If you are searching for a colour code diode chart similar to the 4-band resistor chart, you will not find one. Standard diodes do not use multi-colour bands to indicate their electrical values. Instead, a single coloured band (usually silver, black, or white) marks the cathode (negative terminal), while alphanumeric prefixes printed on the body define the semiconductor material, application, and specifications. In vintage European glass packages, tiny colour dots were sometimes used when text was too small to print.
Below is the complete reference for identifying diodes by their regional prefix standards, cathode band colours, and legacy dot systems, followed by a bench-tested method for identifying diodes when the markings have rubbed off.
Diode Identification Standards: JEDEC, Pro Electron & JIS
Unlike passive components, semiconductor naming conventions are strictly governed by regional standards bodies. The prefix on your diode tells you exactly which standard applies to your region and what the internal junction is made of. The table below maps the three dominant global standards.
| Standard | Primary Region | Prefix Format | Material / Junction Indicator | Common Examples |
|---|---|---|---|---|
| JEDEC | North America / Global | 1N + 3 or 4 digits (e.g., 1Nxxxx) | '1N' denotes one PN junction. Numbers are sequential assignments, not material codes. | 1N4007 (1A Rectifier), 1N4148 (Signal), 1N5819 (Schottky) |
| Pro Electron | Europe (IEC) | 2 Letters + Serial Number (e.g., BYxx, BZxx) | 1st Letter = Material (A=Ge, B=Si, C=GaAs). 2nd Letter = Application (Y=Rectifier, Z=Zener, A=Signal). | BY133 (Si Rectifier), BZX55C5V1 (Si Zener), BAT54 (Schottky) |
| JIS | Japan / Asia | 1S + 3 or 4 digits (e.g., 1Sxxxx) | '1S' denotes a semiconductor junction. Sequential numbering. | 1S92 (Signal), 1S1763 (Fast Recovery) |
| GOST / Soviet | Eastern Europe / Russia | Letter + Number + Letter (e.g., KD202) | 1st Letter = Material (K=Si, G=Ge, A=GaAs). 2nd Letter = Type (D=Rectifier, L=LED). | KD202 (Si Power Rectifier), AL307 (LED) |
How to read the Pro Electron rows: The Pro Electron system is the most descriptive. If you hold a BZX85C12, the B tells you it is Silicon, the Z tells you it is a Zener diode, and the 12 indicates a 12V nominal breakdown voltage. Conversely, a JEDEC 1N4742A is also a 12V Zener, but you must look up the datasheet to know that; the number '4742' carries no intrinsic mathematical meaning. For authoritative cross-referencing of these prefixes, the Electronics Tutorials diode guide provides excellent baseline characteristic curves for these standard families.
Cathode Bands, Legacy Color Dots & Package Markings
When makers ask about a "diode colour code," they are usually looking at the single band on one end of the cylindrical body. This band always indicates the cathode (the N-type material, which connects to the more negative side of the circuit in forward bias). However, the colour of this band varies by package type and manufacturer.
| Band / Dot Colour | Package Type | Meaning & Polarity | Typical Diode Families |
|---|---|---|---|
| Silver / Grey Band | DO-41, DO-15, DO-201AD (Axial) | Cathode (Negative). Standard for most through-hole silicon rectifiers. | 1N400x series, 1N540x series, 1N4148 |
| Black Band | DO-41, SMA, SMB (SMD) | Cathode (Negative). Often used on Schottky or fast-recovery diodes with black epoxy bodies. | 1N5817-1N5819, SS34, UF4007 |
| White Band | SMA, SMB, SMC (Surface Mount) | Cathode (Negative). Standard for SMD packages where the body is black or dark grey. | ES1J, US1M, M7 |
| Blue / Red / Green Dots | DO-35 (Vintage Glass Axial) | Legacy Pro Electron identifier. Used by Mullard/Philips when glass was too small for text. | Vintage BZY88 Zeners, OAA118 signal diodes |
| No Band (4 Pins) | WOB, DIP-4 (Bridge Rectifiers) | No single cathode band. Polarity is marked by a '+' or '-' on the plastic casing near specific pins. | W10M, KBPC5010, DF10M |
Rows People Get Wrong & Testing Faded Markings
Even experienced bench technicians make assumptions based on visual cues that lead to circuit failures. Here are the most common misidentifications regarding diode markings, followed by the definitive method for identifying a diode when the paint has rubbed off.
The Most Common Marking Mistakes
- Mistaking a 5-Band Resistor for a Diode: A 1/4W resistor with a blue body and 5 colour bands is a precision resistor, not a diode. Diodes do not use the EIA colour-band value system. If it has 4 or 5 distinct colour rings spanning the body, it is a resistor.
- Assuming All Black Bodies are 1A Rectifiers: While the ubiquitous 1N4007 is a black cylinder with a silver band rated for 1A, the 1N5822 is also a black cylinder with a silver band but is a 3A Schottky diode. Swapping them based purely on body colour will result in a reverse-voltage failure, as the 1N5822 is only rated for 40V PIV compared to the 1N4007's 1000V PIV.
- Confusing Pro Electron 'B' and 'A' Prefixes: In the Pro Electron system, B means Silicon (e.g., BY133) and A means Germanium (e.g., AA119). Germanium diodes have a much lower forward voltage drop (~0.3V) and lower maximum junction temperatures. Using a vintage 'A' prefix diode in a modern 120°C switching power supply will result in thermal runaway.
How to Safely Interpret Faded or Missing Markings
Heat, flux residue, and age frequently destroy the alphanumeric printing on DO-41 and DO-35 packages. When you cannot read the text, you must rely on the semiconductor physics of the PN junction. According to All About Circuits, the forward voltage drop ($V_f$) is dictated by the semiconductor material, which allows us to categorize unmarked diodes using a digital multimeter (DMM).
Required Tool: A DMM with a dedicated Diode Test mode (symbolized by an arrow and a line). Note that standard DMMs output between 2.0V and 3.0V in this mode.
- Isolate the Component: Remove the diode from the circuit. Testing in-circuit will yield false readings due to parallel impedance paths.
- Set the DMM: Turn the dial to the Diode Test mode.
- Forward Bias Test: Place the Red probe on the anode (the end without the band) and the Black probe on the cathode (the end with the band).
- Read the $V_f$ Value:
- 0.20V to 0.35V: You are holding a Schottky diode (e.g., 1N5819) or a vintage Germanium signal diode (e.g., 1N34A). These are used for high-frequency switching or low-voltage drop rectification.
- 0.50V to 0.75V: You are holding a standard Silicon rectifier or signal diode (e.g., 1N4007, 1N4148). This covers 90% of general-purpose bench components.
- 1.0V to 1.5V: Likely a high-current silicon rectifier, a fast-recovery diode, or an LED (if it emits light during the test).
- 'OL' or '1' (Overload): If it reads OL in both directions, the diode is blown (open circuit). If it reads OL in forward bias but shows a voltage in reverse bias, you are likely testing a Zener diode with a breakdown voltage lower than your DMM's test voltage, or the diode is installed backwards in your probes.
- Reverse Bias Test: Swap the probes (Red to cathode, Black to anode). A healthy diode will read 'OL' (infinite resistance). If it reads a low voltage or near 0.00V, the junction is shorted and the component must be discarded.
By combining the physical package size (which hints at the current rating, such as DO-41 for 1A and DO-201AD for 3A) with the $V_f$ measurement from your DMM, you can safely substitute an unmarked diode with a known JEDEC equivalent without needing to decipher the faded colour code or text.
