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I am looking to build an ac power line conditioner. It was suggested that I use a transformer with dual 6.3v secondaries wired in a common mode choke configuration. This is done by using the in-phase ends of the transformer secondaries. The primaries will not be used in this configuration and are just taped off. Does this sound like a better option verses just using an already manufactured common mode choke for power lines?

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  • \$\begingroup\$ I like toroidal coils, are the commercial chokes also toroids? Something already made sounds smaller and cheaper. \$\endgroup\$ – George Herold Jan 20 '15 at 0:41
  • \$\begingroup\$ I would think a toroidal power choke would be cheaper. Or you could buy just the core and wrap it however you want. \$\endgroup\$ – mkeith Jan 20 '15 at 5:30
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Power line conditioning needs a reasonable voltage insulation between the two coils that make up the common mode choke. If the voltage insulation were not that good, you could get breakdown between the coils and a fire.

I would have thought that a standard off-the-shelf transformer that has two 6.3 volt secondary windings would not have insulation that is rated to expect a few hundred volts between them. After all why should they - they will likely be isolated from the primary by either distance or some other form of breakdown protection.

In short, you cannot rely on this method - get a proper common mode choke that is rated at the AC mains voltages you use.

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Common-mode chocks made with bifilar winding, like baluns and the main use is to reject the common-mode noise.

Typical applications is power line filters, and differential digital data buses (i.e. Ethernet ), and often called “soft isolation”.

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Toroidal transformers can acting as filter because includes R,L and C elements. But winding is not bifilar, and also a high inrush current exhibits (because of absence of gap in magnetic path). In addition saturation of core is possible in high currents.

In toroidal transformer the leakage inductance between primary and secondary is low and acts like a low pass filter (first order) with usual corner frequency 50to 200kHz (20kHz for EI transformers), with very low attenuation rate. In ordinary line filters corner frequency usually is at 1kHz with an attenuation of 60dB at 1MHz.

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  • \$\begingroup\$ What does 'low pass filter (first order) ... with very low attenuation rate' mean? \$\endgroup\$ – user207421 Apr 23 '15 at 10:46
  • \$\begingroup\$ @EJP Smoth (not sharp) attenuation like 6dB/oct, and low phase shift. More here en.wikipedia.org/wiki/Low-pass_filter \$\endgroup\$ – GR Tech Apr 24 '15 at 2:27

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