As a bipolar component, a diode has an anode and a cathode at its two terminals, just like the positive and negative terminals in a circuit power source. This implies that the combination can be either anode + anode or anode + cathode. In fact, both combinations are correct and may coexist in a circuit.
Anode + Anode = Forward bias, allowing current conduction.
Anode + Cathode = Reverse bias, limiting or hindering current flow.
To accurately identify the anode and cathode of a diode is crucial for achieving the design objectives. In this article, TechSparks will introduce three techniques to help you quickly and accurately identify the two terminals of a diode.
Circuit Symbol Identification
Diode symbols appear in circuit diagrams, printed circuit boards, and even directly on components. Recognizing these symbols allows for easy identification of the diode’s anode and cathode. Below are some common diode symbols, and you will notice that, despite variations, they share a commonality: a triangular arrow and a line perpendicular to the circuit.
The direction of the arrow indicates the diode’s anode, while the line represents the cathode, making their differentiation very intuitive.
However, sometimes you may encounter double-arrow situations, as seen in the symbols for DIAC and TRIAC. In such cases, how do you distinguish the two poles of the diode?
In fact, when the diode symbol displays a bidirectional arrow or lacks an arrow altogether, it typically indicates that the diode can operate in both forward and reverse directions. In such cases, there is no need to consider specific anode and cathode distinctions.
This method of identifying the diode’s anode and cathode in circuit diagrams is highly useful, and the same principles apply on PCB. The following illustration depicts the representation of diode symbols on a PCB:
In the manufacturing of diodes, certain distinguishing elements are added to facilitate user identification of the two poles. We can use these features for recognition. For example:
In the diagram below, which represents a rectifier diode, you will notice a difference at the two ends of its body—specifically, one end is encased in a white ring. Similar features are common in other diodes, although the color may vary, with the wrapped end typically indicating the cathode.
In the case of a Schottky diode, the surface often includes distinct markings, such as vertical lines, circular rings, or colored dots. Regardless of the marking method, these symbols denote that end as the cathode. Additionally, the symbol for a Schottky diode may be directly printed on the surface, following the standard circuit symbol recognition method.
For a photodiode, the identification of its two poles is usually simpler. One common method is to observe the metallic encapsulation area, with the raised end being the anode, or to check the pins, where the longer pin is also the anode. For rectangular photodiodes, observe the marking, with the side receiving light indicating the anode.
In the case of a light-emitting diode (LED), the identification of the anode and cathode primarily relies on the pins—where the longer pin is the anode, and the shorter pin is the cathode. If the pin lengths are the same, examine the LED body, with the smaller side being the anode and the larger, bowl-shaped side being the cathode.
For high-power diodes, there is usually no need for additional comparisons, as they are easily distinguishable. The end with threads is the cathode, and the other end is the anode.
And so on…
There are numerous types of diodes, and not all can be listed here. The methods described above for identifying the poles based on the diode’s appearance are applicable in most cases. If you encounter exceptions, it is recommended to use a multimeter or other instruments for detection.
The working principle of a diode restricts the flow of current from one end to the other. Utilizing this principle, we can precisely identify the two poles of a diode using a multimeter:
Before initiating the testing, it’s essential to understand the construction of the multimeter, including the display screen, function switch button, and test probes. The digits or “OL” on the display screen are crucial for determining polarity. While both diode mode and ohmmeter mode can be used for testing, for convenience, it is recommended to set the multimeter to diode test mode. According to the standard electronic color code, typically, the red probe signifies the positive (+) end, while the black probe signifies the negative (-) end.
During testing, touch the two probes to the diode’s two pins, and then observe:
If the display screen shows a diode forward voltage drop (usually around 0.5V-0.7V): This indicates that the diode is in forward bias, and the pin touched by the positive probe is the anode, allowing current to flow.
If the display screen shows “OL” (Over Limit): This signifies that the diode is in reverse bias, and the pin touched by the positive probe is the cathode, preventing current flow.
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