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Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ.

Re: Please explain doubling

209.215.72.246

[ Were would you place your test equipment probes on a common-mode choke to measure this doubled voltage? ]

How do you get the power from the AC line to the transformer? You hook across the hot and neutral, on the output side of the CM choke, but there must be a load present.

[ Is not the canceling out effect of a C-M choke the real reason it is used in the first place. It looks like almost no inductance to diff. signals (low Z) and its rated value of inductance to C-M signals (high z). ]

The reason it is used is to prevent the line current drawn through it from saturating the core. You can get away with a much smaller core, or wind a lot more turns when using a common mode choke. In effect, the loads current draw is cancelled out inside the core.

The current in one winding creates a (I will arbitrarily pick a polarity) plus plus field, and the return current creates a negative field, and they cancel. However, signals other than the 60 cycle power line frequency, such as fast spikes, or HF's, can come down either the hot or neutral only, and will not have a clear cut return path to allow cancellation, yet the fields in the choke core will couple immediately, and the opposite polarity voltage will appear on the other line on the output side of the CM choke.

When I was designing surge suppressors, I checked this very thing out, and sure enough, the CM chokes would do this on surges and spikes. Since they depended on the differential cancellation of the 60 cycle signal, they had no margin at all for surge currents, and would saturate at a very low level. Once the inductor is saturated, not only does the inductance go way down, but the act of going into and coming out of saturation often causes a highly non-linear pulse of energy to be developed on the output of the inductor. This is a separate phenomenon than the above mentioned effect.

Independant inductors on each line avoid all these effects, and still provide the filtering that is helpful (and needed if you want the best surge/spike protection). There is no down side for independant inductors, as long as they can handle the current draws. If they can not, then CM choke type construction will allow use of what would be an inadequate core for independant inductor use. This method of getting by is not the best way, just _a_ way. As with most things audio, getting by does not sound as good as what seems to be overkill.

Jon Risch


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