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The oscillating circuit , Tuning or resonant circuit in the wireless radio receivers

From Online Sciences - April 9, 2018

Oscillating circuit

Oscillating circuit is the electric circuit in which there are interchange of the energy stored in the induction coil as a magnetic field and the capacitor as an electric field , The oscillating circuit consists of induction coil of very small resistance , capacitor , battery and they are connected together through switches a and b .

How oscillating circuit works

When the switch ( a ) is turned on : A momentary current passes from the battery to charge up the capacitor such that the plate connected to the positive pole becomes positively charged while the plate connected to the negative pole becomes negatively charged .

The current stops flowing when the potential difference between the two plates of the capacitor equals the potential difference of the battery , An electric field is build up between the capacitor plates where energy is stored as electric field .

When switch ( a ) is turned off and switch ( b ) is turned on : The capacitor discharges its charge through the inductive coil and therefore a momentarily current passes from the positive plate to the negative plate , that decreases the voltage across the capacitor till it collapses and the electric field between its plates vanishes .

The flowing current through the coil generates magnetic field that stores the energy that was stored in the capacitor before , as electric energy , Initially , there is a high current flowing through the coil due to the high voltage across the capacitor plates .

As this current fades gradually , a forward current is induced in the coil by self induction that draws more charges from the positive plate of the capacitor towards the negative plate .

Consequently , the plate that was negatively charged becomes positive by charged while the other plate becomes negatively charged reversing the capacitor polarity and generating an electric field between them .

The current through the coil ceases and so is the magnetic field till they vanish , Accordingly the energy stored in the coil as magnetic field is transformed into electric energy stored in the capacitor .

Next to that , the capacitor is charged another time through the coil in a direction opposite to the original charge , Thus the charging and discharging of the capacitor is cycled and an electric oscillation of high frequency is produced in the circuit , interchanging the energy alternately between the two fields .

The process of charging and discharging in the oscillating circuit stops after a while , due to the ohmic resistance of the coil and the other circuit wires , a part of the energy is dissipated as heat energy , This result in a gradual decrease in the alternating current in the circuit and the voltage across the capacitor plates , as well , This hinders charging and discharging process causes the current eventually to die away to zero .

However if this loss is compensated by extra charges supplied to the capacitor , the oscillatory action of passing energy back and forward between the capacitor and inductor will continue indefinitely .

The relation between the frequency , XL , XC , R and Z

The ohmic resistance does not change as the current frequency changes , The inductive reactance increases as the AC frequency increases ( XL f ) , The capacitive reactance decreases as the AC frequency increases ( XC 1/f ) , The impedance Z of the circuit decreases till it becomes minimum value when XL = XC which called a state of resonance , After that the impedance Z of the circuit increases as the AC frequency increases .

Tuning or resonant circuit

Tuning or resonant circuit is used in Tuning radio receivers for picking up the signal of a particular station , at a particular frequency , Tuning or resonant circuit consists of :

How it works

When the frequency of the AC supply is varied , the flow of the current intensity varies such that the current value decreases as the difference between frequency of the power supply and the resonance frequency increases , The current value increases as the frequency of the power supply approaches the resonance frequency .

The current value reaches its maximum when the frequency of the supply equals the resonance frequency of the circuit ( when the inductive reactance equals the capacitive reactance ) and the circuit is said to be resonant .

Deduction of the resonant frequency

In the oscillating circuit when capacitive reactance and inductive reactance are equal they cancel out each other , leaving only the resistance of the circuit to oppose the flow of current , thus the current reaches its maximum in that circuit .

The circuit frequency ( f ) can be deduced as follows :

XL = XC

Factors affecting the resonant frequency
Tuning resonant circuit in the wireless radio receivers

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