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What you want is lossy core behavior.

Posted by Al Sekela (A) on April 27, 2007 at 16:28:54

In Reply to: Chokes: toroids versus EI cores for AC power line noise filtration posted by Christopher Witmer on April 26, 2007 at 22:04:06:

The Hammond 193L choke is designed to filter ripple in tube circuit B+ supplies. It would be used after the rectifier and in series with filter capacitors to block AC and pass DC. It is designed to exhibit 5 henries of inductance with 300 mA of DC current passing through it.

The inductance depends on the core magnetic behavior. The core consists of steel plates. Steel has magnetic domains in its microscopic structure, and these domains can only switch direction with finite speed. The domains switching direction are what give the core its high magnetic permeability and the high measured inductance of the choke. Once the imposed AC frequency exceeds the capability of the domains to switch, the effective inductance will fall and the filter will not function as designed.

When used as a parallel AC filter, as Alan recommends, it will block the power frequency and noise with increasing impedance as the frequency rises, until it reaches the speed limit imposed by the core domains. For noise above this frequency, the inductance will drop and the device will look like a resistor (the Ohmic resistance of the winding, which may include skin effect).

This is the secret of how it works. The AC line needs to have a resistive termination to damp electrical ringing on it. An ideal inductor would look like an open-circuit to the ringing tones (and overtones), and do nothing but reflect the noise. An ideal capacitor would look like a short-circuit and also reflect the noise. The choke looks like an inductor to the AC, so that it draws just a little reactive current, but like a resistor to the higher-frequency noise. It dissipates energy and damps the ringing tones.

Similar filters can be made by placing resistors in series with capacitors. The drawback with these is that capacitors all have self-resonance frequencies above which they look like inductors. Thus, a number of different X- or Y-rated capacitors are required to span the frequency range inhabited by AC circuit ringing tones. The capacitors are subject to damage from line spikes, while the choke effectively absorbs them, again due to the limited frequency response of the core.

Toroidal transformers have wider frequency response compared to E-I because of the differences in core materials permitted by how they are made. If you find chokes made the same way, they will have similar wider frequency responses. This will not be what you want for parallel filter applications.

There is another problem. If the core domains do not limit the frequency response, the parasitic winding capacitance will. The capacitance will interact with the inductance to create a high impedance with high internal reactive currents.

I'm not aware of toroidal chokes with 5 or more henries of inductance, that would be suitable for 120 volt AC filters. Where did you see them?

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