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.

1. 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.

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   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.

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