just blew up my 17" monitor

[Keith Lofstrom]

Andy Green <fedora at warmcat.com> writes:

> Thanks for the offer :-)  but explaining the mechanism would be a lot more
> useful... what exactly gets hot and why?  Generally if things are asked to
> operate outside their bandwidth the signal attenuates towards nothing.  It
> would be a badly designed circuit that reacted to this by getting hot and
> letting out the magic smoke.

Back in the days when microprocessors cost something, it was not economical
to put complicated control circuits in monitors, so the raw circuitry
inside the monitor was fed with the raw signals from the video input.

The raw circuitry in the monitor contains a number of coils to make
magnetic fields and transformers and such.  When you put a fixed voltage
across a coil, the current increases over time, increasing the magnetic
field and storing more energy in it.  Eventually the magnetic material
saturates, and the current increases very rapidly.   Lots of current
becomes lots of heat, which is a signal to the black smoke to leave the
box in search of cooler places.

The "change magnetic field over time" process is how the beam is deflected
in a CRT.  If the timing is very different than the monitor is made for,
then the fields and the currents will be wrong.  Black smoke.  

If this seems cruel and a poor design, consider that your TV set is made
to work just this way, and you can still make the black smoke come out of
the TV set if you somehow manage to feed it a fiendishly incorrect video
signal.  Since TV stations and VCRs tend to make correct signals,  your
TV is safe.  The first computer monitors were just modified TV sets.  

Old monitors in a multisync age probably should be discarded.  A friend
of mine lost his house due to a monitor catching fire.  I would not trust
my house to the validity of the signals leaving a video card.  You may
be lucky that your monitor died while you were watching it;  imagine 
what might have happened if something went balky while you were out of
the room.  Monitors like that deserve to die.

Keith

-- 
Keith Lofstrom           keithl at ieee.org         Voice (503)-520-1993
KLIC --- Keith Lofstrom Integrated Circuits --- "Your Ideas in Silicon"
Design Contracting in Bipolar and CMOS - Analog, Digital, and Scan ICs