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Old 18th Nov 2015, 7:50 pm   #21
merlinmaxwell
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Default Re: How much DC on your mains ?

Quote:
Originally Posted by happytiger View Post
No don't try that, you may get high voltages on the transformer if you get near resonance.
Why not try (after suitable calculations) you would be very unlucky (unlikely too) to get resonance in a loaded transformer. A huge electrolytic could be used, they cope with a few hundred millivolts backwards, not to sure about long term though.
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Old 18th Nov 2015, 9:12 pm   #22
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Default Re: How much DC on your mains ?

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Originally Posted by merlinmaxwell View Post
... A huge electrolytic could be used, they cope with a few hundred millivolts backwards, not to sure about long term though.
Thanks for the continued interest folks ! You can buy commercial DC blockers for use with low-power equipment (hi-fi for example) which are based on exactly this. Some use two large electrolytics back-to-back. They do need to be able to handle enough current of course. They can be protected against overvoltage by putting a pair of antiparallel diodes across them. A meaty bridge rectifier with + connected to - and the two ~ inputs connected to one another gives you a two-terminal device with four large diodes in two antiparallel pairs. This will handle fault currents large enough to blow any reasonable fuse in the plug.

Does anyone fancy making a measurement ?

Cheers,

GJ
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Old 18th Nov 2015, 9:34 pm   #23
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Default Re: How much DC on your mains ?

I wonder if this is part of the reasoning behind the wave of ex-BT "mains conditioning units" that appeared on the surplus market a few years back in ratings from 125 to 1,000VA? I got a couple of 250VA ones for a tenner a while back from a BT guy who had a garage-full- "not sure what these are for...."- intending to use them with cabin receiver Eddystones and the like as they are essentially a 1:1 transformer with one side of the output connected to case, i.e. blue and green/yellow commoned on the output socket. There are what appear to serve as a resonator capacitor and an RC snubber on the output winding- presumably, as well as DC component blocking, there's an element of smoothing out the shape of a mains waveform that's been distorted by electronic loading.

I have to say that I was always a tad sceptical of the audiophile mains conditioner market, seeing them as (sometimes very!) expensively dressed up versions of the above devices but if they let buzz-prone transformers run quiet when there's an appreciable DC supply component, then there's at least sound (!) reasoning behind the idea.
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Old 18th Nov 2015, 11:43 pm   #24
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Default Re: How much DC on your mains ?

GJ & Guys, its generally assumed that Electricity Supply Companies just provide pure 50Hz sinusoidal AC to properties. That's not the case these days. Ultra modern digital metering can be read remotely using data transmissions to & from the meter, via the supply mains cabling. For some years, Total Control heating tariffs use a timeswitch control signal superimposed on Radio 4's 198KHz signal. Some scallywags use phase changing methods to reverse the older analogue(rotating disc) meters. Some sub-stations have an amazing array of electronic & radio equipment, not just an old transformer & a couple of oil switches.
Any of this could be going on in your neighbourhood, and resulting in spurious small DC readings in your property.

Regards, David

Last edited by David Simpson; 18th Nov 2015 at 11:44 pm. Reason: spelling mistake
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Old 19th Nov 2015, 7:38 pm   #25
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Default Re: How much DC on your mains ?

I've seen a couple of videos showing what appears to be a transformer which when plugged in seem to stop or reverse the meter I'm guessing this is what your referring to above. Do BT or other telco company's now not bother with conditioning the mains? I saw one of these units hanging by its wires in a cupboard in an office a while ago
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Old 20th Nov 2015, 12:21 pm   #26
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Default Re: How much DC on your mains ?

Well Hannahs Radios, I'm not going to reveal the innards of what is known in the electrical supply industry as a "Black Box" on an open Forum. Its illegal, and affects us all because Revenue Protection is financed by our electricity bills. It was part of my job for a number of years to catch electricity theft scallywags.
Plans have been afoot for a number of years by gas, water, and electricity utilities, to utilise the mains cabling for metering data transmissions. I even heard somewhere that internet trials have commenced. But you'd better ask Forum guys with Telecomms expertise about that.

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Old 20th Nov 2015, 1:03 pm   #27
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Default Re: How much DC on your mains ?

We all live in fear of them wanting to stuff data down the mains even relatively local use of such a system within a house radiates horrible QRM but I'm getting off topic here sorry mods please forgive me
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Old 20th Nov 2015, 2:29 pm   #28
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Default Re: How much DC on your mains ?

I went to an IEE lecture on mains signalling some years ago. The speaker commented on how difficult it was to put into practice, due to the very different transmission characteristics of different supplies, not to mention the different types of installation in different countries, and how difficult it was to find this sort of information. The blocking effect of substation transformers was mentioned. It must have been more than a decade ago, and I don't know what the present state of the art is.
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Old 20th Nov 2015, 3:18 pm   #29
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Default Re: How much DC on your mains ?

All I know is that I've recently been contacted by the electricity supplier of a non-residential property with which I'm involved wishing to replace our meter with a "smart" metering system - which I assume must involve meter-reading signals travelling back and forth through the grid between our property and their monitoring centre.
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Old 21st Nov 2015, 12:03 pm   #30
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Default Re: How much DC on your mains ?

