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What Is a Tank Circuit?

What Is a Tank Circuit?

A tank circuit is an electronic circuit used in many applications, including oscillators, TV and radiosets. In it's most basic form, the circuit consists of just two electronic components, namely acapacitor and an inductor (a coil). In actual application, as opposed to a theoretical design, othercomponents come into play that affect the circuit's operation. These include a resistive load and analternating 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 soundsof talking, but you can't make out any one specific conversation. If you could filter out all of theconversations 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 goodenough to just be able to detect these signals...they must also be filtered to a narrow range, toexclude all the others. When you tune your radio to your favorite station, the circuitry inside the radiois able to hone in on a narrow frequency band and the audio signal extracted from it, so all you hearis the music coming from that one station. With the simple push of a button or the turn of a dial, thatstation 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 frequencyband 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 thetwo components in a periodic cycle. This "resonant" or "tuned" circuit works by energy flowing back

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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" standsfor 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 twometal 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 acharge. An inductor is typically a coil of insulated wire, sometimes wound around an iron core. Whenelectricity passes through the wire, a magnetic field surrounding the coil is built up. The reversehappens when a magnetic field starts to dissipate, namely, the collapsing field causes current to flowin the wire.

How It Works

An audio oscillator made by synthesizers.com

Once energy is supplied to the tank circuit, a cycle begins. The capacitor stores energy in it'sconfiguration of plates. In the circuit, the capacitor's plates are connected to the ends of the inductorcoil. 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 ofenergy, 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 thepolarity (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 othercomponents in the system. The circuit is not a "perpetual motion" machine, and an alternatingcurrent source must constantly drive the tank. Mathematical formulas are used to determine thefrequency of resonance, the narrowness of the frequency band, the values of the inductor andcapacitor, and other parameters. These can be found in any basic electronics text book. Tankcircuits are used in oscillators, radio frequency equipment (such as TVs and radios), and otherelectronic devices.

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