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Eliminating DC on an AC line

Discussion in 'Electronic Basics' started by Harry Houdini, Dec 22, 2005.

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  1. Hi there,

    I was wondering if anyone could point me in the direction of
    instructions on how to rig up a filter to remove DC from an AC line
    (120V, 15A, 60Hz). I've got 0.3 Vrms to get rid of. I understand it
    can be done with zeners.

    Thanks a bunch!
  2. John Fields

    John Fields Guest

  3. Haha! Someone's quick! Smoke and mirrors - good one!

    Assuming that there is substantial DC coming in on my AC line but that
    my description is in error, do you have any ideas on how to remove it?

    Appreciate your help and interest.
  4. John Fields

    John Fields Guest

    What I'd suspect is that you have a load somewhere that's being fed
    by half-wave rectified mains. I don't know what it's likely to be,
    but the easiest way to find out would be to disconnect operating
    devices from the mains while monitoring the mains for DC. When you
    disconnect the right device the DC will go away.
  5. John G

    John G Guest

    Be really interesting to know how you measured 0.3volts DC anyway.
    What did you use for a reference point and what did you use for a
    measuring instrument?

    And anyway Wots the DC doing for you that you are worried about it?
  6. John G

    John G Guest

    Sorry John that was my fault replying to the wrong person I should have
    been one step up.
  7. Jasen Betts

    Jasen Betts Guest

    typically a capacitor is used to block DC

    the sort of capacitor that would pass a usable portion of your 120V 15A 60Hz
    would be extremely large and expensive.

    Is this 0.3V constant?

    possibly you could do something like this:

    D1 15A
    L --+-->|--+--o-_-~-o---
    | |
    N ----------------------

    D1 silicon 0.6V forwards voltage drop
    D2 Shottky 0.3V forwards voltage drop

    the fuse is in case either diode fails open circuit.
    it may be enough to protect your unspecified load from
    80V DC

    before installing that I'd check that your earth and neutral connections at
    the supply are functioning correctly
  8. kell

    kell Guest

    I'd want to know the impedance of the source of that 0.3 volts...
    assuming it's coming in on the line and not the result of some load as
    John Fields suggested.

    All I can think of is shunt the mains with a massive inductor.
    You could just use a transformer and leave the secondary open
    (tape the wires off).
    Put it right before whatever sensitive load concerns you.
    That would shunt any dc, right?
    The series resistance of the transformer primary would have to be
    significantly less than the impedance of your 0.3 volts.
  9. Jasen Betts

    Jasen Betts Guest

    Please check my reasoning below, I don't claim to be an expert here.

    The impedance of the 0.3 will be that of the mains supply
    which is essentially that of the transformer at the other
    end of the supply in series with the wiring but in parallel
    with all the loads.

    I measured the DC resistance (or rather attempted to) of a 1500VA transformer
    (this being the largest I possess) I was unable to see and difference from 0
    ohms on my analogue VOM (so I'll call it less than 0.1 ohms) (and I don't
    trust my Digital meter with inductors))

    Success... I measures a 12VA transformer at 4ohms. (this is for 240V 50Hz
    supply) for 120V 60Hz typical resistance will be lower.

    hmm, into 4ohms, 0.3V is 750mA
    but at 240V 12VA is only 50mA RMS (so 71mA peak),
    so a small inductor like that 12VA transformer is going to saturate,

    with a physically large inductor (like my surplus 1500VA isolating
    transformer) - assuming resistance is inversely propirtionall to the
    VA rating - I get approx 0.03 ohms fir its DC resistance. if the wiring
    before the transformer has enough resistance to keep the DC current below
    the transformer's 8.8A saturation limit it'd work... so for 6A at 0.3V a
    resistamce of atleast .04 ohms is needed, for the shunt inductor
    (transformer) to operate as an inductor.

    This is total resistance so 0.01 ohm in the supply and 0.03 in the
    shunt inductor, the inductor would be under no extraordinary stress,
    but with a setup like that the measured 0.3V woun't be reduced much...

    you need a supply resistance significantly larger than the inductors's
    resistance. EG 0.3 Ohms - that'd get you approx 10:1 reduction down to

    through my 4 Ohm 12VA modem transformer that's 75mA, still above the peak
    current but not by much, it'd probably survive with no load, but what use
    is a transformer that can't drive any load...

    OTOH if I plug the modem transformer into the isolating transformer's output
    there'll be no DC there :)

    of-cource with the DC present I can't get the full 1500VA from te isolating
    transformer's output. but it should be able to spare 12VA for the modem and
    a few for the alarm clock etc...

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