Don't be so sure guys!
Did anybody notice the shiny brand new electronic arc welders- portable as
it comes- now all the rage?
They're transformerless and sold all over the world.
Like somebody already said, all computers have 50% of their power supply
connected straight to the line and they're approved all over the world. And
what about your smart phone wall wart?
Alex 4Z5KS
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Manfred
Mornhinweg
Sent: Tuesday, September 17, 2013 10:00 PM
To: amps@contesting.com
Subject: Re: [Amps] Direct rectification of AC mains to derive the amp VDD,
supply
Leigh,
> I can in principle concur with your 3 salient points here Manfred.
Yes, we concur on the points, and only weigh them differently. No problem
with that. Given the choice, I also find an isolated power supply and
grounded amplifier block to be a cleaner layout - but the economy and
greater efficiency of a direct line supply is attractive too, and for this
I'm willing to use a floating amplifier!
> directly rectifying the AC power mains would never pass regulatory
> approval for commercial equipment sale in most jurisdictions as the
> rules have tightened up in recent years, particularly in respect of a
> DC and harmonic components going back into the mains, and power factor
> requirements mandated by CISPR, CENELEC, EN standards, IEC, etc.
I'm aware of the new power factor standards in some countries, but not of
any of the others you mention. Can you point me at some, to get up to date?
Since I live pretty much at the end of the world, where there is essentially
no electronics industry and no enforcement of any standards whatsoever in
regard to RFI/EMC, I'm a bit out of touch with such standards. If I should
design and build an amp which I also want to publish, I would like to make
it legal in most jurisdictions, even if here at home I can use anything I
like.
> For these reasons we're unlikely to see such a dubious direct AC mains
> rectification scheme deployed in the QRO amp commercial marketplace.
I don't know. But for a simple, inexpensive homebuild amp, I still think
direct line powering is attractive.
The issues one has to address when doing this, as I see it, are the
following:
- Safety. This is easily handled by using good enough insulation in the RF
transformers, along with grounding the amplifier cabinet (of course NOT the
neutral!!!), and using a ground fault interrupter, which could be internal
or external. In my country GFIs are legally required in all homes for all
outlets, so it's not necessary to add an internal one.
- Since you mention it, I will pick it up: DC fed into the power line.
But this is a non-issue, when using a bridge rectifier across the line.
Of course it would be unacceptable to use half wave rectification!
Apparently some people think about half wave rectification when reading
about direct line rectification. Antique radios did that, but modern high
power devices don't!
- Power factor. This is indeed an important factor, not only for legal
reasons, but also because legal limit amps draw so much power that if they
have bad power factor, they require special dedicated feed lines.
In my area any outlet can provide 2200 watts continuously - and that means
2200 voltamperes, or unity power factor. It's usually possible to run a
legal limit amp from them, as long as its power factor is decent, and
perhaps only in SSB mode. With bad power factor and in CW or RTTY, a special
circuit is needed.
But there are important things to consider: First: All the usual, widely
used tube type amps have very poor power factor. Second: A direct off-line
rectifying power supply that has the same ripple as a simple
transformer-based one, also has the same power factor, as long as a series
resistor is used that has the same amount of loss as the windings of a
transformer would have. Note that the efficiency of this supply is still
better than that of the transformer based supply, because the iron loss of
the transformer is avoided.
And third: If a very good power factor is required by law, such as above
0.98, then anyway an active power factor correction circuit is needed.
This needs to be added to ANY kind of power supply! So, a simple, direct
rectification supply turns into a minimal active power correction circuit,
and the amp stage might be powered off a roughly regulated 350VDC line
coming from this PFC circuit. An isolated supply instead would have to start
with this PFC, and then add a complete basic isolated supply! So it's still
simpler to use direct off-line power, even when requiring excellent power
factor.
In my amplifier design I go a middle route: My power supply is non-isolated,
but extremely simple, yet it delivers a well regulated, current-limited DC
output, and has an input power factor that is much better than that of the
average tube type amplifier, although it is not close to 1.
- RF fed into the line. Some people worry about this. I don't. Using basic
RFI filters at the AC input, the amount of RF fed into the AC line by an amp
is at least 50 to 60dB below the power output, even with a very crude,
direct power supply. This amount of RF on the AC line might matter, if the
amp is only used into a dummy load. But if it is used into an antenna
installed in the backyard or on the roof, the power lines will pick up far
more RF out of the air, than directly from the amplifier!
> I would also not wish sacrifice the immense superiority of TLT xfmrs
> for conventional RF xfmrs to provide the necessary robust electrical
> isolation from the potentially lethal AC mains supply.
I see your point, but I wonder why almost all manufacturers of HF radios and
solid state amps use conventional transformers...
There is a common misconception. Many people think that any transformer
wound with a coaxial conductor or with a twisted pair is a transmission line
transformer. But in truth only some of these are. It depends on how these
lines are connected.
> Facilitating robust AC mains electrical isolation in a traditional
> flux coupled RF transformer covering the entire 1.8 to 30 MHz HF range
> is nontrivial and somewhat problematic.
