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How to Indicate Placement Orientation of LED on Your Boards

A proper placement, labeling and orientation of components on a Printed Circuit Board (PCB) helps in reducing some of the common problems. When these things are in place, one can be sure that the components wouldn’t be exposed to short-circuiting or a plethora of other electrical issues.

The two most common components to watch out for are the anode and cathode. Due to the technicalities involved in the placement, you may be unsure of how one of them should be positioned.

In this article, we compare anode vs cathode LED and help you understand the right positioning to use for any of them.

Understanding the Concept of “LED”

Light Emitting Diode, also called LED, refers to the light bulbs comprising of a wide range of tiny diodes. The work or function of these diodes is to emit light when electrical current passes through them.

When compared to the traditional or standard diodes, the LED is known for one feature – the one-way direction. Ideally, the direction or pathway for the electrical current is channeled to one direction.

LED Polarity and the Debate on Anode vs. Cathode Diode

Placement Orientation of LED
Placement Orientation of LED

The two (2) major types of diodes you will find in a LED are anode and cathode. For what it is worth, you want to know the differences between the two. However, it is important to understand the basic – the polarity of the Light Emitting Diode.

LED polarity refers to the direction or pathway through which the electrical current is meant to flow in the diode. Since LEDs’ electrical current are designed to flow in one direction; it is therefore expedient to understand the right way to channel it.

It is on this basis that we introduce you to the two primary diodes – anode and cathode. You will learn about the different functions they perform in a LED.

What is an Anode?

It is a type of Light Emitting Diode (LED) that denotes the positive side of the diode. In this case, it is the terminal in the diode, through which the electrical current is meant to pass into or enter the diode.

What is a Cathode?

It has an opposite function to what the anode does. The function of the cathode in a LED is to provide the medium for the electrical current to leave the diode.

Despite the differences in how they process or facilitate electric current movement, the anode and cathode are two terminals you need in a LED.

How to Tell Anode vs. Cathode

The easiest way to differentiate between the two is to look out for the signs or polarity markings. While the anode has the positive sign (+), the cathode has the negative sign (-).

That is the surface differentiation metric. In the next paragraphs, we will let you into some of the features that will help you differentiate between a common anode vs. common cathode LED.

The Differentiations Based on the Electrical Charge

Since both the anode and cathode deal with processing electrical current; we can use that as a way to differentiate them.

For the anode diode, it attracts a negative electric charge. This is usually a positively-charged electrode and helps the anode to act as an electron receptor. In this instance, it facilitates the occurrence of oxidation, which, in turn, attracts the negative charge.

On the other hand, the cathode attracts cation, which is the same as a positive charge. Despite the attraction of a positive electric charge, the cathode is also susceptible to the negative charging of the electrode.

To this end, it is expedient for the cathode to facilitate the following:

  • It facilitates the generation or production of electrons that create a charge.
  • The negative charge, when created, is used to facilitate the movement of the current from cathode to anode.

Techniques for Identifying Anode and Cathode in a LED


If you are intent on determining which of the diodes is anode and cathode, here are some of the steps or procedures to doing so:

1. Check the Diode’s Length

By merely looking at the diodes, you can tell which is which. The length is a giveaway for doing this.

Ideally, the diode with a shorter pin is the cathode and the one with a longer pin is the anode. Note that the best way to discover this is by looking at the length of the pin at the end of the diode.

2. Check the Notch

You can also use the notch to determine which of the diodes is a cathode and anode. For this exercise, look at the small, flat notch on the side of the Light Emitting Diode.

The lead or pin that is closer to the notch is the cathode while the one that is a bit far away is the anode.

3. Use a Multimeter for the Test

It is possible for you to use a multimeter for the testing or finding out of the anode and cathode. It is better to use this as a last resort, especially if you are uncertain of the outcome of the previous exercises.

Here is a walkthrough on how to get it done:

  • Diode Setting Activation: the first step is to turn on the multimeter to its diode setting. This helps you make a foolproof testing.
  • Probe Usage: once the setting is on, you can then use the multimeter’s probes on each of the LED’s pins.
  • Diode Lighting: essentially, what you are looking for is the diode lighting up on the pins. The pin that on getting touched with the probe lights up the diode, shows that you have matched the positive probe with the anode and the negative probe with the cathode.

Note: it is possible not to get the desired results immediately when using the multimeter for the test. If that is the case, consider switching the diode’s terminals and retry the test.

4. Check the Schematic Symbol

The schematic symbol could also be of help in telling which of the terminals is a cathode and an anode. Ideally, this is possible due to the small line drawn near the cathode’s pin, and which makes a direct correlation with the small, vertical line in the diode circuit’s symbol.

