How to Identify Diode Markings

By John Papiewski
Close-up of LED circuit board.

Diodes are electronic components that permit the flow of electric current in one direction only, shutting current off when it reverses. These handy devices have dozens of important uses in circuits, including as power supplies and radio signal detectors. Because diodes have polarity, their packaging carries distinct markings that help you connect them correctly in a circuit.

Signal Diodes

The smallest discrete diodes are designed to handle about 100 milliamps of current. These devices are typically packaged in tiny glass cylinders with a connecting lead at each end. A stripe on the cylinder marks the diode’s cathode, making the opposite side the anode. With some diodes, you may need a magnifying glass to see the stripe clearly.

Mid-Power Diodes

Diodes used as rectifiers in small power supplies are usually rated between 1 and 5 amps of current, maximum. These components usually have a cylindrical body made of dark epoxy or ceramic with a white or silver stripe marking the cathode side.

High Power Diodes

Diodes designed to carry more than a few amps of current may get very hot during operation, so they come in standard metal packages that bolt to a heat sink for cooling. Although the manufacturer may stamp a diode symbol on the package, indicating the anode and cathode connectors, you can also determine the device’s polarity by the package itself. Component catalogs and specification sheets indicate the case type and how to connect the diode.

Surface Mount Diodes

In recent years, electronics equipment manufacturers have increasingly moved from traditional leaded components to surface-mount devices, or SMDs. They are less expensive than traditional components, lower in cost and are compatible with the high-speed, pick-and-place robots now common in electronic circuit assembly. Like their cylindrical counterparts, SMD diodes have a white stripe that marks the cathode end of the device.

Light-Emitting Diodes

Light-emitting diodes have similar electronic characteristics as plain diodes; they have a cathode and an anode, and they block current in the reverse direction. They are compact, rugged and efficient at emitting light, making them very useful in such applications as simple on-off indicators, numeric displays, video screens and room illumination. Traditional LEDs have a clear or colored epoxy dome with a flat spot indicating the cathode side. Additionally, the cathode’s lead is shorter than the anode’s. LED arrays and other complex packages follow industry standards. However, if the package is not clearly marked, you may have to look up the part’s specifications in a manufacturer’s catalog.

About the Author

Chicago native J.T. Barett has a Bachelor of Science in physics from Northeastern Illinois University and has been writing since 1991. He has contributed to "Foresight Update," a nanotechnology newsletter from the Foresight Institute. He also contributed to the book, "Nanotechnology: Molecular Speculations on Global Abundance."