The standard diode symbol is a solid triangle pointing toward a vertical line. The triangle represents the anode (positive), the vertical line represents the cathode (negative), and conventional current flows in the direction the triangle points. If you are reading a schematic and need to identify a specific semiconductor junction, use the reference table below to match the symbol to its physical component.
The Complete Diode Symbol Reference Table
The following table covers the most common diode variants you will encounter in power supplies, RF circuits, and microcontroller protection networks. The "Cathode Bar Shape" is the primary visual differentiator on schematics.
| Schematic Symbol Description | Component Type | Cathode Bar Shape | Real-World Part Example | Primary Application |
|---|---|---|---|---|
| Triangle + Straight Bar | Standard PN Rectifier | Straight vertical line | 1N4007, 1N5408 | AC/DC power rectification, reverse polarity protection |
| Triangle + Straight Bar + Signal Arrows | Standard Signal Diode | Straight vertical line | 1N4148, 1N914 | High-speed switching, logic gating, clamping |
| Triangle + Hooked Bar | Schottky Diode | Bar bends inward toward the triangle (like an 'S') | BAT54, 1N5819, SS34 | Low forward voltage drop (0.3V) rectification, solar bypass |
| Triangle + Bent Bar | Zener Diode | Bar bends outward away from the triangle (like a 'Z') | BZX55C5V1, 1N4742A | Voltage regulation, overvoltage crowbar circuits |
| Triangle + Bar + Outward Arrows | Light Emitting Diode (LED) | Straight line with two arrows pointing away | Standard 5mm, WS2812B | Visual indication, optical isolation, lighting |
| Triangle + Bar + Inward Arrows | Photodiode | Straight line with two arrows pointing toward it | BPW34, SFH203 | Light sensing, fiber optic receivers, solar cells |
| Triangle + Bar + Extra Parallel Line | Varactor (Varicap) | Straight line with a second parallel line at the cathode | BBY40, 1N5450 | Voltage-controlled capacitance for RF tuning |
Regional Standards and Faded Marking Interpretation
When reading schematics, you will primarily encounter two standardization bodies: IEEE 315 (dominant in North America) and IEC 60617 (dominant in Europe and international projects). Fortunately, the triangle-and-bar diode symbol is one of the few components where IEEE and IEC are virtually identical. Both use the solid triangle and straight bar. However, in older IEC schematics or specific regional military specs, you may occasionally see an outlined (hollow) triangle instead of a solid black one. Electrically, they mean the exact same thing.
On the workbench, physical diode markings often fail. The black cathode band on a DO-41 glass package (like a 1N4148) can flake off, and the silkscreen polarity mark on an SMA or SOD-123 SMD pad can be scorched by rework heat. Never guess polarity on a mains-connected rectifier or a lithium battery protection circuit. A reversed power diode will result in a dead short and potentially an explosive failure.
Instead, use the diode test mode on your digital multimeter. Place the red probe on the suspected anode and the black probe on the suspected cathode. A healthy silicon diode will read a forward voltage drop between 0.500V and 0.700V. A Schottky will read 0.200V to 0.300V. Reverse the probes, and the meter should read "OL" (Over Limit). If it reads near 0.000V in both directions, the junction is shorted; if it reads "OL" in both directions, it is open.
Rows People Get Wrong in Schematic Capture
When drawing schematics in KiCad, Altium, or EasyEDA, it is easy to select the wrong symbol variant from the library. Here are the most common mix-ups and their consequences:
- Zener vs. Schottky Cathode Bars: This is the most frequent error. Remember the mnemonic: the Zener bar bends outward (like the letter Z), while the Schottky bar hooks inward (like the letter S). If you assign a Zener symbol to a Schottky footprint, your SPICE simulation will use the wrong breakdown model, completely invalidating your transient analysis and reverse-recovery timing.
- LED vs. Photodiode Arrows: The arrows indicate photon movement. If photons are leaving the device (emission), it is an LED, and the arrows point outward. If photons are entering the device (reception), it is a photodiode, and the arrows point inward. Swapping these in an optocoupler design will confuse the PCB layout engineer regarding which side is the emitter and which is the detector.
- Standard Rectifier vs. Signal Diode: While both use a straight cathode bar, signal diodes (like the 1N4148) are sometimes drawn with a small circle around the entire symbol in older educational schematics to denote a small-signal glass package. In modern CAD libraries, they look identical to power rectifiers; you must rely on the reference designator (D1 vs D2) and the bill of materials (BOM) to differentiate a 1A 1N4007 from a 300mA 1N4148.
Frequently Asked Questions
What is a diode symbol on a digital multimeter?
The diode symbol on a multimeter dial looks exactly like the standard schematic symbol: a triangle pointing into a vertical line, often with a small plus sign on the anode side and a minus sign on the cathode side. When you turn the dial to this setting, the meter outputs a small, constant test current (usually 1mA to 2mA) through the probes. It then measures the voltage drop across the component. This is the only reliable way to identify the anode and cathode on an unmarked or faded diode, and to verify that the PN junction has not failed short or open.
Which side of the diode symbol is the cathode?
The vertical line (the bar) is always the cathode. The triangle is the anode. Conventional current flows from the anode (triangle) to the cathode (bar). If you are looking at a physical PCB and trying to match it to the schematic, the physical component will have a printed band, a stripe, or a thicker line on the cathode end. For SMD packages like SOD-123 or SMA, the cathode band is usually white or black, contrasting with the body color. Always verify with a multimeter if the board has been subjected to high heat or chemical cleaning, which can strip SMD markings.
How do I identify a SMD diode symbol when the PCB silkscreen is missing?
If the PCB silkscreen (the white paint indicating polarity) is scratched off or missing, look at the copper pads and the component body. For SMD Schottky and rectifier diodes (SMA, SMB, SMC packages), the cathode band is painted directly on the component body. For SOD-123 and SOT-23 packages, the cathode is usually indicated by a single band on the body. If the component is a dual-diode in a SOT-23 package (like a BAV99), you must consult the specific manufacturer datasheet, as pin 1, 2, and 3 configurations vary wildly between series and parallel internal wiring. Use your DMM diode test mode to map the internal junctions against the datasheet's pinout table.
