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Old 17th Nov 2015, 8:55 pm   #1
GrimJosef
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Default How much DC on your mains ?

Having just spent a day or two struggling to find the cause of an occasional buzz in a toroidal mains transformer, which turned out to be DC on my mains supply, I'm curious to know just how much DC there typically is on people's mains. I don't know if my supply is unusually troubled, or actually rather clean (and the transformer is too sensitive), or somewhere in the middle.

Has anyone made any measurements ? Would people be interested in making some, so we can establish a norm ?

The values I had were, I thought, tiny. But they made all the difference to this transformer. Given that they are tiny though it does take just a little effort to measure them. IT ALSO INVOLVES MAKING DIRECT CONNECTIONS TO THE MAINS, SO IF YOU'RE NOT CERTAIN THAT YOU CAN DO THIS SAFELY THEN PLEASE DON'T TRY IT !

The setup I used was a 220k resistor, which I was sure would handle the full mains voltage and the power (a quarter of a watt or so) very comfortably, in series with a 22uF non-polarised capacitor (in my case polypropylene film) which I decided should be rated for more than 10V (there will be several volts of AC across it, as well as whatever DC is on the mains). I connected the spare lead from the capacitor to the neutral pin an IEC plug (the one with the pins) and the spare lead from the resistor to the IEC's live pin. I hooked a DMM set to DC volts across the capacitor. I then plugged an IEC mains lead trailing socket) onto the plug and, last of all, I connected the 13A plug at the other end of the mains lead to a 13A outlet in the wall.

I've watched the DC voltage reading on the DMM all day. It's tended to read between 10mV and 25mV. But yesterday evening it was reading 65-75mV and that was sufficient to make the transformer buzz. I was staggered. So staggered that I made up a synthetic supply with a 1:1 isolating transformer and an adjustable DC source in series so I could turn the DC up and down at will. Sure enough, at 40mV DC the transformer was quiet and at 60mV DC it was buzzing.

So is my 75mV unusually high ? If anyone has measured the DC component on their mains then I'd be very interested to hear how large it was and how much it varied.

Mods - I'm not sure if this is the right place for this thread. But there are passing comments on the problem of mains DC making toroids buzz here https://www.vintage-radio.net/forum/...d.php?p=673158, without any measurements though.

Cheers,

GJ
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Old 17th Nov 2015, 9:10 pm   #2
merlinmaxwell
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Default Re: How much DC on your mains ?

I can't see a way of 'genuine' DC happening on normal mains, but an asymmetric waveform (caused by rectifiers etc.) is more than likely looking like DC to most meters and transformers. An interesting observation GJ.
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Old 17th Nov 2015, 9:17 pm   #3
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Default Re: How much DC on your mains ?

There wont be alot of dc on mains since your 240V comes off a transformer in a substation which is usually pretty close in towns and up a pole in the country. I'd agree with the probability of maybe harmonic distortion from goodness-knows what reactive loads at some point. ISTR a good example in the past was half wave rectified heater chains, though we've generally moved on!?
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Old 17th Nov 2015, 9:52 pm   #4
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Default Re: How much DC on your mains ?

Isn't this rather wooly thinking? Connect a rectifier and lightbulb across the mains, there is a DC current flowing in the tfmr. This will be divided out among the many load devices, but it's 'genuine' DC

Your next challenge is to measure the DC resistance of the supply
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Old 17th Nov 2015, 10:04 pm   #5
GrimJosef
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Default Re: How much DC on your mains ?

That's the beauty of hard measurements. They cut through the wool . The DC component is as real as any other DC and the consequential real-world buzz of the transformer was hard to ignore. I've just been out to check my DMM and it's 110-115mV now. Hmmm.

Cheers,

GJ
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Old 17th Nov 2015, 10:21 pm   #6
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Default Re: How much DC on your mains ?

Yet another good reason for using an isolation transformer to supply sensitive kit.
Rob.
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Old 17th Nov 2015, 10:39 pm   #7
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Default Re: How much DC on your mains ?

well surely supply transformers are 3 phase and not everyone is going to sync their dc usage all on the same phase. Hence harmonic distortion. Bridge rectifiers are still using both halves of the supply cycle
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Old 17th Nov 2015, 10:46 pm   #8
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Default Re: How much DC on your mains ?

Some loads draw DC current from AC mains, and the resultant voltage drop appears as a DC voltage superimposed on the AC supply.

For example, if a load draws more current on the positive half cycles, then the voltage drop between substation and consumer might be 8 volts on the positive half cycles and 7.8 volts on the negative half cycles, or a difference of 200mv.

Some cheap imported appliances draw substantial DC currents from AC mains by use of a silicon power diode in series with a heating element to give reduced heat.
Hair dryers in particular seem to use straight mains on the element for full heat, and half wave rectified for low.
Many later valve TV sets used a half wave rectifier in series with the valve heaters and therefore draw about 0.3 amp DC.
In some countries stairway lighting on delay switches is required to give warning that it is about to go out, normally this is done by half wave rectification.
Failed gas discharge lamps can work as crude rectifiers and the larger sizes can draw many amps of DC.