UK smart meters often use GSM, not mains signalling. Some may form a local mesh network, using some short range radio method, with one meter doing the GSM on behalf of all of them.
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Old 24th May 2020, 8:00 am   #31
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Default DC on the mains???

Just popped over to Diyaudio and noticed a DC mains blocker FS as well as "DC on the mains" in a thread and was perplexed. Apparently toroidal tfmr's don't like DC & hum as a result.

When i searched online I found this article - https://sound-au.com/articles/xfmr-dc.htm which stats that a DC offset can be present of our supplys, this is a new one on me, rubbish from SMPSU's and motors I'd heard about but not DC.

According to the article some old hairdryers and heatguns can cause this, who knew? It's probably nothing new to most of the old hands but thought it might be new to some of us.

Andy.
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Old 24th May 2020, 8:48 am   #32
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Default Re: DC on the mains???

I seem to recall back in the 70's there was concern over the practice of using half-wave rectifiers in AC/DC TV sets. It was said that the millions of TV sets in use were causing a DC offset on our domestic supply. I don't know how true it was or what (if anything) was done about it.
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Old 24th May 2020, 9:05 am   #33
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Default Re: DC on the mains???

Not sure it was half-wave rectifiers, more like thyristor power supplies chopping great lumps out of the mains. These were used in sets that had 'half-mains' live chassis at around 110V....never did like the idea. The manufactures used big chokes on the mains input to reduce the noise fed back into the mains.
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Old 24th May 2020, 9:10 am   #34
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Default Re: DC on the mains???

Half Wave rectifiers did cause a problem for the CEGB but it was relatively short lived in the scheme of things, bridge rectifiers were introduced into the power supplies, it put the chassis at half mains voltage but that was another problem.
I have heard of the audio groups discussing DC on the mains and suitable equipment to buy to cure it. If you canít hear much from the transformer I wouldnít worry about it, more likely poor installation or loose windings will cause more of problem.

Edit, just noticed Sidebands post.
The CEGB complained about it in the middle 60ís from what I recall, made worse when the heater circuits included a diode, that meant only the +ve half cycle was in use. The change over to the power supplies with bridge rectifiers brought their own problems as you state.
I always thought it was just me who didnít like the half mains chassis idea, seems I was not alone.
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Old 24th May 2020, 9:15 am   #35
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Default Re: DC on the mains???

There is a thread here from 2015, not sure how to link to it but search for 'How much DC on your mains' it even has a real measurement!

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Old 24th May 2020, 9:18 am   #36
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Default Re: DC on the mains???

Could have sworn I posted something, but it seems to have vanished. Must've contravened a rule unwittingly.

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Old 24th May 2020, 9:28 am   #37
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Default Re: DC on the mains???

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Old 24th May 2020, 10:01 am   #38
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Default DC on mains

Somewhere, can't remember where or when but fairly recent, there was another discussion about this.

Apparently the real problem of DC on the mains is that an RCD is saturated, only 30mA, and won't work. That is nasty.

There is a B version of an RCD that can cope with limited DC.

If I can find where I read it will post more, but it is of importance.

Last edited by Station X; 24th May 2020 at 10:09 am. Reason: Threads merged.
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Old 24th May 2020, 10:53 am   #39
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Default Re: How much DC on your mains ?

There are four main types of RCD in use (AC, A, F & D) with immunity to various undesirable forms of residual current. See: https://hager.com/uk/support/regulat...n-of-rcd-types

Anyone installing an RCD should consider the likely presence of leakage currents on the protected circuits other than the AC fundamental, and fit a device capable of both responding to such leakage and remaining correctly calibrated in its presence. Note that it is the DC component of the leakage that causes the problem, not that of the total load current, and therefore most likely to reach significant levels if a load leaks asymmetrically, rather than through linear leakage of a supply with a small DC component.

A historical note about egregious dumping of DC into the mains: One of the first attempts in the UK at a fully electronic theatre-lighting dimmer was Strand's 1949 thyratron-based unit that came to be known as 'Woody's Electronic' after J.T.Wood, its designer. Probably to sidestep a patent by Century in the USA who were working towards a similar goal, Wood started out by using three thyratrons per channel, one on each phase, to provide controlled 3-pulse rectification feeding variable voltage DC to the lamps. The entire current used by the installation was DC and the sum of the three line currents, totalling some hundreds of amps of DC, returned via the neutral! Not surprisingly, one of the first installations to be conmissioned (possibly Reykjavik Opera House) resulted in the neutral of the theatre's main supply cable burning out. It had other problems including thyratrons running away, so the company soon returned to electromechanical dimmers pending the arrival of thyristors.
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Old 24th May 2020, 11:10 am   #40
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Default Re: How much DC on your mains ?

Hi!

Quote:

Well Hannahs Radios, I'm not going to reveal the innards of what is known in the electrical supply industry as a "Black Box" on an open Forum. Its illegal, and affects us all because Revenue Protection is financed by our electricity bills. It was part of my job for a number of years to catch electricity theft scallywags.
I remember seeing illicit advertisements for these things in the less salubrious journals!

Wouldn't work on our electricity meter tho', it's a fully electronic one using Hall sensor sensing!

Chris Williams

PS!

We have no grumbles about our electricity bills!
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