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DC on AC supply

nicemark
I have seen a number of videos which demonstrate the problems caused by DC currents on AC supplies with respect to RCD's. An issue which is apparently increasing with increased technology applications.  Please could someone explain how DC gets back into the neutral.


Almost all PSU's in all equipment begin with an isolating transformer. So getting a significant pulsed "dc" signal back that way is not impossible but tricky. (Pulses only occurring on one side of the cycle). 


My thoughts are that the DC is most often "created" by an unbalanced waveform, with a muted positive cycle, giving an overall DC flow.


Please could you direct me to, or give examples of how DC gets into the neutral, and/or just how much of an issue it can be in some homes.
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    A number of cheap domestic appliances draw DC current from AC mains. Hair dryers, hair straighteners, and small cooking appliances. They use full mains onto the element for full power, and mains via a silicon power diode onto the element for half power.

    Older valve TV sets draw DC current from AC mains. The valve heaters are wired in a series chain and connected to the mains via a silicon power diode.

    Valve radios usualy obtain the HT supply from half wave rectification of the mains.

    Older traffic lights and Belisha beacons draw DC current from AC mains at night. During the day the lamps use full mains, at night they are dimmed to reduce glare and prolong lamp life. The dimming is achieved by switching a diode in series with the lamps, a slight line frequency flicker is observable on the lamps at night.
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  • 0
    A number of cheap domestic appliances draw DC current from AC mains. Hair dryers, hair straighteners, and small cooking appliances. They use full mains onto the element for full power, and mains via a silicon power diode onto the element for half power.

    Older valve TV sets draw DC current from AC mains. The valve heaters are wired in a series chain and connected to the mains via a silicon power diode.

    Valve radios usualy obtain the HT supply from half wave rectification of the mains.

    Older traffic lights and Belisha beacons draw DC current from AC mains at night. During the day the lamps use full mains, at night they are dimmed to reduce glare and prolong lamp life. The dimming is achieved by switching a diode in series with the lamps, a slight line frequency flicker is observable on the lamps at night.
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