Can approximately 5V DC on AC Mains cause ground loop issues in devices, many of which are DC audio devices ran by wall-warts.
I've always figured that AC Mains had approximately 0V DC since it usually comes from an AC transformer. If there shouldn't be any DC on the mains what could be a possible cause? Bad mains transformer or some type of DC feedback from devices?
"Alpha"
** No.** What happens afterwards is what matters.
** Any kind of load that draws more current from one half cycle in preference to the other.
This will cause the AC wave to become slightly lop sided and so develop an average DC component.
Usually the cause is quite local - like in the same premises.
I have a 1600 watt hot air gun that operates from one polarity when switched to half power - it causes about 1 volt of DC offset in the premises when in use.
.... Phil
What I've read, in the past, is that the AC mains can develop a significant DC offset, if there's a device drawing power from the mains in a non-symmetrical fashion (i.e. more from one half of the powerline cycle than from the other).
The commonest culprit I know of, was usually called a "lightbulb saver". It's a small disc which is inserted into a light-bulb socket (between the socket and bulb). It contains a diode, and thus forces the (incandescent) bulb to run on half-wave-rectified AC. The bulb runs cooler as a result, uses less electricity, and lasts longer (but you lose a greater percentage of light than you save in electricity, so it's not really a good deal).
The AC mains waveform can become somewhat asymmtrical (in effect, a DC offset) if you have enough devices like this pulling half-wave-rectified power at once (especially if all of the rectifier diodes are "lined up" in the same polarity).
This doesn't create a "ground loop" per se, but it can cause other difficulties. Some devices (e.g. audio amplifiers) are designed with toroidal-core transformers, and these transformers really don't handle DC offsets at all well... they can saturate and start to make an audible buzzing sound.
I don't believe that the DC offset tends to cause such severe problems with EI-core transformers, such as are usually found in wall warts. These cors have an air gap between the sections, and this makes them less sensitive to DC offset and saturation. I could be wrong about this, though - they might run hotter.
--
Dave Platt AE6EO
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In the 220/240 V world, tube televisions used half wave rectification directly from the mains to create something like +200 V anode voltages (PY series tubes, selenium rectifiers, silicon rectifiers).
However, since the mains plugs were non-polarized, the load was more or less symmetrical in most cases.
While I have done such experiments (with 1N4004 diodes), I have never seen any commercial products doing such things. According to my experiments, using half wave rectification with incandescent lamps below 250 W (@230 V) will cause unacceptable flicker.
In a 220 V world with dozens or hundreds of consumers on a single transformer secondary and nonpolarized plugs, this has never been an issue.
A long time ago, car battery chargers used a crude half-wave rectifier with current limiting by a lamp. Yes, note it was not line isolated.
At a battery dealer/shop [gone from the auto world, but still around for golf carts, etc] this DC offset was significant enough to cause some pole pigs to saturate and announce their displeasure [aka kaaboom].
(I can't even recall the rectifier but it predated selenium stacks.)
-- A host is a host from coast to coast.................wb8foz@nrk.com & no one will talk to a host that's close........[v].(301) 56-LINUX Unless the host (that isn't close).........................pob 1433 is busy, hung or dead....................................20915-1433
Ooogh. Makes today's concerns about industrial power factor and harmonic currents seem trifling by comparison :-)
Copper oxide, perhaps?
--
Dave Platt AE6EO
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Vacuum tube rectifier. Extremely common in the USA was the 35W4 used in direct line powered AM radios. The other tubes in the set included 12AX7 and the 50C5. The three tube filaments were connected in series and wired across the AC line through the power on/off switch.
-- Michael Karas Carousel Design Solutions http://www.carousel-design.com
On a sunny day (Mon, 20 Jun 2011 23:08:22 +0300) it happened snipped-for-privacy@downunder.com wrote in :
With a single rectifier you get sqrt(2) * 220V from 220V AC without a series regulator. That would have required at least one extra tube.
The old U type radios (150 mA heaters wired in series) may have had a single recifier.
All P type (300 mA wired in series) *color* TVs I have seen had a Si bridge rectifier. As color did not start here until 1967, si bridges were common.
