Caps, Caps, Caps

Capacitor is one of the most wonderful passive devices invented so far. Its applications have permeated into every nook of circuits. The construction and working of Capacitors are dealt in texts.

Though the concept is as simple as ‘2 conducting plates separated by a dielectric’, several types of capacitors have sprung up.

In college labs, we usually find Electrolytic Capacitors and Ceramic Capacitors.

Why the name? An electrolytic cap uses a conducting liquid(electrolyte) as one of its conducting plate.

Where to use? Typically in low frequency applications, like when you deal with the 50 to 60 Hz signal from power lines. Also, it can handle higher current compared to ceramic caps. Say, you build a 5V power supply for your project, in the rectifier following the transformer, electrolytic caps will be used as filters.

Range: nF to uF

Why the name? Ceramic capacitors are called so because they use ceramic dielectrics.

Where to use? They are used in high frequency applications, unto several megahertz. Amplifiers, oscillator circuits etc are examples.

Range: upto mF

Next is an interesting and advanced cap type called ‘Supercapacitors’ (alias EDLC). They are characterized by their ability to store large amount of charge in a small volume. They have capacitances of several farads. Some have crossed 1000 Farads. These caps store so much charge that they are effectively replacing Batteries in some applications. One highlight feature of these batteries is that they get charged rapidly.

Heard of ‘Capabus’? Electric bus with no permanent connection to power lines. When it stops in a bus stop, it charges its SuperCap, and uses this energy to drive till the next bus stop. Supercaps can be charged in the few seconds for which the bus stays in the bus stop. Do u think batteries could be charged that fast?

Not just bus, even in Formula 1, the KERS (Kinetic Energy Restoration System) introduced in 2009 season, is based on Supercaps to store the energy released during braking, to accelerate later.

It is also used in Regenerative Braking of typical electric vehicles.

Among electrolytic caps, there’s a notable variant called “Tantalum” caps. They have very stable capacitance. Also they offer higher capacitance per unit volume (more compact for a given value of capacitance) and are hence used in miniature applications such as cellular telephones. Eventually they are very expensive.