On 9/17/2013 11:59 AM, Manfred Mornhinweg wrote:
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!
There are several fundamental issues here, and your post indicates that
you're generally aware of most of them. Nearly all relate to safety, and
are written into the electrical Codes (laws) that govern buildings. They
are not "dumb" or out of touch with reality -- rather they are based on
very solid good engineering practice. So for the benefit of others who
might be aware of them, here they are.
#1 -- In most parts of the civilized world, in any SYSTEM, there must be
one, and ONLY one, bond between neutral and ground (the Green wire in
North America, Protective Earth in EU, etc.) .In North America, that
bond must be at the point where the SYSTEM is established. In most
homes and small businesses, there is only one system, and it is
established where power enters the building. This point is called "the
service," or "the service entrance." This point also requires a
physical connection to the soil.
A new system is established by a transformer, either one in a UPS, or an
isolation transformer, or a step-down or step-up transformer. Thus, the
neutral must be bonded to the Green Wire (or Protective Earth) at the
secondary.
#2 -- ALL grounds in a facility MUST be bonded together.
#3 -- ALL exposed metal parts that "could be energized" MUST be bonded
to the Green Wire (or Protective Earth).
#4 -- In most of the civilized world, load current is permitted ONLY on
the Phase (hot) and neutral conductors. It is both illegal and a major
violation of good engineering practice to connect a load between a
phase (hot) and the Green wire (or Protective earth).
#5 -- For purposes of EMC, all cable shields MUST be bonded directly to
the shielding enclosure of equipment to which they are terminated, and
all balanced cables must either filtered or bypassed.
#6 -- For purposes of lightning safety, cable shields MUST be bonded to
earth.
Most safety codes make equipment with more stringent construction
requirements exempt from the Green wire. In North America, this is
called Class II equipment, and it has either no exposed metal parts, or
is double insulated so that there is no possibility of electrical
shock. Those consumer devices with direct rectification are built with
no exposed chassis, and no external connection to ground (or a
transformer coupled connection).
These fundamental requirements combine to make direct rectification of
the mains more than a bit impractical if you care about safety and
freedom from EMC issues. Your post, which outlines the sort hoops one
must jump through to get there, suggests that the cure may be worse than
the disease.
73, Jim K9YC
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