1 Sep
1999
1 Sep
'99
2:24 p.m.
At 07:08 PM 9/1/99 +0100, Tony wrote:
TVs : Certainly in Europe, almost all modern TVs have
an isolated
chassis. Mainly because all our TVs have a 'SCART socket' which carries
line-level audio, composite video, RGB, etc. It's a lot easier to isolate
the PSU (most TVs have an SMPSU anyway) than to isolate the dozen or so
signals on that socket. I can't comment on US-TVs, though -- are they
really still hot-chassis?
I quess "hot chassis" isn't as clear as it once was. TV's
don't have mains
60Hz power transformers like they used to. Are they many with 50Hz ones there?
What they have is a mains rectified DC supply, typically 170V for 120V AC
that runs the horiz. output stage. Much of the rest of the TV runs off the
flyback transformer windings, which can be considered isolated. This is
like SMPSU's except in a TV, the filter cap, horiz. output transistor, etc.
may be scattered around the TV, and are all "hot", unlike many computer
SMPSU where the mains side circuits are obviously apart from the output
side. Of course, the horiz. driver has to somehow be powered to start up
the TV, so may be hot as well.
As for a "chassis" it has about disappeared. There is only the PC board,
the CRT, and a pod with the controls, all in a plastic cabinet. Maybe
ground could be considered the return of any inputs, or the CRT outer
coating (not always the same circuit point)
-Dave