Capacitance is the ability of an object to store an electrical charge. While these devices’ physical constructions vary, capacitors involve a pair of conductive plates separated by a dielectric material. This material allows each plate to hold an equal and opposite charge. This stored charge can then release as needed into an electrical circuit. A capacitor may be an electrical component, but many objects, such as the human body, exhibit this ability to hold and release a charge. As we’ll note, this ability can be advantageous.
Capacitance Equation
The basic formula governing capacitors is:
charge = capacitance x voltage
or
Q = C x V
We measure capacitance in farads, which is the capacitance that stores one coulomb (defined as the amount of charge transported by one ampere in one second) of charge per one volt. While a convenient way to define the term, everyday capacitors aren’t big enough to store a single farad, so we often display ratings in terms of microfarads (μF, or millionths of a farad), or even picofarads (pF, or trillionths of a farad).
From this definition, you might assume that a capacitor is a type of rechargeable battery, storing charge to use later. However, a capacitor’s characteristically low charge capacity compared to conventional battery cells generally makes them ill-suited to prolonged use as a power source. The other characteristic that makes them disadvantageous for prolonged power delivery is that a capacitor’s voltage is directly proportional to the amount of stored charge, evidenced by rearranging the terms in the above equation to:
V = Q/C
Conventional batteries hold a somewhat steady charge until depleted, making them more appropriate in many situations.
Power Smoothing and Time Constant
Prolonged usage aside, capacitors do a very good job of evening out momentary drops in power. The time constant tau indicates this capability. Tau equals resistance times capacitance:
τ = RC
Tau indicates the amount of time in seconds that it takes a voltage to decay exponentially to 37 percent of its original value. At five times this number, the capacitor is considered fully discharged. If a capacitor attaches across a voltage source that varies (or momentarily cuts off) over time, a capacitor can help even out the load with a charge that drops to 37 percent in one time constant. The inverse is true for charging; after one time constant, a capacitor is 63 percent charged, while after five time constants, a capacitor is considered fully charged.
Image: PartSim Drawing by Jeremy S. Cook
For example, if you had a circuit as defined in Figure 1 above, the time constant of the RC circuit is:
1000 ohms x 47 x 10-6 farads
This time constant works out to .047 seconds. When we disconnect the 5V source seen here, it takes .047 seconds to drop to 1.85V, and five times this, or .235 seconds, to discharge. If the capacitor charged up to 5V, that process would also take .235 seconds. You can use a larger capacitor to increase these numbers depending on the situation or load in question.
What Else is a Capacitor Used For?
Making an intermittent voltage supply closer to a desired constant voltage is a capacitor’s most fundamental purpose. Here are several more ways to use a capacitor:
- AC to DC conversion. The DC output tends to vary sinusoidally in this important “smoothing” application.
- Coupling. A standard capacitor allows AC to pass and stops DC.
- Decoupling. Capacitors can also eliminate any AC that may be present in a DC circuit.
- RF signals and older radios. You can adjust variable “tuning” capacitors to change the station — you can even build your own radio as an educational tool this tutorial
- Timers. Use the time it takes a capacitor to charge to a certain level to trip other parts of the circuit. As with RF tuning, integrated circuits and microcontrollers have largely replaced capacitive timing functions.
- Touchscreens. Though exotic when compared to other circuits described here, a capacitive touchscreen is an extremely common way to use a capacitor. These devices sense the change in capacitance at a point on a display device and translate it into coordinates on an X-Y plane.
- Microscopic capacitors. These devices serve as data storage units in Flash memory. Considering the innumerable number of bits in Flash memory, microscopic capacitors contain the largest number of capacitors in use today.
Capacitors in Series and Parallel
Capacitors, like resistors, can combine in parallel or series within a circuit. However, the net effect is quite different between the two. When done in parallel, combining capacitors mimics adding each capacitor’s conductor and dielectric surface area. In parallel, the total capacitance is the sum of each capacitor’s value.
Capacitance in series reduces the total amount of capacitance, such that the total capacitance of these components in total will be less than the value of the smallest capacitor value. The equation is given by:
1/CT = 1/C1 + 1/C2 + 1/Cn
Series usage is less common than parallel configurations, but dividing the voltage applied to each component has some limited uses.
Leyden Jar: History of Capacitors and Their Structure
The first capacitor was called the Leyden Jar. These early charge storage devices were full of water and served as conductors, but they eventually evolved into a glass bottle with metallic foil coating the inside and the outside of the bottle. The foil acts as conductors separated by glass, which acts as a dielectric material. The two segments store charges between them until connected.
Today’s capacitors come in many shapes in sizes, but at their core, they have two electrically conducting “plates” separated by a dielectric insulating material. The governing equation for capacitor design is:
C = εA/d,
In this equation, C is capacitance; ε is permittivity, a term for how well dielectric material stores an electric field; A is the parallel plate area; and d is the distance between the two conductive plates.
Image: By Eric Schrader via Wikimedia Commons
You can split capacitor construction into two categories, non-polarized and polarized.
- Non-polarized capacitors are most like the theoretical capacitor we described earlier. They contain a pair of conducting plates separated by a dielectric and they can connect to a source voltage in either electrical orientation. Ceramic capacitors contain several plates stacked on top of one another to increase the surface area, while a ceramic material forms the dielectric between the positive and negative poles. Film capacitors wrap these plates against each other, and the dielectric film is usually plastic.
- Polarized capacitors are electrolytic. An electrolytic capacitor’s anode can form an insulating oxide layer that acts as a dielectric. Because this oxide layer is extremely thin, the denominator in the C = εA/d equation is very small, thus enhancing these components’ capacitance. Additionally, the surface area component can be quite high per component volume because the anode material (generally aluminum, tantalum, or niobium) can be rough or porous.
You could classify a supercapacitor as a type of electrolytic capacitor, though a supercapacitor’s charge storage method involves the arrangement of ions in an electrolytic solution between two electrodes to form a double layer of charged ions. This arrangement gives an extremely high charge compared to traditional electrolytic and non-polarized capacitors but also results in a slower charge and discharge rate as well as a typically lower breakdown voltage. Because of this slow speed, a supercapacitor isn’t appropriate for filtering applications. One might even argue supercapacitors are in a class all unto themselves, and supercapacitor technology merits its own research.
Capacitor Specifications
A capacitor’s most basic rating is its capacitance, as we’ve mentioned. Capacitance specifies a capacitor’s charge-holding capability per volt. Beyond that, you can specify a capacitor by the following:
- Working Voltage: The voltage above which a capacitor may start to short and no longer hold a charge
- Tolerance: How close to the capacitor’s charge rating the actual component will be
- Polarity: Which lead is meant to connect to a positive lead, and which goes to a negative in the case of polarized capacitors
- Leakage Current: How much current will seep through a dielectric, gradually discharging a capacitor over time
- Equivalent Series Resistance (ESR): The capacitor’s impedance at high frequencies
- Working Temperature: Temperature range at which a capacitor is expected to perform nominally
- Temperature Coefficient: Change in a capacitor’s charge holding performance over a specified temperature range
- Volumetric Efficiency: While not always considered or explicitly specified, this factor indicates how much capacitance the component exhibits for a certain volume
For how capacitors indicate these values, check out this guide to capacitor code markings.
A Fundamental Passive Component
Along with resistors and inductors, capacitors act as one of the fundamental passive components that form the circuits we use every day. While the concept of two opposite charges on plates is simple, their application, and the wide variety of manufacturing techniques and form-factors available, is not. The good news is that whatever your charge storage issue, there’s probably a capacitor out there that will fit your application perfectly.