L7_TUNED CIRCUITS.pdf
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Transcript of 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