Peter,
Much as I respect Manfred, I think he is "pushing things" a bit far -
at least for "Joe Q. Ham".
I can't help but keep defending my point of view!
Consider the following situations:
1.- A ham amplifier that uses the directly rectified mains, or perhaps
the 350V output from a power factor correction circuit (non-insulated)
to power a floating RF power module, insulated from the rest of the
world by its RF transformers. The whole thing housed in a metal
enclosure, connected to safety ground.
2.- A ham amplifier that uses a conventional transformer-input power
supply to power an RF module that's connected to the metal enclosure.
This enclosure is connected to safety ground.
3.- A ham amplifier that uses a switching power supply with power
transistor circuitry directly connected to the line, and a 50kHz
transformer providing insulation, to power an RF module that's connected
to the metal enclosure. This enclosure is connected to safety ground.
4.- A washing machine that powers its motors, heater, valves, level
sensors, etc, directly off the mains, with insulation provided inside
each device. Housed in a metal enclosure, connected to safety ground.
Peter, can you tell me where's the difference, in terms of safety,
between these four devices? Each of them has a single level of
insulation between the mains and ground, each of them involves windings
in a transformer or motor, and lots of wiring, and each of them uses the
grounded metal enclosure to drain off any leakage current, and trip a
ground fault interrupter or even the main circuit breaker, if necessary.
There is no fundamental difference between any of them.
But there is some difference in the details: The washing machine has
many more possible points of insulation failure, than any of the
amplifiers. Also the washing machine operates with water, will be
touched by people with wet hands, possibly even standing on a wet floor,
and will usually be operated by a housewife, who cannot be expected to
know anything about electrical safety. Even Joe Q. Ham knows more about
this! So the washing machine is BY FAR the most dangerous of these four
devices!
And which one is the safest? The ham amplifier with the switching power
supply has a small transformer packed full of magnet wire. I know from
experience that it takes some good design and construction to achieve
4kV of insulation in such a transformer. If badly designed or built, the
insulation can fail by creepage at much lower levels.
The amplifier with the conventional power supply, just as the washing
machine, use iron cores inside the magnet wire windings. So, in addition
to direct wire-to-wire shorts in a transformer, these devices can suffer
from wire-to-core shorts.
The amp with the 50kHz transformer is only slightly better, because the
ferrite used for power conversion is slightly conductive.
But the amp using the RF transformers for insulation is the safest of
all, because its transformer cores are non-conductive, and the windings
that need to be insulated have only very few turns, and have enough room
around them to accomodate a thick insulation.
Surprise! ;-)
I think that the reluctance of some people to accept line-connected
electronics stems only from gut feeling, and not from a cool-headed
analysis of the advantages and disadvantages. Unfortunately all
electronicians are taught to think in a grounded frame: There is ground
in every circuit, everything is referenced to ground, bypassed to
ground, dumped into ground, etc. Ground is our good friend who swallows
all bad signals, can be trusted to be always at zero volts, and so on.
It can be difficult for ground-dependent electronicians to accept the
concept that a ground point isn't necessary at all for electronics, and
that actually that single ground point that joins one side of ALL power
and signal lines is a really big problem, simply because it's
technically impossible to keep all grounded spots at exactly the same
potential! It would be much healthier if electronic designers would
consider BOTH poles of each signal and supply line, and handle both of
them intelligently, instead of joining one pole of each signal and
power, and calling that point "ground". This means differential signals
throughout a circuit, and floating supplies, and the lack of a common
point that could be called ground. This whole electronic circuit would
be treated exactly like the wiring in a house: There would be one, and
only one, point where the whole circuit would be connected to physical
ground, to avoid problems of static build-up and the like. But there
wouldn't be any single chunk of metal to which hundreds of components
connect in common.
Many electronicians can understand this, and even like it, but in their
daily life and work they fall back into the old concept of using
single-ended signals and referencing everything to ground. And that's a
real pity!
Manfred
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