Capacitors are circuit components that store electric charge when a potential difference is applied across their terminals. This ability to store charge is called capacitance (C) and is measured in farads (F). One farad is equivalent to one coulomb of charge stored per volt. Now you don’t see one-farad capacitors because that is a large capacitance. Capacitor values are commonly expressed in microfarads, nanofarads, or even picofarads.
Basic Construction
Capacitors are typically constructed using two conductive plates separated by an insulating material called a dielectric.
The dielectric serves as an insulator, preventing the direct flow of current between the plates and enhancing the capacitor’s ability to store charge. In an electrolytic capacitor, the dielectric material is typically a thin layer of aluminum oxide (for aluminum electrolytic capacitors) or tantalum pentoxide (for tantalum electrolytic capacitors).
The plates of a capacitor can be viewed as electrodes that establish an electric field within the dielectric material, facilitating the storage of electric charge.
Now, the type of dielectric material and the physical characteristics of the plates (area and spacing) influence the capacitance. Common types of capacitors also include parallel-plate, ceramic disc, and tubular capacitors.
Factors Affecting Capacitance
- The area of the plates: Capacitance is directly proportional to the area of the plates.
- The spacing between the plates: Capacitance is inversely proportional to the distance between the plates.
- The type of dielectric between the plates.
Polarized Vs. Non-polarized Capacitors
- Polarized capacitors (like electrolytic capacitors) have a positive and a negative lead and must be connected in the correct orientation in a circuit. If reversed, they can fail or even explode.
- Non-polarized capacitors (like ceramic capacitors) do not have polarity and can be connected in any direction.
Types of Capacitors
Not all capacitors are the same, and choosing the right type for your project can make all the difference.
Ceramic Capacitors
These capacitors are the most common and versatile. They’re non-polarized, meaning they can be placed in a circuit without worrying about which side is positive or negative. They are used for high-frequency applications like filtering noise.
They are constructed using a ceramic disc as the dielectric material between two metal plates. A metal coating is applied to both sides of the ceramic disc, forming the plates of the capacitor. Ceramic capacitors are small and generally have a capacitance of less than 1 µF.
Electrolytic capacitors
Electrolytic capacitors have a tubular construction and use a thin gauze soaked in an electrolyte solution instead of a solid dielectric. They can store more energy than ceramic capacitors, but they are polarized, meaning they have a specific positive and negative side. They are often used in power supply circuits to smooth out voltage.
Note: Electrolytic capacitors must be connected with the correct polarity. Most electrolytic capacitors have a clear marking, usually a stripe along the side of the body. This stripe typically has a minus sign (-) printed on it, indicating the negative terminal.
Tantalum Capacitors
Tantalum capacitors are known for their stability and reliability in small packages. They are often used in low-power devices. However, tantalums can be more expensive and may be sensitive to over-voltage, so handle them with care.
Please note that each type of capacitor has specific strengths and weaknesses. For example, electrolytic capacitors offer high capacitance but are prone to leakage currents, while ceramic capacitors are stable but limited in capacitance values.
Applications in Circuits
Capacitors have numerous applications in electric circuits, making them essential components in many electronic devices. They are used for filtering, timing circuits, and energy storage.
Filtering
This is perhaps the most common use of capacitors. In a power supply circuit, capacitors smooth out voltage fluctuations or “ripple voltage” from the rectified AC voltage, providing a cleaner DC output. Without this, your circuit could experience voltage dips and spikes, causing erratic behavior.
You can connect multiple capacitors in parallel to increase the overall capacitance and provide better filtering.
Coupling and Decoupling
Capacitors are often used to couple AC signals between stages of an amplifier or decouple noise from the power supply. In audio circuits, capacitors block any DC offset from being amplified and ensure that only the desired AC audio signal passes through.
Decoupling capacitors typically consist of a combination of a large-value electrolytic capacitor and a small-value capacitor, often ceramic. The large capacitor handles low-frequency ripple, while the smaller one deals with high-frequency noise.
Timing and Oscillation
Capacitors, in combination with resistors, can create timing circuits. The classic example is the RC circuit (see figure below), where the capacitor charges and discharges at a predictable rate, creating a time delay or oscillation. This is the principle behind many oscillators (such as this 555 oscillator circuit), like those found in radio transmitters or clock circuits.
Energy Storage
Capacitors also function as energy storage devices, capable of delivering bursts of energy, as seen in flash photography.
Charging and Discharging Capability
Although capacitors can charge and discharge quickly, in an RC circuit, the process is not instantaneous. Instead, the capacitor charges gradually, following an exponential curve, until the voltage across it matches the supply voltage. Likewise, when the capacitor discharges through a resistor, the voltage across it doesn’t drop immediately. It decreases slowly, also in an exponential manner, over time.
The rate at which this happens is determined by the combination of the capacitor and resistor in the circuit. This rate is called the RC time constant, where R is the resistance, and C is the capacitance.
The formula is simple: τ = R × C, where τ (tau) is the time constant. In practical terms, this means the capacitor charges to about 63% of the supply voltage in one time constant and nearly fully charges after about five times constants.
Why does this matter? This is useful for designing a timing circuit, such as introducing a delay before an action occurs or controlling the speed at which an LED blinks.
Troubleshooting
Common capacitor faults include dielectric breakdown (open circuit), increased leakage current, and internal shorts.
- Bulging or Leakage: Electrolytic capacitors, in particular, are prone to failure due to aging. A physical inspection can reveal bulging, cracks, or leakage of the electrolyte. This is a clear sign the capacitor needs replacement.
- Erratic Circuit Behavior: If your circuit is behaving erratically—such as flickering LEDs, distorted audio, or power fluctuations—capacitors could be the culprit.
- Heat: Sometimes, overheating capacitors, especially those in power circuits, can indicate excessive current flow or an internal failure.
However, please note that it’s difficult to recognize the signs that a capacitor might be faulty. In this case, you can use an ohmmeter or a capacitance meter to check for these faults. Many modern digital multimeters come with a capacitance measurement function, but even without it, you can still check for basic issues like shorts or opens.
See Also
FAQs
Why do Capacitors Block DC but Pass AC?
Capacitors block DC because, after they are fully charged, they stop allowing current to flow. For AC signals, however, capacitors continually charge and discharge as the alternating current changes direction, effectively allowing the AC signal to pass through.
What Does Voltage Rating in Capacitors Mean?
Electrolytic capacitors have markings such as “100μF 25V”. This indicates capacitance (100μF) and voltage rating (25V). The voltage rating is the maximum DC voltage that can be applied continuously to a capacitor without breaking down the dielectric.
Can a Capacitor be Used to Store Energy like a Battery?
Capacitors can store energy, but unlike batteries, they discharge very quickly. They are best suited for short-term energy storage or for applications that require rapid energy release.
Next Steps
- Explore Capacitors in Series and Parallel.
- Learn more about the Diode.
Creator and Editor at AnitoCircuits.com based in Toronto