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Old 18th Nov 2015, 7:03 pm   #20
trsomian
Hexode
 
Join Date: Oct 2010
Location: Box End, Beds. UK.
Posts: 271
Default Re: How much DC on your mains ?

The point here is that even a few millivolts over many seconds build up to quite a lot of volt seconds of DC after a while, hence the saturation of the core, as signified by the increased hum. Presumably the transformer in question has a fairly sharp transition to into saturation; some do, but some don't.

An interesting academic experiment would be to run the transformer via two reverse parallel connected diodes, one diode with a little resistance in series with it. Get the resistor right and one could adjust out the offset, though only at one specific load current, and only until the DC voltage changed.
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