Although it could sometimes be faint, it is one of the best ways to determine the cathode and anode terminals in the diode.

Anode vs. Cathode Diodes: The Major Differences

Besides the length of the pins and the markings, here are a couple of additional differentiators between these two diodes:

1. Type of Terminal

The anode acts as the positive terminal, while the cathode as the negative terminal.

2. Type of Charge

The anode acts as the donor of electron, while the cathode plays the role of electron acceptor/receptor, in the sense that it assists in the generation of the charge that moves the current from cathode to anode.

3. Electric Current Flow

The anode terminal is the direction from where electrical current moves into the LED, while the cathode is the direction for the current to leave the diode.

4. Positivity and Negativity

Due to the types of charge that they generate and the current flow; anode and cathode are denoted by the type of current they process.

On the one hand, we have the anode serving as the positive terminal and the cathode serving as the negative terminal of the LED.

5. Oxidation vs. Reduction

Anode facilitates the oxidation that leads to the creation of a negative charge. On the other hand, reduction occurs in the cathode.

6. The Pin’s Length

The length of the pins can also help you find out which of the terminals is a cathode and which is the anode.

Sometimes, it could be difficult to determine this due to the fact that the pins may have been subjected to manual cutting. In that case, they tend to have a similar length.

However, you can easily find out the cathode by close examination of the outer casing’s edge. Here is how to go about it:

  • Find out if one side of the outer casing is flat and lined up with the diode.
  • Find out if the other side of the casing extends slightly.
  • If those parameters are met, there is a chance that the cathode is the pin nearest to the flat edge.

What Color is the Cathode and Anode?

Color could be an important consideration when looking to differentiate between cathode and anode terminals in a diode.

The simplest rule is to check the signs or markings. Usually, the positive side of the diode, the anode, is marked with the red color, while the negative is marked with a black color.

Tackling Ambiguity and Inconsistency in LED Placement and Orientation

There is a chance that the standards set out for the placement and or orientation of the Light Emitting Diodes (LEDs) are not followed.

Some of the standards are:

  • Using the positive sign (+) to mark or denote the anode and the negative sign (-) for the cathode.
  • There should be an arrow or marking moving from the diode’s pin to the vertical line in the diode’s circuit symbol.

However, the tendencies of ambiguity and inconsistency is common due to the following reasons:

Interchanged Diode Markings

The standard is to use the letter “A” to denote an anode in a diode and the letter “K” to denote the cathode.

However, depending on the manufacturer, this standard is not always followed. For example, some manufacturers could use a different alphabet.

Nevertheless, one of the benefits of using the alphabet “K” for the cathode instead of the letter “C” is to prevent the mistakes of taking it to be a capacitor.

Discarding of the Markings

As mentioned in the previous paragraphs, it is possible to use the positive sign (+) and the negative sign (-) to denote the anode and cathode terminals for the diodes.

However, there is a tendency for ambiguity, which often leads to the discarding of these markings. In their place and depending on the manufacturers’ preferences, a dot and a bent outline could be used to denote the anode and cathode terminals.

THT vs. SMT Anode and Cathode Orientations

The LEDs assembled via the Through Hole Technology (THT) and the Surface Mount Technology (SMT) have distinct orientations.

The placement options for the THT include:

  • Marking of the Through Hole Device (THD) with a line at the cathode.
  • The usage of a longer pin or a flat edge for the Light Emitting Diodes (LEDs).

The Surface Mount Technology (SMT) process spells out the marking of the end of the cathode in the LED device. However, some manufacturers do not follow this and the ones that do, are not consistent with the implementation.

LED Component Orientation Benefits

Why should we place the Light Emitting Diodes, such as cathodes and anodes appropriately on the device?

Here are some of the reasons as to why you should do that:

  • It enables the proper alignment or positioning of the components in the right direction on the board or device. This way, it makes the soldering of the pins simpler.
  • An excellent LED polarity aids the connection of the diode to the right voltage source. This is an important consideration for making the supply of electric current possible.

LED & Resistor: When Do You Need the Latter?

Light Emitting Diodes (LEDs) require a specific voltage. However, if this voltage is exceeded, there is a chance that it will negatively impact the device’s performance. To this end, you would need to use a resistor to help put things back in good shape.

Here is how to maximize the use of the resistor:

  • Place the resistor between one of the diode’s leads or pins and the connection to either the Ground (GND) or the supply voltage.
  • The right amount of resistance should be used, as it helps to regulate the Light Emitting Diode (LED)’s capability to function optimally, and without burning itself out.

Final Words

Using a Light Emitting Diode (LED) helps in streamlining the flow of electricity, extending the lifespan of devices and conserving energy.

Using the tips in this article, you should now be able to place and orient the anode and cathode diodes appropriately in a LED device.

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