Hi Mike,

> >FWIW, my primary desktop just needed a new power supply.  A transistor blew
> >after 5+ years of being on almost continuously.  Anybody out there ever
> >hear of "inrush current" and the damage it does?!?!?!
>
> Almost every switch mode supply I've seen use Barium Titanate inrush
> current limiters - these are great devices - we used them on the output
> of a handheld inverter they protected the transistors very well indeed.

They are a serious liability if you have a 0.5 to 2 second drop out.
The typically very effective NTC thermistors that go close to short circuit
a few seconds after switch on.  Problem is when they are hot/short-circuit
and you have a dip, the primary switcher capacitor discharges and when the
power comes back the inrush current is now NOT moderated by the
thermistor and relies on the slo-blo of the fuse not to rupture.

Some switchers also enter a protection mode with dips of 0.1 to 0.5
seconds when the voltage does not reach zero and allow the switcher
IC to properly reset (perrenial PIC and other micro problem) and I have
had an occasion where 5 out of 30 computers latched up after one
particular dip.  Power cycling them clears the fault.

> I still would not ever leave a switchmode supply unattended - especially
> knowing that even switchmode transistors are susceptible to static
> damage during assembly and you never know when a fire could be caused.

There are hundresds of thousands of switchers that are unatended
all over the world in all sorts of industries, The Internet lives
on them and I would say that at least half of them are unatended
overnight and weekends.  I think you are beeing a bit too cautious
unless you have a national monument that cannot handle any risk of
fire.

We have some 30 PCs and they are on all the time and most of them
have not been even reset since the UPS was installed about 3 months
ago.  We run DOS, Win3.1, NT, Linux, SCO. Under heavy load the
Linux boxes do have resource leaks and need 60 day restarts, under
light load one cannot tell.  When running NT with non Microsoft
applications one sometimes also has resource leaks but stopping
and starting of offending services is sufficient to clear the problem.
The SCO box runs without reset unless The disk fills up with log
files, then I reboot it after making space.  My Win4W 3.11 crashes
at least 5 times per week, normally when using a web browser or
graphics application, I expect this and save before loading images.

> So if I wished to have a PC in a unattended environment I'd design my
> own supply with a heck of a lot of safeguards...

What I find is that the FAN is very important.  I have had switchers
fail in service but only those that do not have a fan or if the
fan has failed.  Dust is the main enemy of a fan.

Cheers
--
Kalle Pihlajasaari   kalle@ip.co.za   http://www.ip.co.za/ip
Interface Products   P O Box 15775, DOORNFONTEIN, 2028, South Africa
+ 27 (11) 402-7750   Fax: 402-7751    http://www.ip.co.za/people/kalle

DonTronics, Silicon Studio and Wirz Electronics uP Product Dealer