A diode is a simple device that allows current to flow in one direction only. It is composed of a P-type semiconductor and an N-type semiconductor, which form a P-N junction interface. This creates a space charge region and an intrinsic electric field on both sides. Different manufacturers use different color codes for the anode and cathode of diodes, making it difficult for beginners to distinguish between the two. This article explains how to identify the anode and cathode of a diode. For a regular diode, the colored end marks the cathode.

For light-emitting diodes (LEDs), the longer leg is the anode (positive), and the shorter leg is the cathode (negative). If the legs are the same length, the larger internal structure is the cathode, and the smaller one is the anode. Some LEDs have a flat side; the lead nearest to the flat side is the cathode.

 

Using a multimeter: Connect the red probe to the positive terminal (+) and the black probe to the negative terminal (-). When measuring an LED with a multimeter, the low resistance range may not work; instead, use the RX10K range. Touch the probes to the diode's leads. If the resistance is low, the black probe is connected to the anode. If the resistance is high, the black probe is connected to the cathode. When using LEDs with TTL components, typically a 470-ohm series resistor is used to prevent damage.


A crystal diode consists of a PN junction, two electrode leads, and a package. The leads are connected to the PN junction and encased, forming the crystal diode. The symbol for a crystal diode is V. The conduction direction of the PN junction is from the P-type semiconductor to the N-type semiconductor, i.e., P to N (P is the anode, N is the cathode). The PN junction conducts in the forward direction and blocks in the reverse direction, allowing unidirectional conduction.

On printed circuit boards (PCBs), diodes are typically identified by silk-screen markings:

1. The end with a notch is the cathode.
2. The end with a horizontal line is the cathode.
3. The end with white double lines is the cathode.
4. The end in the direction of the triangle arrow is the cathode.
5. For through-hole diodes, the end with a small circle is the cathode, and the large circle is the anode. When soldered vertically, the component body is within the anode ring.
6. For through-hole LEDs, the square hole indicates the first pin, which is the anode.

Regardless of the color code used by different manufacturers, the positions of the anode and cathode are fixed. You can determine the anode and cathode using an ohmmeter or by observing the leg lengths. For surface-mount diodes, observe the colored stripe; the side with the stripe is the cathode, and the other side is the anode. By mastering these methods, you can quickly and accurately identify the anode and cathode of a diode.