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Old 10th April 2004, 11:54 AM   #1
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Default Whats the purpose of inductors in circuits (not crossovers)

Whats the purpose of having inductors in Circuits, like on PCB's? Is it to limit high frequencies from going through the circuits? And I'm referring to actuall air-core inductors, not like a transformer or something.

Its just that I don't see inductors in circuits nearly as much as caps, so I didn't know if was for a special purpose.
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Old 10th April 2004, 01:25 PM   #2
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Mostly used for filtering in an amp...especially Class A amps...some are used for swithiching like in an SMPSU...depends on what u are looking at really...
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Old 10th April 2004, 01:47 PM   #3
sreten is offline sreten  United Kingdom
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Default Re: Whats the purpose of inductors in circuits (not crossovers)

Quote:
Originally posted by Hybrid fourdoor
Its just that I don't see inductors in circuits nearly as much as caps, so I didn't know if was for a special purpose.
You'd expect to see lots of inductors in a graphic equaliser.

The reason you don't is a circuit called a "gyrator" is used to
form a virtual inductor by using capacitance in the feedback.

This avoids two major problems of inductors at audio frequencies,
the very high values needed for circuits with impedances in the
kohms range, consequently the high cost, and the hum pickup
these inductors would also cause.

sreten.
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Old 10th April 2004, 07:49 PM   #4
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If you really want to get back to basics - an inductor resists change in current flow whereas a capacitor resists change in voltage.

Reactance is the measure of impedance (the ability to impede current flow) of both inductors and capacitors. For simplicities sake, think of reactance as resistance - resistance being the measure of impedance of a resistor - with one major difference, the effect of frequency.

Looking at the formula for reactance (X) of an inductor (XL):

XL = 2.pi.f.L (2 x 3.14159 x frequency x inductance)

From this, one can see that as the frequency (f) increases, so does the reactance - making it harder for current to flow.

The opposite is true of a capacitor, the formula for reactance of a capacitor (XC) being:

XC = 1/(2.pi.f.c) (1 over [2 x 3.14159 x frequency x capacitance])

From this, one can see that as the frequency increases, the reactance decreases - making it easier for current to flow. Exactly the opposite of an inductor.

This should explain the use of inductors and capacitors in audio signal circuits - filtering.

As far as inductors go, this is their only real function (filtering) when talking about amplifiers.

As far as capacitors go, they not only perform filtering, but also play a major role in providing a clean (ideally ripple free) DC supply to the amplifier.

As mentioned above, capacitors resist change in voltage, which is what one wants in a (amplifiers) power supply as the transistors used have a SOA (safe operating area) of voltage of which they work within.

As BJTs are current driven devices (as opposed to FETs) and inductors resist change in current flow, one can see that there is no point in trying to resist the flow of current to a BJT in an amplifier while it is trying to drive a loudspeaker in response to the input signal - speaking from a supply of
power point of view ofcourse. Coming from a signal point of view, that's exactly what one wants an inductor to do - stop a particular frequency range from 'getting through'...

Large inductors are used in switchmode power supplies... but that's another story...

Hope that helps...

Nic.
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Old 11th April 2004, 12:13 AM   #5
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