Some specialist theatre lighting dimmers are actually designed to give a half wave rectified output so as to drive one lamp on the positive pulses and another lamp on the negative pulses, thereby giving two control channels from one dimmer and cable run. If only one channel is used then several amps of DC can be drawn.
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Old 17th Nov 2015, 11:15 pm   #9
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Default Re: How much DC on your mains ?

so why doesnt the effect average out over each phase? Doesn't the random power factor of multiple mixed ac & dc loads come into play? Industrial users of equipment containing rectifiers capable of distorting the mains (even symmetrically across positive and negative cycles..which is usual for any single rectifier load capable of distortion) have to undertake to mitigate the effect by installing large reactors or phase shifting transformers or otherwise they get fined.
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Old 18th Nov 2015, 12:56 am   #10
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Default Re: How much DC on your mains ?

In the days of half wave rectifiers in TVs only conducting on each half cycle could indeed upset the symmetry of the mains supply but I think most things use bridge rectifiers now. The main problem currently is RF noise.

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Old 18th Nov 2015, 12:58 am   #11
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Default Re: How much DC on your mains ?

I suspect that often effects do cancel out, but in some circumstances even a single load drawing several amps of DC current from AC mains can be significant.
A relatively long AC main and the transformer winding supplying it might have a resistance of 0.1 ohm, which at say 250 amps load might equate to 250 volts at the transformer and 225 volts at the far end.
A load drawing 4 amps DC near the far end of this main would result in about 400mv of DC voltage drop which as noted in the first post can be significant.

Hairdryers are normally only used briefly but a failed high wattage discharge lamp might draw several amps a DC for week or more.
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Old 18th Nov 2015, 8:54 am   #12
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Default Re: How much DC on your mains ?

An electronically controlled AC load can generate a net DC current when it conducts a bit more current during one half-cycle, than the other half-cycle. A diode bridge with different on-voltages of diodes would be a minor example - eg. any smps. There are reports of large power equipment using thyristor regulation that can cause a problematic net DC current flow due to the subtle on-voltage differences between back-to-back regulating SCRs. In a domestic/commercial envrionment, that may be commercial heating control elements, old battery chargers - perhaps welders.
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Old 18th Nov 2015, 11:22 am   #13
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Default Re: How much DC on your mains ?

I was called to a problem with intermittent AC motor control in Kent. The problem was only present for short periods, so it involved sitting drinking coffee with an eye on the scope. Eventually the culprit showed itself. At the peak of one half-cycle, a surprisingly large chunk was being taken out of the mains (around 20% for around 5 m.s.) This was causing the triac controller to go mad. The culprit was traced to a plating organisation on the same industrial estate. It would not take much treatment of this sort to result in an asymmetric waveform having an apparent DC value.
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Old 18th Nov 2015, 12:33 pm   #14
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Default Re: How much DC on your mains ?

The turn-on current in to AC devices such as transformers and simple capacitive filter DC power supplies, or switchmodes without softstart, will exhibit a step sawtooth style DC transient.
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Old 18th Nov 2015, 4:20 pm   #15
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Default Re: How much DC on your mains ?

This is quite a good read,
http://sound.westhost.com/articles/xfmr-dc.htm
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Old 18th Nov 2015, 5:37 pm   #16
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Default Re: How much DC on your mains ?

I learn something everyday and now see how DC can be superimposed on the AC mains. Thank you all. I suppose a solution would be to feed the transformer via a capacitor.
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Old 18th Nov 2015, 6:39 pm   #17
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Default Re: How much DC on your mains ?

No don't try that, you may get high voltages on the transformer if you get near resonance.
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Old 18th Nov 2015, 6:46 pm   #18
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Default Re: How much DC on your mains ?

The problem with DC on the AC mains is that you could saturate the transformer core, in fact back when TV sets used half wave rectification the DC load was significant at the sub station transformer ,this would reduce the KVA rating of the transformer and of course it would buzz loudly. Back in those days you were not encouraged to ensure that the chassis was always connected to neutral (to try to balance the load) plugs were reversible to enable this, however most engineers preferred to leave the repair with chassis connected to neutral, (except with DC mains).
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Old 18th Nov 2015, 6:52 pm   #19
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Default Re: How much DC on your mains ?

Back in the '60s, I went on a short course on Process Control, my interest being in firing pottery. We had a tech rep gave us a talk, he was from a US company, West I think, maybe Leeds and Northrup. Anyway, part of his talk was of power control of electric furnaces. No longer big saturable reactor / tapped transformer, but thyristor control. However, he cautioned us that many manufacturers were using phase angle firing, which upset the mains as it only fired on one part of the positive going waveform, and as such "screwed up" the mains supply. Their particular controller, however used something called "Cycle syncopation" and this was wonderful, not affecting the mains.
I suppose this was an early example of zero crossover switching. If I go and dig out my notes, I may still have the product sheet he gave us all.
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Old 18th Nov 2015, 7:03 pm   #20
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Default Re: How much DC on your mains ?

The point here is that even a few millivolts over many seconds build up to quite a lot of volt seconds of DC after a while, hence the saturation of the core, as signified by the increased hum. Presumably the transformer in question has a fairly sharp transition to into saturation; some do, but some don't.

An interesting academic experiment would be to run the transformer via two reverse parallel connected diodes, one diode with a little resistance in series with it. Get the resistor right and one could adjust out the offset, though only at one specific load current, and only until the DC voltage changed.
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