Back to Capacitor Basics

This article is published by EE Power as part of an exclusive digital content partnership with Bodo’s Power Systems.

The humble capacitor, a key passive component, is an essential item used in virtually every electronics-based design. As students, we learn their prime circuit function, but rarely do we fully appreciate some of the factors that make them a less-than-ideal component. 

 

Figure 1. The current flows of a capacitor through charge and discharge cycles from a direct current battery. (Source: Mouser Electronics). Image used courtesy of Bodo’s Power Systems [PDF]

 

Introducing the Capacitor

The ability to hold or store an electrical charge in an electrostatic field is termed capacitance. An electrostatic field exists when a voltage exists between two points, such as two flat metal plates. A simple circuit to demonstrate how an electrostatic field is created is illustrated in Figure 1. When the charge switch is closed, the graphs highlight the current flows and voltage across the capacitor as it is charged from the battery. Once the electrostatic field between the plates has reached a maximum, the current reduces to zero. Opening the charge switch and closing the discharge switch reverses the process, with an initial high current flow that diminishes as the electrostatic field dissipates. Note that the circuit’s resistor equates to the circuit wiring’s minimal yet measurable resistance.

With a DC source, once charged, there is no current flow through the capacitor. However, an alternating current (AC) can flow through a capacitor, albeit with a lag or phase difference due to the changing charging cycles. The detailed physics of a capacitor are beyond the size constraints of this short introductory article, so a curious reader should consult a technical source such as those highlighted in the footnotes.

Capacitors are typically constructed using single or multiple pairs of parallel metal foil plates separated by an insulating dielectric material. The plates’ physical dimensions and the dielectric material’s electrical properties determine the capacitor’s value. The unit of capacitance is the Farad. A Farad is a relatively high value of capacitance for many small signal electronic circuits, so much smaller values such as microfarads (µF), picofarads (pF), and nanofarads (nF) are common. Large ‘supercapacitors,’ with values of tens and hundreds of Farads, are used for energy storage in many transportation applications.

A few typical use cases of capacitors include:

• Signal coupling of audio and radio frequencies to remove DC signal bias
• Decoupling of voltage transients to remove unwanted electrical noise from power supply lines
• In signal conditioning circuits together with inductors or resistors to create low pass, high pass, or bandpass filters, capacitors are available in various construction methods such as axial, radial, surface mount, and multi-capacitor integrated packages.

Capacitors fall into two specific groups: Non-polarised electrostatic capacitors and polarised electrolytic capacitors. Electrolytic capacitors usually have higher values than electrostatics and, since they are polarised, must be inserted correctly into a circuit. Electrostatic types have smaller capacitance values and may be inserted either way around into a circuit.

 

Capacitor Terminology

Like many electronic components, each has distinct features, and in this section of the article, we explain the key terms you will encounter.

Rated capacitance value: Measured in Farads and ranging from a few nanofarads to hundreds of Farads, they are usually available in popular EIA series values used for other passive components, such as E6 and E12. The capacitance value is usually quoted at 25°C and a specified frequency.

Tolerance specification: Together with the capacitor’s value, its tolerance indicates the likely variation from the stated nominal value—for example, 220pF ±10 %. Standard tolerances include ±5 % and ±10 %. Electrolytic capacitors typically have a larger tolerance range of up to ± 20%.

 

Figure 2. The EIA capacitor codes for marking capacitor value, tolerance, and working voltage. (Source: Mouser Electronics). Image used courtesy of Bodo’s Power Systems [PDF]

 

Working voltage: This indicates the maximum DC voltage the capacitor can withstand for continuous operation and may include an upper-temperature limit. The Electronics Industry Association (EIA) specifies coding groups for marking the value, tolerance, and working voltage on capacitors (Figure 2). Note that this is the maximum of a DC bias voltage with any superimposed AC waveforms. Common working voltages are 10 VDC, 16 VDC, and 25 VDC.

Dielectric types: Several popular dielectric types are available; the choice of dielectric significantly influences the capacitor’s characteristics and, consequently, the types of applications it suits. Popular types of dielectric materials are aluminium, tantalum, and ceramic. The article’s next section explains more information on how the dielectric type influences capacitor usage.

Working temperature and temperature coefficient: All capacitors have a maximum working temperature, which is significant for electrolytic capacitors since their service life reduces with increasing temperature. A capacitor’s temperature coefficient indicates how the temperature changes impact its capacitance value. Although the amount that the capacitance change is small, it is still a consideration for some applications. The coefficient is stated as parts per million per °C. Figure 3 illustrates the capacitance change curve against the temperature of a Murata ceramic radial leaded capacitor.

 

Figure 3. The variation of capacitance value against temperature for a typical ceramic capacitor. (Source: Murata). Image used courtesy of Bodo’s Power Systems [PDF]

 

DC Leakage Resistance: An ideal capacitor would not leak any direct current across the insulated plates, but internal leakage is a real-world characteristic of any capacitor. Consequently, a small proportion of the capacitor’s charge slowly leaks away. Leakage also causes a small current flow through the capacitor when charging. A capacitor’s datasheet will indicate the equivalent leakage resistance, which is a DC measurement. It is typically quoted in MΩ.

