In any diode symbol or physical package, the cathode (negative side) is indicated by the vertical bar on the schematic symbol or the painted band/notch on the physical body. Current flows from the anode (triangle/positive) to the cathode (bar/negative) during forward bias. The anode connects to the P-type semiconductor material, while the cathode band marks the N-type material.
Master Diode Symbol & Physical Polarity Table
The following reference table maps the most common diode types across both major schematic standards and their physical through-hole and surface-mount (SMD) markings. Use this to cross-reference a PCB silkscreen or schematic symbol with the physical component on your bench.
| Diode Type (Example Part) | ANSI/IEEE 315 Schematic Symbol | IEC 60617 Schematic Symbol | Physical Marking (Through-Hole) | Physical Marking (SMD) |
|---|---|---|---|---|
| Standard Rectifier (1N4007) |
Triangle + Straight Vertical Bar | Triangle + Straight Vertical Bar (often enclosed in a circle) | Silver/White Band on Cathode end (Black epoxy body) | SMA/SMB/SMC: White/Black Band on Cathode end |
| Small Signal (1N4148) |
Triangle + Straight Vertical Bar | Triangle + Straight Vertical Bar | Black Band on orange glass body (Cathode) | SOD-323/SOD-123: Band on Cathode end |
| Schottky (SS34 / 1N5819) |
Triangle + Bar with 'S' hooks on ends | Triangle + Bar with 'S' hooks or inward bends | Silver Band (Cathode) on black epoxy or metal can | SMA/SMB: Band on Cathode. SOT-23: Pin mapping required |
| Zener (1N4733A) |
Triangle + Bar with bent 'Z' wings | Triangle + Bar with bent 'Z' wings | Black Band on glass body (Cathode) | SOD-123/SMA: Band on Cathode end |
| Dual Common Cathode (BAV70) |
Two Anodes pointing to one shared Cathode bar | Two Anodes pointing to one shared Cathode bar | N/A (Rare in through-hole) | SOT-23: Pin 3 is shared Cathode, Pins 1 & 2 are Anodes |
Rows People Get Wrong (and Standard Variants)
Even experienced makers misinterpret specific diode symbols and physical markings. Here are the most common errors and how to resolve them based on your regional schematic standard.
The 'Band Means Positive' Fallacy
The most frequent mistake is assuming the painted band on a physical diode represents the positive terminal because it looks like a 'plus' ring or a prominent marker. The band is always the cathode (negative). In forward bias, the anode (unmarked end) must be at a higher potential than the cathode (banded end) by at least the forward voltage drop (typically 0.6V for silicon).
Zener and Schottky Symbol Confusion
- Zener Diodes: The 'bent wings' on the cathode bar of a Zener schematic symbol do not change the polarity. The bar is still the cathode. The wings simply indicate that the diode is designed to operate in the reverse breakdown region.
- Schottky Diodes: The 'S' hooks on the bar denote a metal-semiconductor junction rather than a P-N junction. The polarity rule remains identical: the bar side is the cathode.
- LEDs: Light Emitting Diodes add outward-pointing arrows to the standard symbol to indicate photon emission. The triangle and bar remain exactly the same; the flat edge on a 5mm physical LED dome corresponds to the cathode (bar).
Regional Schematic Standards: ANSI vs. IEC
If you are reading schematics from different regions, you will encounter two primary standards. According to All About Circuits semiconductor references, the US-based ANSI/IEEE 315 standard uses a solid, filled-in triangle for the anode. The European/International IEC 60617 standard historically used an outlined (hollow) triangle, and sometimes encloses the entire diode symbol in a circle to denote a discrete semiconductor package. In both standards, the vertical line intersecting the point of the triangle is unequivocally the cathode.
When working with high-voltage microwave or CRT rectifiers (e.g., CL01-12), the physical package is often a long epoxy cylinder with no visible band. These contain multiple series diodes internally. The cathode is usually marked by a beveled edge, a printed dot, or a chamfered corner. Never guess polarity on HV stacks; a reverse-bias failure will result in a catastrophic short and potential arc flash.
SMD Package Polarity & Pinout Guide
Surface-mount diodes drop the long wire leads, making polarity markings smaller and highly dependent on the JEDEC/EIAJ package standard. Below is the data-dense reference for the most common SMD diode footprints.
| SMD Package (JEDEC) | Typical Dimensions (L x W) | Polarity Indicator | Common Diode Types Found |
|---|---|---|---|
| SMA (DO-214AC) | ~4.5mm x 2.5mm | Wide painted band on one end (Cathode) | SS34, 1N4007S, US1M |
| SMB (DO-214AA) | ~4.3mm x 3.6mm | Wide painted band on one end (Cathode) | SS54, SMBJ (TVS) |
| SMC (DO-214AB) | ~6.8mm x 5.8mm | Wide painted band on one end (Cathode) | SK34, SMCJ (TVS) |
| SOD-123 | ~2.7mm x 1.6mm | Painted band on Cathode end | 1N4148W, BAV16W |
| SOT-23 (3-pin) | ~2.9mm x 1.3mm | No band. Pin 3 is usually Cathode (check datasheet) | BAV99 (Series), BAT54 (Schottky) |
SOT-23 Edge Case: Unlike two-terminal SMA/SMB packages, 3-pin SOT-23 packages (like the BAV99 dual series diode) do not use a painted band. Instead, you must locate Pin 1 using the small etched dimple or notch on the PCB silkscreen, then count counter-clockwise. For a standard BAV99, Pin 1 is Anode 1, Pin 2 is Anode 2, and Pin 3 is the shared Cathode. Always verify against the specific manufacturer's datasheet, as ON Semiconductor and Nexperia may use different internal die configurations for the same package outline.
Safe Interpretation When Markings Are Faded or Missing
Physical diode markings degrade. The black paint on a 1N4148 glass envelope can flake off after years of thermal cycling, and the white epoxy band on an SMA Schottky diode can be obscured by flux residue or conformal coating. When visual inspection fails, use your multimeter to definitively identify the cathode.
The Multimeter Diode Test Procedure
- De-energize the Circuit: Never test diode polarity in a live circuit. Capacitors must be discharged, and the diode should ideally be lifted from the PCB (or at least one leg desoldered) to prevent parallel circuit paths from skewing the reading.
- Set the Meter: Turn your multimeter dial to the Diode Test mode (usually indicated by a diode symbol: −|▷−).
- Forward Bias Test: Place the Red probe on one terminal and the Black probe on the other.
- If the meter reads a voltage drop between 0.2V and 0.3V, you are testing a Schottky diode. The Red probe is on the Anode, and the Black probe is on the Cathode.
- If the meter reads between 0.5V and 0.7V, you are testing a standard silicon diode. Again, the Black probe is on the Cathode.
- If the meter reads 1.2V to 2.5V, you are likely testing an LED or a high-voltage silicon stack.
- Reverse Bias Verification: Swap the probes. The meter should now read OL (Over Limit) or display a '1' on the far left of the screen, indicating infinite resistance. The terminal currently touching the Black probe is the Anode; the terminal touching the Red probe is the Cathode.
Diagnosing Failed Components
If your multimeter reads 0.00V (or a dead short with continuity beep) in both directions, the diode has failed short-circuit. This is common in power supply rectifiers that have experienced thermal runaway or voltage spikes exceeding their Peak Inverse Voltage (PIV). If the meter reads OL in both directions, the diode has failed open-circuit, usually due to a blown internal bond wire from overcurrent. In either failure state, the component must be discarded; polarity identification is no longer relevant as the P-N junction is destroyed.
