The cathode (negative side) of a standard diode is always marked with a physical band, stripe, or printed line, while the anode (positive side) is the unmarked lead. On a schematic, the cathode is the vertical line and the anode is the flat base of the triangle. Conventional current flows from anode to cathode when forward-biased. Because surface-mount packages and high-power rectifiers use wildly different physical conventions, relying on memory alone leads to reverse-bias failures. Use the reference table below to map your physical component to its correct pinout.
Complete Diode Package and Pinout Reference Table
| Package Type | Common Part Numbers | Anode (Positive) ID | Cathode (Negative) ID | Pin Count / Config |
|---|---|---|---|---|
| DO-41 (Axial Leaded) | 1N4001-1N4007, 1N5817 | Unmarked wire lead | Wide painted band near lead | 2 Pins (Through-hole) |
| DO-35 (Glass Axial) | 1N4148, 1N914, BZX55 | Unmarked wire lead | Black band (or multiple color bands for Zeners) | 2 Pins (Through-hole) |
| SOD-123 / SOD-323 (SMD) | 1N4148W, BAT54, BZT52 | Unmarked side of plastic body | White or black printed stripe on top/side | 2 Pins (Surface Mount) |
| TO-220-2L (Power) | MUR860, MBR1045, STPS20H100 | Left pin (viewed from front, tab up) | Right pin AND the metal mounting tab | 2 Pins + Tab (Tab = Cathode) |
| KBPC / WOB (Bridge Rectifier) | KBPC5010, W10M, KBU806 | Marked with '+' (DC Output) | Marked with '-' (DC Output); AC marked with '~' | 4 Pins (2 AC, 1 DC+, 1 DC-) |
| SOT-23 (Dual SMD) | BAV99, BAT54C, BAV70 | Depends on internal config (see notes) | Depends on internal config (see notes) | 3 Pins (Dual diode configurations) |
What Each Row Means in Practice
- DO-41 / DO-35: The classic through-hole standard. The cathode band is physically closer to the cathode lead. If the paint is chipped, the cathode lead is often slightly thicker or has a different metallic sheen at the glass seal, but always verify with a meter.
- SOD-123/323: The stripe is almost always on the cathode. However, on ultra-miniature packages like SOD-523, the stripe can be incredibly faint. Look for the asymmetrical chamfer on the plastic body; the chamfered edge usually denotes the cathode.
- TO-220-2L: The metal tab is internally bonded to the cathode pin. This is critical for heatsinking: if you mount a TO-220 diode to a shared metal chassis without an isolating mica or silicone pad, you will short the cathode to ground.
- Bridge Rectifiers: Never guess the AC pins. The two pins marked with '~' or 'AC' are the anode/cathode junctions. The '+' pin is the common cathode output, and the '-' pin is the common anode output.
Schematic Symbols and Regional Standards
When reading schematics, the symbol variant depends on the drafting standard used by the engineer. While the basic diode symbol is largely harmonized globally, specialty diodes (Zeners, Schottkys, LEDs) have distinct regional variations.
| Feature | ANSI/IEEE 315 (North America) | IEC 60617 (Europe / Global) |
|---|---|---|
| Basic Diode | Solid filled triangle with a straight vertical bar. | Unfilled (outline) triangle with a straight vertical bar, often enclosed in a box. |
| Zener Diode | Straight bar with bent 'Z' shape at the ends. | Vertical bar bent into an 'N' or 'U' shape at the ends. |
| Schottky Diode | Vertical bar with inward hooks at both ends. | Similar to ANSI, but sometimes drawn with a distinct 'S' curve on the cathode bar. |
| Part Numbering | JEDEC standard (e.g., 1N4007, 1N4148). The '1N' denotes one junction. | Pro Electron standard (e.g., BA159, BYW98, BZX84). First letter = material (B=Si, A=Ge). |
Which standard applies to you? If you are reading schematics from US-based firms, open-source hardware from North America, or older textbooks, expect ANSI/IEEE 315 symbols and JEDEC part numbers. If you are working with European automotive electronics, industrial PLC schematics (Siemens, ABB), or modern global datasheets from Nexperia and STMicroelectronics, IEC 60617 symbols and Pro Electron part numbers dominate. According to the All About Circuits semiconductor guide, modern CAD tools like Altium and KiCad often blur these lines by offering both symbol libraries, so always cross-reference the part number rather than relying solely on the symbol shape.
Rows People Get Wrong and Faded Marking Recovery
Diode misidentification rarely happens with a fresh, clearly marked 1N4007. It happens in the repair queue when dealing with SMD codes, Zener voltage bands, and heat-faded glass packages.
The 'Rows People Get Wrong' Notes
- Zener Color Bands: On DO-35 glass Zeners (like the 1N47xx series), the bands indicate the voltage, not just the cathode. A 1N4733A (5.1V) has three bands. The rule is: the band closest to the lead is the cathode, but if there is a distinctly wider band or a black isolation band, that marks the cathode. Always verify with a datasheet.
- SOT-23 Dual Diodes: A 3-pin SOT-23 SMD diode is a trap for beginners. The marking code (e.g., 'A7') does not tell you the internal wiring. A BAV99 is a series pair (Pin 1 = Anode 1, Pin 3 = Cathode 1/Anode 2, Pin 2 = Cathode 2). A BAT54C is a common cathode pair (Pins 1 and 2 are anodes, Pin 3 is the shared cathode). You must look up the specific marking code in a SMD codebook.
- TO-220 Tab Assumptions: While the tab is the cathode on 95% of standard rectifiers (MUR860), on some specific voltage regulator ICs or specialized power packages that look identical, the pinout shifts. Never assume the tab is cathode without checking the manufacturer's mechanical drawing.
Safe Interpretation When Markings Are Faded or Missing
When a DO-35 glass diode has been baked by a nearby power resistor and the paint has flaked off, or an SMD diode has no visible stripe, do not guess. Use the diode test function on your digital multimeter (DMM).
- Set your DMM to the diode test mode (usually indicated by a diode symbol and a soundwave).
- Place the red probe on one lead and the black probe on the other.
- A standard silicon diode (1N4007, 1N4148) will show a forward voltage drop (0.500V to 0.700V).
- A Schottky diode (BAT54, 1N5817) will show a lower drop (0.150V to 0.350V).
- Reverse the probes. A healthy diode will read 'OL' (Open Loop) or '1' on the display, indicating infinite resistance.
- The lead connected to the red probe when you get the 0.6V reading is the Anode. The lead connected to the black probe is the Cathode.
For comprehensive testing procedures and safety thresholds, refer to the Fluke guide on testing diodes, which details how to interpret marginal forward voltage drops that indicate a degrading, rather than fully failed, silicon junction.
By combining physical package identification with standard-specific schematic reading and empirical DMM verification, you eliminate the guesswork that leads to reverse-bias short circuits and blown traces on your workbench.