I have done it, and found no problems at all with the insulation. Simple
teflon tubing is good enough, as long as it is kept from being cut by some
metal edge. Instead the problem is achieving flat enough, low loss response
over that frequency range, when working with higher voltage supplies. There
is a problem with the total wire length required, compared to the shortest
wavelength, to push the flux density low enough. I developed a transformer
that handles legal limit, goes from
1.8 to 30MHz (but barely - at 35MHz it turns resonant!), provides a true
center point, and provides safe line voltage insulation, while being compact
and quite inexpensive. It took me some tinkering, though.
> Achieving the requisite isolation with coupling / blocking capacitors
> is a risk as they have a propensity to fail...and not in a failsafe
> manner :-(
Yes. I wouldn't go that route.
> In summary such schemes whilst seemingly attractive and tempting to do
> on the surface of things have many ugly hairs growing on them that
> make them unattractive.
I'm almost fully determined to go through it and turn my experiments into a
complete amplifier. I just don't know when... since I spend my time posting
stuff on this forum! ;-) If I ever finish my amp, I will of course report
on it.
> BTW Manfred, I really like your succinct piece titled "Output
> architectures of conventional class AB push-pull amplifiers" at:
>
> http://ludens.cl/Electron/mosfetamps/amps.html
>
> I concur with what you say here about how circuit design
> misinformation is perpetuated.
Good. One more member in the club! What I would like to hear is the reasons
anyone who did this "the wrong way" had! I still wonder if all who use the
wrong configuration do so simply because they copy older wrong designs
without understanding them, or if they see a good reason to use that
configuration!
Peter,
> In some countries, connecting the neutral to an outside earth could be
> extremely dangerous, leading to fires or even electrocution.
In Chile, connecting the neutral to earth inside any equipment would
instantly trip the ground fault interrupter. The neutral is grounded at a
single point, that being at the source of the circuit: The step down
transformer, or the generator, or the inverter, or whatever is powering the
circuit. That's before the GFI.
> Where
> a Protective Multiple Earth system is used, a ruptured neutral could
> pass many amps down any earth lead, or lead the coax outer to float up
> to full mains voltage. This is why it is not permitted to bring any
> conductor within the unipotential zone unless it is bonded to the
> system earth, and earth leads need to be able to carry large fault
> currents.
Chile has a double-safe standard there: It requires GFIs, so that the fault
current can never get large, but at the same time it is required that
protective earth conductors be as thick as the main conductors.
In olden times, GFIs didn't even exist, and earth ground in power outlets
was unknown here. Outlets were two prong, non-polarized, most tube radios
had one side of the line connected to the chassis, the back covers usually
fell off and got lost, and if a knob pulled off, the shaft poking out had a
50% chance of being live at 220V, depending which way the power plug had
been connected. Wire insulation was mostly rubber, which crumbled and fell
off after ten or twenty years. In retrospect, it's surprising that so few
people were electrocuted.
Compared to that, a teflon-insulated device, with its metal case grounded,
and powered through a GFI, is extremely safe!
>
> Where PME is not used and the neutral is earthed somewhere else, you
> can still get quite a large potential between neutral and earth. When
> I lived on a housing estate in Swindon where the whole estate was fed
> from a substation where the neutral was earthed, on a Sunday morning
> with washing machines, electric cookers, immersion heaters etc all
> blasting away, I could get 25 volts at 500mA - free - between a decent
> ground and the mains neutral. No RCD (GFI in the US) there - if there
> was one fitted, earthing the neutral to a decent external earth would
> be very safe - you would have no electricity supply because the RCD
> would trip!! At this QTH, at the end of the line feeding other houses
> and a half mile of wire from here to the transformer, the neutral is
> usually around 1 volt above earth potential.
So that's much the same as here. When I still lived in the city and bought
my electricity, I typically got about 4V between ground and neutral. Now I
make my own power, and the grounding spot of the neutral is right under my
house, so I get only a fraction of a volt between ground and neutral - but
If I join them at an outlet, the GFI still trips!
> So any system earthing the neutral is not a good idea and may even be
> illegal in some places.
Agreed. But just to make sure all readers get it: Powering an amp directly
from the line does not require earthing the neutral. Instead the entire amp
module floats at line potential.
> If one used TLT instead of conventional transformers, the blocking
> capacitors would need to be Class X, and for 160m, at least 10nF. But
> Class X capacitors are not designed for handling large amounts of RF
> current (ca. 3 amps at 500 watts) , and are not as low inductance as
> one would want for 10 metre operation.
Exactly.
> Cascaded RF transformers is, to my mind, asking for trouble.
Some amps use an unbalanced to unbalanced transformer to step up the
impedance, and then cascade a balun to provide coax output. That works
perfectly. I see no problem with cascading RF transformers, other than the
added cost and loss. Usually it is possible to find a single transformer
solution, which is more cost-effective.
> From memory, an isolating transformer
> to meet the regulations needs to withstand 2kV, although I could be
> wrong on that.
I have seen a 4kV rating in many things, such as the required insulation
between primary and secondary sides for cellphone chargers, or the specs of
safety-rated optocouplers. So I design my own stuff for 4kV insulation. My
legal limit HF output transformer comfortably handles 4kV. This is probably
overkill, as it is the requirement for equipment that has no grounding on
the secondary side. When there is grounding, probably insulating for 2kV is
plenty. Even 1kV might be enough!
Manfred
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