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What is this Capacitor Between Hot and Cold Chassis for?


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They're for RF suppression.

Notice how the capacitor and resistors are connected from the 0V of the low voltage DC rail to the high voltage rail?

The capacitors divert any high frequency signal which has leaked across the transformer's interwinding capacitance back to the mains. The resistors leak any static charge built up on the capacitors to prevent ESD and damage to them.

The capacitors must be Y1 rated and the resistors should be high voltage, each one should be able to withstand 4kV peak and be flame retardant, in order to fulfil the requirements of double insulation.

Here's some information about supression capacitors, note the withstand voltage of the class Y1 capacitors.
http://www.epcos.com/web/generator/Web/Sections/ProductCatalog/Capacitors/FilmCapacitors/PDF/PDF__EMIGeneralStandard,property=Data__en.pdf;/PDF_EMIGeneralStandard.pdf
http://www.okaya.com/images/noise/1100.pdf
   

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