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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Ballasting Resistors
These will take the form of two equal resistors (a few ohms each), inserted in series with each half-winding. Though this is often recommended, despite the small associated power loss, and is in principle useful, especially for short cables, the truth is nobody is really using ballasting resistors. In Midspans, however, there is a greater need to use such techniques, since there is no ballasting effect from the DC resistance of the (equal) winding halves of a transformer. Note that common-mode filters are often inserted in series with the PoE rails, and these also help in this regard, often unknowingly. Though the filter is primarily present for EMI filtering and common-mode noise rejection, its windings provide ballasting ...

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