On 3/17/2017 6:07 AM, Dave B via Elecraft wrote:
(Often low cost brittle plastic things from
the Far East.)
These are *all* low cost plastic things from the far east!
In this case, as the PSU is intended to be used with a device that has
exposed metallic parts connected to one side of the DC lead, I suspect
(I admit that I do not know for sure) that the AC input to that PSU
should include a safety ground, connected to (among other things) the
transformer inter-winding shield.
The safety regulations pertain to the power supply, not the target
device. So, it doesn't matter whether the radio has exposed conductive
parts. Safety ground on the AC line input is not necessary either,
however, design requirements change if you don't have it. The concern is
isolation between high voltage line input and low voltage output, and
the result of component failures. This specifies clearance & creepage
distances, insulation types, rating of capacitors which bridge across
line or from line to load, etc.
[K9YC: I should say "protective earthing" rather than "ground" so I
don't get in trouble for improper use of the term!]
The shield (winding or foil) between windings in the transformer is not
for safety or necessarily connected to earth. This is for EMI and
reduces common mode noise passing from primary to secondaries. An
adapter like the KX33 would be flyback topology, so a transformer shield
(if present) would be connected back to the primary switching MOSFET's
source pin, not ground.
Incidentally, the safety regulations are pretty well harmonized between
USA and most of Europe. For example, this type of power adapter falls
under UL60950 which is identical to IEC60950.
73,
Josh W6XU
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