Equivalent Series Resistance and Impedance: Figure 4 illustrates a real-world model of a capacitor. The internal resistance (IR) is the leakage resistance highlighted above. Parasitic ESR is the equivalent series resistance of the capacitor, including any connecting leads or terminals. C represents the electrodes of the capacitor, and ESL is the equivalent series inductance of the leads and plates. ESL is typically small, up to 10nH, but at higher frequencies becomes an important attribute. These parameters determine the capacitor’s impedance (Z) characteristics and frequency response.

Self-resonant Frequency (SRF): A capacitor’s SRF results from its resistive, capacitive, and inductive attributes, forming a resonant circuit. At the SRF, the capacitor’s impedance (|Z|) drops to its lowest value before increasing again (Figure 5). The SRF characteristic is fundamental for higher frequency applications, particularly radio frequencies. Up to the self-resonant frequency (fr), capacitive reactance dominates, and inductive reactance dominates beyond the SRF.

 

Figure 4. A real-world model of a capacitor. (Source: Murata). Image used courtesy of Bodo’s Power Systems [PDF]

 

Figure 5. The graph of impedance (|Z|) against frequency, in Hertz, for a typical capacitor. (Source: Murata). Image used courtesy of Bodo’s Power Systems [PDF]

 

Ripple Current: The ripple current specification of a capacitor is vital for high-current power supply applications. A large ripple current can cause internal heating inside the capacitor, increasing its operating temperature and reducing reliability and service life.

 

Capacitor Types

The dielectric material typically defines the capacitor’s type. Electrolytic capacitors include aluminium and tantalum.

Aluminium capacitors: Most are polarised, with capacitance values ranging from 1 µF to tens of Farad. Working voltages are typically up to 500 V. They exhibit a high-capacity density and are capable of rapid charging and discharge. However, they are prone to dry out at higher operating temperatures, increasing leakage and changing the capacitance value. They also have a relatively low self-resonant frequency, limiting applications to below 100 kHz.

Ideal applications:

• Power supply filtering
• Energy storage
• Low-frequency coupling
• Non-critical timing circuits

Not suitable for:

• RF/wireless applications
• Critical timing circuits
• Phase shift applications
• Triggering circuits

Tantalum capacitors: Polarised, with values ranging from 0.1µF to 1,000F and 2V to 50V working voltages. Exhibit higher capacitance density but are much more expensive than aluminium capacitors. They typically have a low ESR characteristic, are more temperature stable, and do not dry out. However, they are susceptible to reverse voltages and will usually fail as a result.

Ideal applications:

• Long-duration timing
• Energy storage
• Power supply filtering
• Low and medium frequency bypassing

Not suitable for:

• Wireless/RF applications
• Cost-sensitive applications
• High voltage circuits
• Large AC signals

Ceramic capacitors: Ceramic electrostatic capacitors are extremely popular and typically low cost, with a wide range of values from less than one pF to more than 500nF and working voltages typically up to 1,000VDC. All have low ESR and good RF performance. There are several classifications of ceramic capacitors, NP0/C0G, X7R, and Y5V/Z5U.

NP0/C0G: < 1pF to 1µF, very temperature stable, tight tolerance X7R: Good balance of stability v. temperature, larger values Y5V/Z5U: Wide value range but poor temperature stability.

Ideal applications:

• low cost
• bypass
• coupling
• RF/wireless applications

Not suitable for:

• Some ceramic capacitors become microphonic
• Very low-frequency filtering
• Frequency generation circuits

Film-based capacitors: Several types of capacitors are constructed using spiral metallised films and foils, including polycarbonate, polyester, polystyrene, and polypropylene. They share similar attributes of tight tolerances, low leakage, low ESR, and good temperature stability. Values from tens of pF up to 1,000µF, with working voltages from 10VDC to over 1,000VDC.

Ideal applications:

• High voltage circuits
• audio to RF applications
• Circuits requiring temperature and voltage stability

Not suitable for:

• space constrained designs
• cost-sensitive applications
• wide variety of package formats

Mica capacitors: With excellent tolerance and temperature stability characteristics, mica capacitors are available for working voltages up to 1,000V and capacitance values from one pF to 10nF.

Ideal applications:

• Frequency generation circuits
• Voltage-controlled oscillators
• RF filters

Not suitable for:

• Applications that require a variety of package types

Polymer capacitors: Capacitors created using polymer and aluminium and polymer and tantalum have quickly gained market adoption since their introduction in the mid-1980s. Exhibiting better characteristics than aluminium and tantalum capacitors, excellent temperature stability, and no microphonic acoustic noise, they are suitable for a wide variety of applications.

 

Capacitors are Essential Electronic Components

Capacitors are a fundamental component used in virtually every electronic circuit. This short article explains the key terminology engineers should become familiar with, along with the popular capacitor types. With this knowledge, engineers can decide which capacitor type best fits their design.

 

Footnotes and References

Mouser article - MRA172 - Capacitors 101

Mouser Capacitor Infographic

Does any supplier have a Capacitors 101 guide(s) to link to?

ARRL Handbook RSGB Radio Communication Handbook

Horowitz and Hill. The Art of Electronics

 

Featured image used courtesy of Adobe Stock

This article originally appeared in Bodo’s Power Systems [PDF] magazine.