Isolated AC grounds and S-38s

Richard E. Robinson rerobins at NEWMAIL.UNCC.EDU
Thu Feb 19 11:55:15 EST 1998

 Bob, KE6F, wrote

>    Most if not all modern solid state color televisions are direct line
>without any isolation transformer of any kind.

True, no transformer but the AC is isolated on the input.  Most if not all
TVs and computer monitors use switching power supplies.  There is no
input transformer off the AC line, but this is an isolated AC common which
is *not* at AC ground. Look at the schematic of a monitor or modern TV and
you'll see that the AC side of the power supply is not at AC
ground.  The symbol is usually a "point down" triangle not the common
ground symbol.  This indicates a common not a ground.  Try hooking an
oscilloscope with the chassis at AC ground to the AC common of a swithcing
power supply. You'll be buying a new 'scope probe if you're lucky and a new
'scope if you're not.  That's why you should always "lift" the ground of a
scope whenever you work on the primary side of a switching power supply or
any AC/DC gear.  Actually I run the monitor or AC/DC gear and the 'scope
off the isolation transformer with nothing connected to the AC ground.

The 117VAC is bumped up by a voltage doubler to roughly 250VAC then
switched at a freq of a couple of hundred KHz by a transisitor into the
primary of the power transformer.  Hence the term switching power supply.
This switching transformer replaces the old linear 117VAC input transformer
we are all familiar with. The DC side can be operated at AC ground, but the
AC side will always be isolated from chassis or AC ground if a swithcing
power supply is used.  I've fixed more TVs and computer monitors than I
care to think about.  Thank goodnes those days are over.

Modern TVs and computer monitors are a whole different ball game from the
old AC/DC receivers of old.


Rick kf4ar

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