Eliminating DC on an AC line

--- 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.

-- John Fields Professional Circuit Designer

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.

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--- | | +--|

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?

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

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.

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...

Bye. Jasen