Capacitors

 

Stare into the sky most days and you'll see some huge capacitors floating over your head. Capacitors (sometimes known as condensers) are energy-storing devices that are widely used in televisions, radios, and other kinds of electronicequipment. Tune a radio into a station, take a flash photo with a digital camera, or flick the channels on your HDTV and you're making good use of capacitors. The capacitors that drift through the sky are better known as clouds and, though they're absolutely gigantic compared to the capacitors we use in electronics, they store energy in exactly the same way. Let's take a closer look at capacitors and how they work!

Photo: A typical capacitor used in electronic circuits. This one is called an electrolytic capacitor and it's rated as 4.7 μF (4.7 microfarads), with a working voltage of 350 volts (350 V).

What is a capacitor?

Photo: A small capacitor in a transistor radio circuit.

Take two electrical conductors (things that let electricity flow through them) and separate them with an insulator (a material that doesn't let electricity flow very well) and you make a capacitor: something that can store electrical energy. Adding electrical energy to a capacitor is called charging; releasing the energy from a capacitor is known as discharging.

A capacitor is a bit like a battery, but it has a different job to do. A battery uses chemicals to store electrical energy and release it very slowly through a circuit; sometimes (in the case of a quartz watch) it can take several years. A capacitor generally releases its energy much more rapidly—often in seconds or less. If you're taking a flash photograph, for example, you need your camera to produce a huge burst of light in a fraction of a second. A capacitor attached to the flash gun charges up for a few seconds using energy from your camera's batteries. (It takes time to charge a capacitor and that's why you typically have to wait a little while.) Once the capacitor is fully charged, it can release all that energy in an instant through the xenon flash bulb. Zap!

Capacitors come in all shapes and sizes, but they usually have the same basic components. There are the two conductors (known as plates, largely for historic reasons) and there's the insulator in between them (called the dielectric). The two plates inside a capacitor are wired to two electrical connections on the outside called terminals, which are like thin metal legs you can hook into an electric circuit.

Photo: Inside, an electrolytic capacitor is a bit like a Swiss roll. The "plates" are two very thin sheets of metal; the dielectric an oily plastic film in between them. The whole thing is wrapped up into a compact cylinder and coated in a protective metal case. WARNING: It can be dangerous to open up capacitors. First, they can hold very high voltages. Second, the dielectric is sometimes made of toxic or corrosive chemicals that can burn your skin.

You can charge a capacitor simply by wiring it up into an electric circuit. When you turn on the power, an electric charge gradually builds up on the plates. One plate gains a positive charge and the other plate gains an equal and opposite (negative) charge. If you disconnect the power, the capacitor keeps hold of its charge (though it may slowly leak away over time). But if you connect the capacitor to a second circuit containing something like an electric motor or a flash bulb, charge will flow from the capacitor through the motor or lamp until there's none remaining on the plates.

Although capacitors effectively have only one job to do (storing charge), they can be put to all sorts of different uses in electrical circuits. They can be used as timing devices (because it takes a certain, predictable amount of time to charge them), as filters (circuits that allow only certain signals to flow), for smoothing the voltage in circuits, for tuning (in radios and TVs), and for a variety of other purposes. Largesupercapacitors can also be used instead of batteries.

Capacitors and capacitance

The amount of electrical energy a capacitor can store is called its capacitance. The capacitance of a capacitor is a bit like the size of a bucket: the bigger the bucket, the more water it can store; the bigger the capacitance, the more electricity a capacitor can store. There are three ways to increase the capacitance of a capacitor. One is to increase the size of the plates. Another is to move the plates closer together. The third way is to make the dielectric as good an insulator as possible. Capacitors use dielectrics made from all sorts of materials. In transistor radios, the tuning is carried out by a large variable capacitor that has nothing but air between its plates. In most electronic circuits, the capacitors are sealed components with dielectrics made of ceramics such as mica and glass, paper soaked in oil, or plastics such as mylar.

Photo: This variable capacitor is attached to the main tuning dial in a transistor radio. When you turn the dial with your finger, you turn an axle running through the capacitor. This rotates a set of thin metal plates so they overlap to a greater or lesser extent with another set of plates threaded in between them. The degree of overlap between the plates alters the capacitance and that's what tunes the radio into a particular station.

How do we measure capacitance?

The size of a capacitor is measured in units called farads (F), named for English electrical pioneer Michael Faraday (1791–1867). One farad is a huge amount of capacitance so, in practice, most of the capacitors we come across are just fractions of a farad—typically microfarads (thousandths of a farad, written μF), nanofarads (thousand-millionths of a farad written nF), and picofarads (million millionths of a farad, written pF). Supercapacitors store far bigger charges, sometimes rated in thousands of farads.