The anode is the positive terminal where conventional current enters; the cathode is the negative terminal where current exits. On a schematic, the diode symbol anode cathode orientation is a triangle (anode) pointing to a vertical bar (cathode). Physically, the cathode is marked by a painted stripe, a bevel, or a specific SMD code. Current flows only from anode to cathode when the anode voltage exceeds the cathode voltage by the forward threshold (typically 0.6V for silicon).

Complete Diode Symbol & Pinout Reference Table

Use this table to instantly map schematic symbols to physical component terminals. The 'Line Bar' always represents the cathode.

Diode Type Schematic Symbol Description Anode Identifier Cathode Identifier Common Part Numbers
Standard Rectifier Triangle pointing to a straight line Triangle base Straight line bar 1N4007, FR107, M7 (SMD)
Zener Triangle to line with bent outward ends Triangle base Bent line bar BZX55C5V1, 1N4733A
Schottky Triangle to line with inward hooks Triangle base Hooked line bar 1N5819, SS34, BAT54
LED Triangle to line with two outward arrows Triangle base Straight line bar 5mm Standard, WS2812B
Photodiode Triangle to line with two inward arrows Triangle base Straight line bar BPW34, SFH203
TVS (Unidirectional) Triangle to line with a second opposing triangle Opposing triangle base Shared center line P6KE15A, SMAJ5.0A

Physical Markings & Regional Standard Variants

While the schematic symbols are globally harmonized under IEC 60617 and IEEE 315, physical component naming and marking conventions vary by the standard body that registered them. Knowing these prefixes helps you identify a diode's purpose when the datasheet is missing.

Pro Tip: The prefix tells you the origin and general class, but the numbers dictate the specs. Always cross-reference the full alphanumeric code with a manufacturer datasheet (like Vishay's 1N400x series) before substituting parts.
  • JEDEC (US/Global): Uses the 1N prefix (e.g., 1N4148, 1N4007). This is the most common standard for discrete through-hole and axial diodes worldwide.
  • Pro Electron (European): Uses two-letter prefixes. BA indicates a signal/mixer diode, BD a power/rectifier diode, BZ a Zener diode (e.g., BZX85C12).
  • JIS (Japanese): Uses the 1S prefix for high-frequency and switching diodes (e.g., 1S92, 1S1763).
  • SMD Marking Codes: Surface-mount packages (SOD-123, SMA, SMB) are too small for full part numbers. A standard 1N4148 in a SOD-80C package is marked LL4148 or simply A6. A standard 1A rectifier in SMA is marked M7. You must look up the specific SMD code via the manufacturer's coding matrix (see Nexperia's SMD code database).

The 'Rows People Get Wrong' Notes

Even experienced builders make orientation mistakes with specific diode sub-types because their operational bias contradicts their physical symbol intuition.

1. Zener Diodes Are Used 'Backwards'

In a voltage regulation circuit, a Zener diode is operated in reverse breakdown. This means the cathode (the striped end) connects to the positive voltage rail, and the anode connects to ground. If you wire it like a standard rectifier (anode to positive), it will just act as a normal diode with a 0.6V drop, failing to regulate your target voltage.

2. The LED 'Anvil vs. Post' Myth

Old textbooks claim that inside a standard 5mm LED epoxy dome, the larger internal metal piece (the anvil) is the cathode, and the smaller piece (the post) is the anode. Do not rely on this. Modern manufacturing, especially with cheap overseas 5mm LEDs, frequently reverses these internal structures. Always identify the LED cathode by the flat spot on the plastic rim or the shorter wire lead. If both are trimmed, use a multimeter.

3. Photodiodes and Reverse Bias

Like Zeners, photodiodes (e.g., BPW34) are typically used in reverse bias for light sensing. The cathode connects to the positive supply, and the anode connects to the transimpedance amplifier input. Wiring them forward-biased turns them into weak solar cells rather than fast optical sensors.

Safe Interpretation When Markings Are Faded or Missing

Heat, flux residue, and age can obliterate the cathode stripe on axial diodes. Never guess orientation in a power supply; a reversed rectifier on a mains transformer secondary will cause a dead short and catastrophic failure. Use a digital multimeter's diode test mode to definitively identify the terminals.

Safety First: Never test a diode in-circuit while power is applied. Disconnect power, discharge all filter capacitors with a bleeder resistor, and ideally lift one leg of the diode out of the PCB to prevent parallel circuit paths from giving false readings.

Follow this procedure, referencing standard Fluke multimeter testing guidelines:

  1. Set your multimeter to the Diode Test mode (usually indicated by a diode symbol).
  2. Touch the Red probe to one terminal and the Black probe to the other.
  3. Forward Bias Reading: If the meter reads between 0.500V and 0.700V (for silicon) or 0.200V and 0.300V (for Schottky/Germanium), the terminal touching the Red probe is the Anode, and the Black probe is on the Cathode.
  4. Reverse Bias Reading: Swap the probes. The meter should now read OL (Over Limit) or 1 (depending on the brand). This confirms the diode is blocking reverse current and is not shorted.
  5. Failure Modes: If it reads 0.000V or beeps continuously in both directions, the diode is shorted. If it reads OL in both directions, the internal junction is open (blown).

Decision Tree: Selecting and Orienting Your Diode

Use this if-then matrix to lock in the exact component and orientation for your specific circuit requirement.

Circuit Requirement Decision Path Concrete Part Pick Orientation Rule
Mains AC to DC Rectification (1A max) Need high reverse voltage (>800V) and standard recovery time. 1N4007 (1000V, 1A) Cathode (stripe) points toward the positive DC output / filter capacitor.
Low-Voltage DC Buck/Boost Catch Diode Need fast switching and low forward voltage drop to prevent thermal runaway. SS34 (Schottky, 40V, 3A) Cathode (stripe) points toward the switching node (inductor/MOSFET junction).
High-Speed Logic Level Shifting / Clipping Need nanosecond reverse recovery time; current is under 200mA. 1N4148 or LL4148 (SMD) Cathode points toward the lower potential or ground reference.
Voltage Rail Regulation / Clamping Need to hold a node at a specific voltage via reverse breakdown. BZX55C5V1 (5.1V Zener) Cathode (stripe) connects to the POSITIVE rail. Anode to GND.
Protecting a 12V DC Motor from Back-EMF Need a flyback path for inductive kickback when the transistor switches off. 1N5819 or 1N4001 Cathode (stripe) points to the positive supply; Anode connects to the motor's switched ground side.

By matching the schematic symbol to the physical marking standard and verifying with a multimeter when in doubt, you eliminate the most common cause of PCB rework: reversed polarity semiconductor placement.