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Common mode filter choke...
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Old 27th November 2003, 07:37 AM   #11
ashwin is offline ashwin  India
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Exclamation carefull!

Originally posted by JojoD818

I also have a bunch of those small toroidal looking chokes that came from pc psu. I was hoping I can re-wound it and make use of it in filtering mains or my rails just before the bridge rectifier.

you must ensure that those chokes are really meant for common mode filtering Check the winding directions on both coils on that torroid. If they are not wound for common mode use, then the flux in the ferrite core would cause huge currents to flow in the core, heating it up and cracking it. The heat could burn the insulation off the windings as well, and cause a mains short. I know because this is exactly what happened to me! I wound the coils for a common mode choke the wrong way and had some nice fireworks

As I understand it, a common mode choke must have negligible flux in the core during normal operation (i.e. the fluxes from the two windings must cancel each other). Only in the presence of common mode noise is some net flux generated in the core. What this means essentially is that the choke acts as a short for differential mode AC, and a high inductance for common mode AC line noise, thus allowing it to pass the normal (diff-mode) AC and block the common mode noise.

- Ashwin
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Old 27th November 2003, 02:59 PM   #12
JojoD818 is offline JojoD818  Philippines
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Common mode filter choke...

thanks for the insight. I myself am very careful especially when tinkering with mains wiring.

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