Everything You Need to Know About Capacitor Polarity

A capacitor is an electronic component used for storing and releasing electrical energy, consisting of two conductive materials (commonly referred to as electrodes or plates) and a layer of insulating material (called a dielectric). When a capacitor is connected to a power source, the voltage applied by the power source creates an electric field between the two electrodes, allowing the capacitor to perform its function. But how should you connect a capacitor to a circuit? Do you need to consider its polarity? In this edition of TechSparks, we will delve into the topic of capacitor polarity, including the concept of polarity, identification techniques, and more. Let’s explore!

Polarity Differentiation in Capacitor

In the field of electronic components, the term “polarity” refers to whether a component has distinct positive and negative terminals. If so, it means that current can only flow through these components in a specific direction. Reversing the polarity during installation can lead to poor performance or even explosions, posing a risk to the entire circuit and device.

So, do capacitors have polarity?

The answer is yes. Capacitors used in electronic devices can be categorized into polarized capacitors and non-polarized capacitors based on their polarity. Therefore, designers need to identify and correctly install capacitors’ polarity during circuit design to ensure the circuit’s normal operation and long-term stability.

Polarized and Non-Polarized Capacitor

The fundamental distinction between polarized and non-polarized capacitors lies in their internal structure and operating principles. Polarized capacitors typically consist of two metal layers: an anode and a cathode. The anode is typically made of metal, while the cathode is composed of an electrolytic material. By forming an insulating oxide layer on the anode of polarized capacitors, they exhibit distinct positive and negative polarities, thereby restricting the flow of current in a specific direction. In contrast, non-polarized capacitors have a relatively simple structure, consisting of two electrodes and a dielectric layer. The dielectric layer material can be ceramic or polyester, allowing bidirectional flow of current, thus eliminating the need for a specific positive or negative polarity.

Characteristics of Polarized Capacitors:

  • Possess distinct positive and negative polarities and require correct polarity during installation.
  • The insulating oxide layer restricts the flow of current direction, making them widely applicable in DC circuits.
  • Primarily used in circuits requiring larger capacitance values, such as power supply filtering.

Characteristics of Non-Polarized Capacitors:

  • Do not require a specific positive or negative polarity and can be installed arbitrarily.
  • The dielectric layer allows bidirectional flow of current, eliminating the need to consider polarity.
  • More suitable for AC circuits, such as audio signal processing and coupling circuits.

Which Capacitors are Polarized

To achieve specific functions and purposes, different types of capacitors are chosen and used in your designs. So, which capacitors are polarized, and which ones are not?

Typically, electrolytic capacitors and tantalum capacitors are polarized. You can find positive and negative polarity markings on the capacitor’s casing, and it’s important to pay attention to these markings and connect the circuit correctly when using them. On the other hand, ceramic capacitors and film capacitors are non-polarized and can be installed in any direction. Here’s a brief overview of these capacitor types:

Electrolytic Capacitor

These capacitors use an electrolyte as the dielectric and consist of metal plates, electrolyte, and dielectric. The electrolyte plays a crucial role in storing charge and maintaining the polarization state of the capacitor. They are known for their high capacitance and voltage ratings and are commonly used in filtering, voltage regulation, and coupling circuits.

Electrolytic Capacitor

Tantalum Capacitor

A special type of electrolytic capacitor, tantalum capacitors are renowned for their use of tantalum as the anode material, providing high capacitance and low ESR (Equivalent Series Resistance). They are suitable for applications requiring high performance and stability, such as mobile phones, computers, and medical devices.

 

Tantalum Capacitor

Ceramic Capacitor

These capacitors use ceramic material as the dielectric, offering advantages like fast response, good stability, and high temperature stability. They are commonly used in high-frequency and high-temperature environments in circuits such as RF circuits, filtering circuits, and tuning circuits.

Ceramic Capacitor​

Film Capacitor

Film capacitors use thin film or metallized plastic film as the dielectric and offer excellent frequency response, temperature stability, and durability. They are frequently used in precision circuits, such as audio amplifiers, antenna matching circuits, and filters.

Film Capacitor

How to Identify Capacitor Polarity

Visual Inspection Method

This is perhaps the simplest way to identify capacitor polarity. You can find clear markings on the capacitor casing indicating its polarity, such as:

For through-hole capacitors, polarity can be identified using the following methods:

  • Observing markings on the side of the capacitor.
  • Observing the length of the capacitor’s leads.

Through-hole capacitor polarity identification

For surface-mount capacitors, polarity is typically indicated by a prominent black or colored marking on the top, as shown below:

Tantalum electrolytic capacitor polarity determination

Multimeter Identification Method

In some cases, visually identifying capacitor polarity may be unreliable or difficult, such as when the capacitor leads are bent incorrectly or when the outer packaging is damaged. In such situations, using a multimeter to identify capacitor polarity is the most reliable method.

You need to set the multimeter to resistance mode, then connect the red probe to one terminal of the capacitor and the black probe to the other terminal. If the resistance reading starts low and then increases, the terminal connected to the red probe is positive. If the resistance reading remains low or at zero, switch the probes.

Is Identifying Capacitor Polarity Enough to Ensure Safety

Skilled designers know that the PCB manufacturing process starts with design and encompasses the entire workflow from schematic library design, schematic design, PCB footprint design, to PCB layout design, ultimately outputting Gerber files for manufacturers to complete assembly and production. Any error in this process can lead to PCB failure.

However, sometimes even when all steps appear to be correct, issues may still arise. For example, you receive a custom PCB and find that a capacitor is faulty. Upon inspection, you confirm that the capacitor’s silkscreen and soldering polarity are correct, and the voltage rating is appropriate. At this point, many people feel puzzled and unsure where the problem lies.

Experienced designers may shift their focus to the design itself since it has been determined that there are no soldering errors on the PCB. After carefully reviewing the schematic and PCB files, they might discover that while the schematic is correct, the silkscreen polarity on the PCB is reversed compared to the theoretical polarity on the schematic. In such cases, simply reversing the capacitor’s orientation and re-soldering it can resolve the issue.

The root cause of this error usually lies in the incorrect mapping of the capacitor’s pin numbers between the schematic library and the PCB footprint. For example, if the capacitor’s positive and negative pin numbers in the schematic are 1 and 2 (or 2 and 1), but the PCB footprint has the pin numbers as 2 and 1 (or 1 and 2), there is no 1-to-1, 2-to-2 pin mapping relationship, leading to a correct schematic but an erroneous PCB.

In circuit design, the schematic represents the theoretical form, while the PCB is the actual physical construction. They connect through the one-to-one mapping of pin numbers. Therefore, when designing circuits, it is crucial to ensure the pin mapping relationship between the schematic library and the PCB footprint library for each component, especially for polarized components with dual terminals.

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