 package. However, in the marking guide provided by Vishay there is no single mnemonic for all packages; it varies by the specific package and directs users to the individual component datasheets to make sure they are using the correct markings. For instance, the axial 1N4148 and 1N4448 from Nexperia have the letters “k” and “a” identified in the drawing for that particular package view (1N4148; 1N4448).

The examples above are not universal guidelines for bands, dots, bevels, or molded outlines; they are defined and specific for those particular parts. Before confirming that you are using the correct package, review the drawing provided to determine whether it indicates a top view, bottom view, or terminal view. A correct package interpretation can still fail if the footprint pad mapping or assembly rotation is wrong.

What Changes When a Diode Is Reversed?

What happens when a diode is reversed will depend on many factors, including the diode family, circuit topology, applied voltage, current limiting, and rated specifications. Not all reversed diodes are harmless or destroyed; you need to determine first whether or not the circuit you are using is designed to operate with reverse bias.

Typically, an ordinary diode blocks current under reverse bias but can exhibit leakage current, and reverse-voltage limits still apply. If the reverse voltage reaches breakdown, excess current can damage or degrade the junction. Toshiba distinguishes this risk from Zener operation (How Do Diodes Work?).

A Zener diode can operate in its reverse-biased state when a series resistor or other current-limiting element is used to keep the breakdown current within the manufacturer’s rated dissipation limits (What Is a Zener Diode?). Protection diodes may also use reverse-biased configurations for a defined function; their symbols represent the diode family, while the manufacturer’s datasheet indicates the safe operating conditions.

The shortcut “positive always goes to the anode” may not hold true for circuits specifically designed to operate a diode in reverse bias.

Conclusion

A diode symbol does not represent a complete part specification; rather, it indicates the family of diode and its A/K orientation. Always read the cathode bar first, determine the intended bias based on the circuit voltages, then reference the selected part’s specifications and package mapping via the manufacturer’s datasheet and CAD data.