Some very early BW TVs used a PY82 tube rectifier.
All these sets worked at a much higher anode voltage than 200V DC.
That was the All-American Five design, i.e. it had five tubes. The post-WWII variant was as follows (from Wikipedia,
"After the Second World War the set was redesigned to use miniature
7-pin tubes and the line up became:Cheers
Phil Hobbs
regulator.
While 310 V would be the _no_load_ voltage on a capacitor after a diode with zero voltage drop, the real situation was quite different.
With tube rectifiers and selenium stacks, the voltage drop was quite significant. Those rectifiers do not handle well large peak currents, thus it is out of the question to just have a large electrolytic capacitor after the rectifier. In practice, choke input filtering (or at least CLC filtering) had to be used to extend the rectifier conduction angle and hence avoid large peak currents. A form of power factor corrector :-).
Some radio receivers did not have permanent magnets in the loudspeaker, but an electromagnet coil was used to create the static magnet field, in which the actual voice coil moved. The electromagnet coil doubled as the power supply choke :-).
Due to these factors, the normal loaded anode supply voltage was about
200..250 Vdc for AC/DC radio/TV receivers with a half wave rectifier.recifier.
U series is 100 mA.
I got an AC/DC receiver from my grandmother, who had previously been living in an apartment building with 220 Vdc. It had UY1N half wave rectifier, two UCH?? triode/heksode and an UL?? power pentode. Working inside AC/DC receivers can be a bit dangerous, since the chassis can be at full mains phase voltage. Quite quickly one learns to make marks on the plug as well as close to the socket, so you know, which way to insert the plug :-).
rectifier.
Due to the late introduction of colour TV in Europe, only a few generations of TV sets were made with tubes, in 1972 receivers with CRT as the only tube were available.
Take a look at for example
With 220 Vac input, a 220 Vdc supply line from the first capacitor is available with high ripple voltage. After the S1 choke and a few large capacitors, 205 Vdc is available at low ripple.
For radio tubes, the Telefunken 1964 Tascenbuch uses an example configuration for UY85 with 220 Vac mains and 215 Vdc anode supply at
110 mA with a 100 uF capacitor.
Hardly; a 35W4 has a current capacity of 60ma or so. We are talking multiple tens of amps. THAT's why it was an issue with the pole pig's saturation.
That said, it may have been a vacuum tube. I recall seeing something like a 100 watt lamp glowing but can't recall if that was the rectifier or current limiter; or both.
-- A host is a host from coast to coast.................wb8foz@nrk.com & no one will talk to a host that's close........[v].(301) 56-LINUX Unless the host (that isn't close).........................pob 1433 is busy, hung or dead....................................20915-1433
On a sunny day (Tue, 21 Jun 2011 22:52:02 +0300) it happened snipped-for-privacy@downunder.com wrote in :
regulator.
Yes I had one with a speaker like that, it was not even a superhet, but multi tuned medium wave long wave, tubes like REN1004. Weight was very very heavy, sound was terrible.
Yes you are right, the 'diode' was also lossy.
recifier.
See how fast one can forget those things? I had one of those U types too for a short while.
Those were the 'cheap' radios. We had a real radio with a transformer, and AZ1 rectifier (2 diodes in a tube).
Yea, well the rule in the workshop is to have a separation transformer. In the later color sets with the si bridge the chassis was always live, no matter how you plugged it in. I am talking about Philips K6, The K8? was the first all transistor, and had a switchmode supply (with bridge rectifier of course).
rectifier.
I am not sure if there was a choke in the old U radios, that would have been too expensive. IIRC they even left out the speaker output transformer, and used 800 Ohm speakers directly in the anode of the audio output stage, at least for some of the cheap Ptube type BW TV sets.. Decent more expensive radios had a transformer in the power supply, and an EL84 with transformer in audio out.
