L7_TUNED CIRCUITS.pdf

8/9/2019 L7_TUNED CIRCUITS.pdf

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TUNED CIRCUITS

Series and Parallel Tuned Circuits


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Introduction

• Inductance – In electromagnetism and electronics, inductance

– is the property of a conductor by which a change

in current in the conductor "induces" (creates)

a voltage (electromotive force) in both the conductor

itself (self-inductance)

• Self-Induction – the production of an electromotive force in a circuit

when the magnetic flux linked with the circuit changes

as a result of a change in current in the same circuit.


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• If a conductor, maybe in the form of a coil was

placed in a changing magnetic field, a current

could be made to flow in it; this is calledinduction.


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• So imagine a coil, and imagine that we apply a

voltage to it. As current starts to flow, a

magnetic field is created. But this means that

our coil is in a changing magnetic field, and

this induces a current in the coil.

• The induced current runs contrary to the

applied current, effectively diminishing it.

(self-induction)


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• What happens is that the self-induction delaysthe build-up of current in the coil, but eventually

the current will reach its maximum and stabilizeat a value only determined by the ohmicresistance in the coil and the voltage applied.

• We now have a steady current and a steadymagnetic field.


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• During the build-up of the field, energy was

supplied to the coil, where did that energy go?

It went into the magnetic field, and as long asthe magnetic field exists, it will be stored

there.


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• Now imagine that we remove the current

source. Without a steady current to uphold it,

the magnetic field starts to disappear, but thismeans our coil is again in a variable field

which induces a current into it.


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• This time the current is in the direction of theapplied current, delaying the decay of the

current and the magnetic field till the storedenergy is spent.

• This can give a funny effect: Since the

coilmust

get rid of the stored energy, thevoltage over it rises indefinitely until a currentcan run somewhere!

• This means you can get a surprising amount of

sparks and arching when coils are involved. If thecoil is large enough, you can actually get anelectric shock from a low-voltage source like an

ohmmeter.


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• Self-inductance is measured in henry (H or

Hy). A henry is almost as enourmous value as

a Farad, and coils are often measured in milli,

micro and even nanohenry.


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Tuned Circuits

• A coil is a component that effectively stores

current. A capacitor is a component that

stores voltage. If we connect those two

together, we get interesting results. Imagine a

coil (L) and a capacitor (C) connected in

parallel:


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• Video Supplement 1

– http://www.youtube.com/watch?v=v3-

HwZMThzQ

• Video Supplement 2

– http://www.youtube.com/watch?v=t5yMPyNkdkI

http://localhost/var/www/apps/conversion/tmp/scratch_4/The%20LC%20Circuit_(360p).flvhttp://localhost/var/www/apps/conversion/tmp/scratch_4/The%20LC%20Circuit%20(part%20II)_(360p).flvhttp://localhost/var/www/apps/conversion/tmp/scratch_4/The%20LC%20Circuit%20(part%20II)_(360p).flvhttp://localhost/var/www/apps/conversion/tmp/scratch_4/The%20LC%20Circuit_(360p).flv


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• Let us assume we charged the capacitor with avoltage before connecting it to the coil, so now

we apply that voltage to the coil. This means acurrent starts to flow, and a magnetic field iscreated. As the magnetic field builds up andreaches maximum, an increasing amount ofcurrent flows in the circuit.

• Eventually, the capacitor becomes discharged;the voltage across it is zero, but now a magneticfield has been built in the coil and a current isflowing in the circuit, the charge has been

transferred and transformed: From a voltagecharge in the capacitor to a current charge in thecoil.


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• As the voltage is now gone, the current starts

to decay, but the self-induction keeps it

running till the magnetic field has been spent.This current charges the capacitor, now with

the opposite polarity as before; the charge is

transferred back to the capacitor:


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• When the current stops, the voltage starts a

new current in the opposite direction, etc. etc.

The charge swings like a pendulum back andforth between the coil and the capacitor,

changing polarity twice in each cycle.

• If there were no losses, this would go on

forever. In real life, some energy is lost in each

cycle, and unless the lost energy is somehowreplaced, the tuned circuit will ring out, much

like a bell:


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• Like a pendulum, the tuned circuit swings at acertain frequency, here determined by thevalues of the components. The formula issimple:


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Types of Tuned Circuits


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References

• http://www.hans-

egebo.dk/Tutorial/tuned_circuits.htm

• https://en.wikipedia.org/wiki/LC_circuit
http://www.hans-egebo.dk/Tutorial/tuned_circuits.htmhttp://www.hans-egebo.dk/Tutorial/tuned_circuits.htmhttps://en.wikipedia.org/wiki/LC_circuithttps://en.wikipedia.org/wiki/LC_circuithttp://www.hans-egebo.dk/Tutorial/tuned_circuits.htmhttp://www.hans-egebo.dk/Tutorial/tuned_circuits.htmhttp://www.hans-egebo.dk/Tutorial/tuned_circuits.htm

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L7_TUNED CIRCUITS.pdf

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