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Building a circuit with no power transformer ?
In article writes:
I think I understand what you're asking, and I think I know the answer.
You hook up the 120VAC mains to the primary of a 1:1 power transformer.
The secondary has 120V across it. But no, there is no 120V circuit
that you can complete with your body between a secondary lead and a
primary lead. Well, there is, but it's only due to the inter-winding
capacitance of the transformer, which is small, and will not let very
much 60Hz current through at all. But the 120VAC *secondary* circuit
is ISOLATED from the primary circuit. They're not connected.
James has a legitimate questions. Since the low side of the power
supply is normally signal ground, which in most cases is also chassis
ground, and with a safety ground, therefore earth ground, there IS a
possible path between the hot side of the power supply and earth
ground. Haven't you ever measured voltages around a tube socket by
clipping the negative lead of the voltmeter to the chassis? No
difference (assuming a good safety ground) between that and clipping
it to a ground rod outside the window.
The safety ground is there for one reason, and that's to assure that
if for some reason the hot side of the power supply (or the power
line, in the case of a transformerless unit) does manage to get
connected to the chassis, it will find its way back to ground through
a lower impedance path than through someone who touches the chassis
while it's in that state. Presumably a fuse will blow before long (or
the power cord will melt, or if it's bigger than the wiring in the
house, the house will catch fire).
--
I'm really Mike Rivers - )
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