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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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    You are making another error Andy.

    Quite possibly... I'd read about a problem with permanent magnets holding contacts closed in RCDs somewhere and thought I'd understood the mechanism - I think my head reckoned that since the magnetic field didn't reverse, then you wouldn't get a reversal of the current in the secondary (the sense coil) - it may well vary in a sinusoidal kind of way, but without actually crossing zero. Hence the electromagnet would have a varying 'pull' but wouldn't actually get around to 'pushing' (or vice versa) that would be needed to counteract the permanent magnet. But I think you might have a point - the secondary current should vary with the rate of change of flux - so when the transformer's magnetic field collapses, the secondary current should reverse. If that's right then a transformer isn't so much a sine-wave-in, sine-wave-out (as per many diagrams) but sine-wave-in, cosine-wave out.


    Apologies to all for any confusion.


    I'll have to try and find what I read and try again - perhaps it was as Mike suggests as design where the L & N were wrapped directly on the electromagnet without a conventional toroid involved and I missed that detail.


       - Andy.
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  • 0
    You are making another error Andy.

    Quite possibly... I'd read about a problem with permanent magnets holding contacts closed in RCDs somewhere and thought I'd understood the mechanism - I think my head reckoned that since the magnetic field didn't reverse, then you wouldn't get a reversal of the current in the secondary (the sense coil) - it may well vary in a sinusoidal kind of way, but without actually crossing zero. Hence the electromagnet would have a varying 'pull' but wouldn't actually get around to 'pushing' (or vice versa) that would be needed to counteract the permanent magnet. But I think you might have a point - the secondary current should vary with the rate of change of flux - so when the transformer's magnetic field collapses, the secondary current should reverse. If that's right then a transformer isn't so much a sine-wave-in, sine-wave-out (as per many diagrams) but sine-wave-in, cosine-wave out.


    Apologies to all for any confusion.


    I'll have to try and find what I read and try again - perhaps it was as Mike suggests as design where the L & N were wrapped directly on the electromagnet without a conventional toroid involved and I missed that detail.


       - Andy.
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