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Old 20th February 2012, 01:22 AM   #1
percy is offline percy  United States
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Default DC offset on power line

I have read numerous threads about building a power line DC blocking filter but I am still looking for answers to some questions. I am building a RFI/EMI filter for the shack and am debating whether to include a dc filter with it or not. So -

How can you really detect if there is DC offset on the line ? I dont have a 'scope.

Besides transformer buzz what other effect could a dc offset possibly have on the performance of the equipment - say an amp or a dac(read clocks and PLLs sensitive to voltage supply).

How much offset is really something that needs to be addressed ? 0.5v ? 5v ? more ? less ?

Last edited by percy; 21st February 2012 at 12:22 PM.
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Old 20th February 2012, 01:35 AM   #2
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Isolation transformer - Wikipedia, the free encyclopedia
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Old 20th February 2012, 05:56 AM   #3
h_a is offline h_a  Europe
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Quote:
How much offset is really something that needs to be addressed ? 0.5v ? 5v ? more ? less ?
Well, that mostly depends on the transformers you use. Some enter saturation pretty quickly and start to humm as you already wrote. Others tolerate more.

Typically one sees about 1V or less dc on mains, but of course that strongly varies.

Hannes
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Old 20th February 2012, 07:05 AM   #4
Bonsai is offline Bonsai  Taiwan
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I recommend you go onto Rod Eliots ESP site - he has a very good write-up on mains DC and a circuit on how to fix it.

I had this issue when I was in Japan, and my transformer rattled very loudly (2KVA torroid) - and it only happened when the gas dryer or dishwasher were running - seems the loads were very unbalanced.
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Old 21st February 2012, 07:32 AM   #5
sek is offline sek  Germany
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I'm using a regular diode bridge and two antiparallel capacitors (as described by Rod Elliot) since a while. I have power supplies in wooden (speaker) enclosures, the boxes really emphasize the hum...

It works as expected, any toroidal transformer I connect it to instantly becomes dead quiet.

I find it most impressing, though, that Elliot measures over 52VA idle losses in his 500VA transformer, of which more than 48VA were due to core saturation as a result of just 264mV mains DC!

Cheers,
Sebastian.
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Old 21st February 2012, 10:47 AM   #6
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DC offset in your distribution line? So where would it come from? It would HAVE to be after the supply TX in your street - what possibly would cause this?
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Old 21st February 2012, 12:22 PM   #7
percy is offline percy  United States
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Thanks.
Still seeking an answer to this question though - all of you who tried the dc filter and quieted the transformer might be able to offer some input on this.


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Originally Posted by percy View Post
Besides transformer buzz what other effect could a dc offset possibly have on the performance of the equipment - say an amp or a dac(read clocks and PLLs sensitive to voltage supply).
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Old 22nd February 2012, 10:05 AM   #8
sek is offline sek  Germany
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DC offset in your distribution line? So where would it come from? It would HAVE to be after the supply TX in your street - what possibly would cause this?
Yes, except - depending on how and where you live - it's not necessarily the transformer just in one's street. In my case the local station supplies a couple of blocks full of apartment buildings and local shops.

What would cause the offset? All sorts of rectifying loads, motors, switching supplies, variable-frequency drive (VFD) circuits, etc.

Quote:
Originally Posted by percy View Post
Thanks.
Still seeking an answer to this question though - all of you who tried the dc filter and quieted the transformer might be able to offer some input on this.
I don't yet see where the original question is coming from. Why would you expect other effects besides core saturation in trafos?
OTOH, lack thereof could of course be beneficial to the supplied circuitry.

Transformer buzz isn't quite the EMI source, I suppose.

Cheers,
Sebastian.
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Old 23rd February 2012, 07:34 AM   #9
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So it is a pulsed DC more or less? IN basic terms, if you were to supply a device which has a different power factor to the device consuming current only on one polarity then you wouldn't notice it?
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Old 23rd February 2012, 10:39 AM   #10
DF96 is offline DF96  England
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Any device which takes more current on one mains phase than the opposite phase creates DC on the mains. Basically this could be anything which has a directly-connected (i.e. no transformer) half-wave rectifier. In the past this would have been (in the UK) almost all TVs and many valve radios - fortunately back then there were few toroidal transformers to get upset by DC. My guess is nowadays it is more likely to be a large number of small devices.

The DC voltage you see happens because of two things:
the transformer down the street is delivering 0V DC,
the resistance in the wiring (and the transformer) gives a DC voltage drop.
Hence you get DC. Because you need resistance to generate this DC voltage it probably means that the cause is near you on the mains - one of your neighbours, or possibly even in your own house! Not necessarily next door, but maybe three doors away as you will be on the same phase.
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