What Is a Tank Circuit?

A tank circuit is an electronic circuit used in many applications, including oscillators, TV and radio sets. In it's most basic form, the circuit consists of just two electronic components, namely a capacitor and an inductor (a coil). In actual application, as opposed to a theoretical design, other components come into play that affect the circuit's operation. These include a resistive load and an alternating current source.

The Concept Of Filtering

Picture yourself sitting in an audience with a hundred people holding conversations among themselves, waiting in anticipation for the curtain to go up and a show to begin. You hear the sounds of talking, but you can't make out any one specific conversation. If you could filter out all of the conversations except one, then you could clearly make out what is being said.

Significance

Similarly, right now, you are being bombarded with hundreds, perhaps thousands, of RF (radio frequency) signals from television stations, radio stations, CB (citizen band) radios, amateur radios, police and emergency transmissions, weather data from satellites, and the list goes on. It's not good enough to just be able to detect these signals...they must also be filtered to a narrow range, to exclude all the others. When you tune your radio to your favorite station, the circuitry inside the radio is able to hone in on a narrow frequency band and the audio signal extracted from it, so all you hear is the music coming from that one station. With the simple push of a button or the turn of a dial, that station is silenced, and another now fills the room with music, as if by magic. The same applies to "tuning" to different "channels" on your TV set.

Function

In it's simplest form, a basic electronic circuit that is used to "tune" a specific frequency or frequency band is made up of two components, a capacitor and an inductor. These two are connected in "parallel" to create a circuit known as a "tank" circuit (see accompanying schematic diagram).

History

The name "tank" comes from the fact that this circuit stores energy. Depending on the electrical values of the capacitor and the inductor, an alternating current can go back and forth between the two components in a periodic cycle. This "resonant" or "tuned" circuit works by energy flowing back and forth between the capacitor and the inductor, and back again. The tank circuit is also called an "LC" circuit. In the field of electronics, "L" represents inductance measured in henries, and "C" stands for capacitance, measured in farads.

The Components

In its traditional form, a capacitor is made up of two sheets of aluminum foil wound together with a layer of wax paper or other non-conducting material between them. It can be imagined as being two metal plates on top of each other, separated by a non-conducting material (which, in some cases, can be air). When voltage is applied to the plates (one positive, one negative), the capacitor stores a charge. An inductor is typically a coil of insulated wire, sometimes wound around an iron core. When electricity passes through the wire, a magnetic field surrounding the coil is built up. The reverse happens when a magnetic field starts to dissipate, namely, the collapsing field causes current to flow in the wire.

How It Works

Once energy is supplied to the tank circuit, a cycle begins. The capacitor stores energy in it's configuration of plates. In the circuit, the capacitor's plates are connected to the ends of the inductor coil. As current begins to flow out of the capacitor (it's voltage starts dropping) and into the inductor, a magnetic field builds up around the coil. Although the capacitor will quickly become empty of energy, current will continue to flow in the inductor caused by the effect of the energy in the magnetic field. This current will start to send current back into the capacitor, although this time the polarity (the "plus" and "minus") will be reversed. The cycle then repeats, over and over, at a period (frequency) that is determined by the values of the inductor, the capacitor, and a few other components in the system. The circuit is not a "perpetual motion" machine, and an alternating current source must constantly drive the tank. Mathematical formulas are used to determine the frequency of resonance, the narrowness of the frequency band, the values of the inductor and capacitor, and other parameters. These can be found in any basic electronics text book. Tank circuits are used in oscillators, radio frequency equipment (such as TVs and radios), and other electronic devices.