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A choke is a real-world inductor.

Posted by Al Sekela (A) on May 1, 2007 at 17:13:29

In Reply to: Help me-I'm the New Kid in Town posted by nightdoggy on May 1, 2007 at 10:18:17:

An ideal inductor would have impedance that rises with frequency for any frequency:

Z = j * omega* L,

where j is the square-root of -1, omega is the angular frequency (twice Pi times the frequency in Hz), and L is the inductance expressed in units of henrys.

The Hammond 193L is a 5 henry choke designed for use as a filter element in a tube amp B+ supply. It has a core made of laminated steel plates, and is designed to exhibit 5 henrys of inductance with a DC current of 300 milliamperes. It will exhibit somewhat more than 5 henrys with zero DC current through it.

The winding insulation is robust and suitable for exposure to the AC line voltage. The coil resistance ("DC" impedance) is 57 ohms.

The steel plate core limits the high frequency performance. Magnetic domains in the steel have to switch their magnetic direction in response to changes in coil current in order for the choke to exhibit inductance. If the frequency of the coil current exceeds the ability of the domains to follow, the core will effectively disappear and the inductance will drop to the value that the coil alone would have due to its geometry.

This makes the choke an effective filter for RF noise present on the AC line. The inductance limits the 60 Hz current to 64 millamperes or so on a 120-volt line, but the core speed limitation reduces the impedance to RF noise.

Note that all transformers with similar construction have similar frequency limitations and can be used as RF noise filters. However, secondary windings on transformers may ring due to parasitic capacitance unless they are loaded with non-inductive resistors. Unless you are a dedicated tinkerer with access to cheap surplus transformers, the choke is the cleanest way to experiment with this kind of filtering.

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