18 Feb
2004
18 Feb
'04
1:10 p.m.
On Tue, 2004-02-17 at 15:45, Mark Tapley wrote:
(But I bet they derate that single cap :-)
>Seriously, I think you just have to change them
often, eg. decadely.
>(That's a perfectly cromulent word.)
"Decadely" I accept, it makes sense by
analogy with "weekly".
But "cromulent"? Communication is not being served. Got a synonym I
can look up?
Sheesh, don't all you people watch THE SIMPSONS? How do you make it
through the week?! cctalk content: it's > 10 yrs old now.
I think
spacecraft programmers know about this sort of thing, but you
could probably design a power supply that presupposed possible failures
like this, but basically no one but us lunatics care about electronic
objects lasting more than five, never mind 10 or 20 years.
It's not guaranteed, even there. There are spacecraft instruments in
build with single capacitors in a circuit where fail-short means the
instrument can't turn on. Same job could have been done with two
capacitors in series. Then if either fails short, the circuit still
works. (It might exceed its timing spec, but it'll work.) In fact I
have caused that addition to be made in some instruments and power
supplies, but the design engineers don't seem to learn easily. And
there are some I didn't get to review until "too late" in the design
cycle. Sigh.
Good trick. Design-for-failure isn't a popular subject in
cost-sensitive, quick-to-market business, but it's fascinating work.
I'm working with 5th-year EE students who don't even know how to make
real-world input protection networks, or do real bypassing, but boy can
they crank out SPICE simulations of CMOS gates. You should see their
solenoid driver designs -- >shudder<. Never heard of inductive power
resistors. Reading the fine print in capacitor specs? Fat chance. We're
all doomed.