LED Anode and Cathode
July 29, 2026
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Source: lideda
When working with LEDs, correctly identifying the anode (+) and cathode (-) is essential. Connecting an LED with the wrong polarity will prevent it from lighting (although most standard LEDs are not damaged by brief reverse connection within their ratings).
What Are the Anode and Cathode?
Anode (+): The positive terminal of the LED. It should be connected to the positive side of the power supply through a current-limiting resistor or LED driver.
Cathode (-): The negative terminal of the LED. It connects to the negative side (ground) of the circuit.
Current flows from the anode to the cathode when the LED is forward biased.
How to Identify LED Polarity
1. Through-Hole LEDs
The easiest way to identify polarity is by looking at the leads:
Long lead = Anode (+)
Short lead = Cathode (-)
You can also inspect the LED package:
The flat edge on the plastic body indicates the cathode (-).
Inside the LED:
The larger internal metal cup is usually the cathode.
The smaller internal electrode is the anode.
2. SMD LEDs
SMD LEDs do not have long and short leads, so manufacturers use polarity marks such as:
Green or black line
Triangle mark
Dot or notch
Chamfered corner
The exact marking varies by package type (2835, 3030, 3535, 5050, etc.), so always refer to the manufacturer's datasheet.
Using a Multimeter
A digital multimeter with a diode test mode can quickly identify polarity.
Set the meter to diode mode.
Connect the red probe to one terminal.
Connect the black probe to the other terminal.
If the LED lights faintly or displays a forward voltage (typically 1.8–3.6 V depending on color), then:
Red probe → Anode
Black probe → Cathode
If nothing happens, reverse the probes.
Typical Forward Voltage
| LED Color | Typical Forward Voltage |
|---|---|
| Red | 1.8–2.2 V |
| Yellow | 2.0–2.2 V |
| Green | 2.0–3.2 V |
| Blue | 2.8–3.4 V |
| White | 2.8–3.6 V |
| UV | 3.0–3.8 V |
Common SMD LED Polarity Marks
| Package | Common Polarity Mark |
|---|---|
| 2835 | Green line or notch indicates cathode |
| 3030 | Corner notch or printed mark |
| 3535 | Chamfered corner or dot |
| 5050 | Triangle, notch, or printed polarity symbol |
Always verify the polarity using the component datasheet before assembly.
Common Mistakes
Assuming all SMD LEDs use the same polarity marking.
Forgetting the current-limiting resistor.
Confusing the flat edge with the positive terminal (the flat edge is usually cathode).
Installing LEDs backwards during PCB assembly.
Summary
Anode = Positive (+)
Cathode = Negative (-)
Through-hole LEDs: Long lead = Anode, Short lead = Cathode.
Flat edge on the package usually indicates the cathode.
SMD LEDs use package-specific polarity markings.
When unsure, use a multimeter or consult the manufacturer's datasheet.
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