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How do ferrite cores work?

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    #2
    Stoner
    Registered Users Posts: 10,952 ✭✭✭✭Stoner


    I worked with these a long time ago on switching mode electric power supplies. this is how they were used at the time

    The PSUs were certified to work from 190 Vac to 250Vac. However they were being used on a grid that operated at 180Vac a lot. This meant more current was drawn to compensate for the lower voltage to deliver the same power.

    This extra current was causing some of the components to blow, so there were some newer components, I was working on the filtering etc.


    However all this work required extensive retesting of the unit, particularly for Canada, they didn't want to release a second version of the unit, they just wanted to make one type that covered multiple regions. (same company produced 300 different types of TV at the time!!)


    When is was all complete, the unit was producing excessive Electro magnetic fields, over and above the allowed amounts.

    the final revised product required a 2 ferrite rings on the mains lead, for suppression. the position of the ring, or tore as they call them, with respect to the plug top was significant, as a junior that was my job (or what i helped do), measuring the field intensity within a range of distances from the plug top and the amount of rings. Selecting the correct unit was important, as was the amount of rings, going from 1 to 2 made a difference, but from 2 to 3 was not significant. Back then you didn't get leads with built in rings with the distance from plug top moulded into place. So the units were shipped with two rings and documentation saying to add them to the lead and the distance they should be from the plug top.



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