DC offset on power line

percy · 12:22 PM

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 ?

alexberg · 20th February 2012, 01:35 AM

Isolation transformer - Wikipedia, the free encyclopedia

h_a · 20th February 2012, 05:56 AM

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

Bonsai · 20th February 2012, 07:05 AM

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.

sek · 21st February 2012, 07:32 AM

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.

riotpack · 21st February 2012, 10:47 AM

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?

percy · 21st February 2012, 12:22 PM

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.

Quote:

Originally Posted by percy

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).

sek · 22nd February 2012, 10:05 AM

Quote:

Originally Posted by riotpack

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

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.

riotpack · 23rd February 2012, 07:34 AM

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?

DF96 · 23rd February 2012, 10:39 AM

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.

20th February 2012, 01:22 AM	#1
percy diyAudio Member Join Date: Apr 2004 Location: MN	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.

20th February 2012, 07:05 AM	#4
Bonsai diyAudio Member Join Date: Aug 2006 Location: Taipei, Taiwan	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. __________________ bonsai Amplifier Design and Construction for MUSIC! http://hifisonix.com/

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Max allowable DC offset for power amplifier	diy didi	Solid State	5	8th January 2012 08:48 PM
DC offset in Class A SE Power Amp	ervinl	Solid State	15	8th August 2010 10:07 AM
Audio line DC offset	foryourdemons	Analog Line Level	3	27th March 2010 08:43 AM
DC offset after power disconnected	Pooley	Chip Amps	10	25th September 2008 11:33 PM
DC offset on household ac line ?	percy	Chip Amps	9	24th January 2006 09:30 AM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

20th February 2012, 01:35 AM	#2
alexberg diyAudio Member Join Date: Oct 2009	Isolation transformer - Wikipedia, the free encyclopedia

21st February 2012, 07:32 AM	#5
sek diyAudio Member Join Date: May 2003 Location: Berlin	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.

21st February 2012, 10:47 AM	#6
riotpack diyAudio Member Join Date: Sep 2011 Location: Brisbane	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?

23rd February 2012, 07:34 AM	#9
riotpack diyAudio Member Join Date: Sep 2011 Location: Brisbane	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?

23rd February 2012, 10:39 AM	#10
DF96 diyAudio Member Join Date: May 2007	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.