I once took an old P series BW (70 degrees deflection, very much like in that diagram), and replaced all stages one by one with transistor circuits:-) Just for fun, to see if I could do it, had to put some new primary turns on the horizontal output. The only transistor I could get that could handle the voltage was some Motorola for car ignition. Dead slow Ge, but I did get a good picture. That was a first before you could buy all transistor sets except the small Sony portable, I used some of their circuits. Build the whole thing, including the tuner on a 100 x 160 mm euro card. Driving this huge 43 cm old tube :-) Give me a handful transistors, diodes, some ferrite cores, a CRT, transformer wire, deflection yoke, and I will build you a working BW set without having to look anything up. A bet I always wanted to make :-) These days with digital no way, you need chips. Have you seen my scope TV?
Or perhaps mercury rectifiers.
A single mercury tube rectifier could deliver power to a whole trolley bus line :-).
The mercury pool at the bottom was connected to the +600 Vdc trolley bus line, while the six anodes were connected to the centre tapped three phase transformer secondaries.
I believe you are referring to the tungar bulb rectifier. They were used in the thirties in many battery chargers. Mercury vapor type bulb, IIRC. They were also used in theaters to provide dc to the projector arc lamps in the thirties and early forties. Tom
DING DING DING.... That's exactly what it was...Thanks.
Were they mercury vapor? I don't think so. Mercury vapor rectifier have a pretty glow, but can't be bounced around. I worked on RF heaters that used same.
-- A host is a host from coast to coast.................wb8foz@nrk.com & no one will talk to a host that's close........[v].(301) 56-LINUX Unless the host (that isn't close).........................pob 1433 is busy, hung or dead....................................20915-1433
On a sunny day (Wed, 22 Jun 2011 00:01:27 +0300) it happened snipped-for-privacy@downunder.com wrote in :
Right, the last time I looked at one mercury tube was kept is a special screened room with a small sliding window for checks, to prevent the people against X ray radiation.
In the theater I worked at in the 60's the projector arc lights used 6 tungar bulb rectifiers off a 3 phase circuit. The carbon plate electrode would glow red.
The audio amp used mercury vapor rectifiers. You had to turn on the mercury vapor filaments a couple minutes before the high voltage.
screened room
radiation.
Are you really sure about this ?
While it makes perfectly sense to put a mercury rectifier into a separate room in order to avoid contamination due to an explosion or harmful UV radiation due to the mercury excitation.
If you are afraid of X-rays, keep the CRT anode voltage below 25 kV In order to avoid the 25 keV radiation, TV receivers were used with 25 kV shunt regulators, such as the PD500 power triode, in order to avoid these those rays.
On a sunny day (Thu, 23 Jun 2011 15:10:48 +0300) it happened snipped-for-privacy@downunder.com wrote in :
screened room
radiation.
Oh, yes, it was at Van Gelder Papier, in the Netherlands. That was (is?) a paper manufacturer. It powered their machines, they had their own power plant. I do not remember the voltage, was there just visiting. It was a BIG tube, pool of mercury at the bottom, and that nice blue glow.
All depends on the voltage.
I must have had so many xrays, I had a TV repair shop, and at one time we had a collection of those HV rectifiers and ballast diodes with nice coloured glass from the radiation. I remember in the TV studio (worked there too), at one time the firemen came with a measurement device to measure xrays at the front of the CRTs. I did hear it was way too high for their liking.
I did normally close the HV cage when working on those monitors and TVs, but some radiation you always get. Considering how old I have gotten it cannot be that harmful. On college of mine I heard worked with that HV cage open for some time, his face was all red on one side. I dunno if he is still alive, sort of a Darwin award thing,
There is some site on the internet IIRC that has a guy making Xray pictures of stuff with a PD100, or maybe it was the rectifier.
It is in a way a pity that CRTs have disappearing, your own personal linear accelerator:-) I have played a lot with those, made scope with one too. Still have a nice CRT colour monitor in the attic :-)
It's a bit more complex than that; the TV problem was with a particular fluorescence, of one of the rare earth elements that made a good phosphor. That particular rare earth also made circa 20 keV X-rays, too penetrating for convenient thicknesses of glass to shield.
The target atom that the electron hits will determine the X-ray energies produced, and anything above 2 kV is worth worrying about: the faceplate of a CRT is very good shielding for the lower energy X-rays, and the colored glass of rectifier tubes may well have been formulated for this reason, too. for X-ray shielding, too.
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