5V DC on AC Mains

[Alpha]

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?

 

[Phil Allison]

"Alpha"

Can approximately 5V DC on AC Mains cause ground loop issues in devices,

** No.

I've always figured that AC Mains had approximately 0V DC since it usually
comes from an AC transformer.

** What happens afterwards is what matters.

If there shouldn't be any DC on the mains what could be a possible cause?

** 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

 

[[email protected]]

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).

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.

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).

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.

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).

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.

 

[David Lesher]

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).

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.)

 

[Michael Karas]

Copper oxide, perhaps?

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.

 

[[email protected]]

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.

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.

The old U type radios (150 mA heaters wired in series) may have had a single 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 .

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.

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.

Some very early BW TVs used a PY82 tube rectifier.
http://www.radiomuseum.org/tubes/tube_py82.html

All these sets worked at a much higher anode voltage than 200V DC.

Take a look at for example
http://frank.pocnet.net/instruments/Philips/HR/TV/TV-docs/21CX211A/21CX211A.pdf
the power supply is on page 11.

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.

 

[David Lesher]

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.

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.

 

[[email protected]]

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.

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.

 

[David Lesher]

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.

 

[Dennis]

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.

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.

 

[[email protected]]

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.

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.

 

[josephkk]

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.

Not so much Xray as short wave UV, 253 nm and 185 nm. Xray production
usually requires much higher anode voltages (many kV).

 

[josephkk]

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?

A bit of thimking. Assuming you have a 100 A service at 120/240 V (US
standard). The limit feeder drop is 3% or 3.6/7.2 V or maximum Z of about
0.3 Ohm. 5 V(dc) flowing 0.3 Ohm gives about 16 A. You should be able to
find that with circuit breakers tripping or fuses blowing.
If that is not happening you may very well have a measurement error, where
something is causing a false reading on your meter. Try looking at it
with an oscilloscope to see what other signals are there.

 

[Phil Allison]

"John Larkin"

5 volts doesn't sound reasonable. What DC voltmeter range were you
using? You may be seeing voltmeter overload.

** The OP did not say he actually read any such value an a meter - but in
general, you cannot read the DC component of a 120 or 230 volt AC supply
with just a DMM set to DC volts.

A little bit of AC filtering is needed first - say a 1Mohm resistor and a
2.2 uF film cap in series across the supply, then read the DC voltage on the
cap with a DMM.

Allow 10% more for the effect of loading from the 10Mohm input of the meter.

I'd be surprised if you see more than 0.5 to 1 volt.



..... Phil

 

[David Lesher]

Mercury Vapor rectifier.

As "hifi-tek" said several days ago; it's the tungar lamp
rectifier that I was thinking of...

I've never thought of mercury vapor rectifiers as
high current/low voltage devices.

 

[David Lesher]

They were used to power subway trains. They can be builkt to about
any scale they needed. I've never seen a low current mercury vapor
rectifier, and doubt they would be practical.

The classic that I have used was the 866A; good for a few KV but
only a few hundred mA.