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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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In Reply to: Are we talking about the same thing??? posted by cheap-Jack on May 16, 2007 at 08:21:25:
Let's first look at a capacitor, since it seems a little easier to intuit the mechanics. Then we’ll look at a choke.A capacitor is a high-pass filter, so when placed across the hot and neutral it lets high frequency noise "short" across the line, thus acting as a high frequency noise filter--the so-called "Auricap tweak".
But as the frequency rises, at some point the inductance caused by the capacitor leads (as small as it is) exceeds the capacitor's capability to store and release energy at that frequency, and thus its action as a capacitor is compromised. The point where these two effects are equal is the capacitor's resonant frequency for that lead length. As frequency rises beyond this, impedance then *increases* with frequency, until at very high frequencies the capacitor is letting very little signal across and not really acting like a capacitor at all. As a (high) frequency-dependent resistor it is impeding high frequencies--or in other words exhibiting the high frequency blocking properties of an *inductor* (Excellent article at http://scitation.aip.org/journals/doc/PHTEAH-ft/vol_44/iss_2/106_1.html.)
For an inductor (choke), an analogous effect happens. An inductor is a low pass filter. So we would expect that a choke placed across hot and neutral would let DC "short", but would offer increasing impedance to rising frequencies. In fact, we rely on this, otherwise a choke across the line really would short the 60Hz wave. But as frequencies rise further still (into the MHz range), the capacitance in the windings plays an increasingly larger relative role. Above the inductor's resonant frequency, further increases in frequency are more affected by the capacitance in the windings than the turns of the inductor. One could say that the electrostatic effects are dominating the electromagnetic. Thus the choke acts increasing like a capacitor--or essentially a (high) frequency-dependent resistor allowing energy at those frequencies to short across the line.
So counter-intuitively, placing an inductor across the line may be more effective at "shorting"--that is filtering--very high frequency noise.
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Follow Ups
- When is an inductor not an inductor? When it is a capacitor... - Rabelais 21:51:51 05/16/07 (0)
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