On Fri, 08 May 2009 14:55:18 +0100, Eeyore
wrote:



Spehro Pefhany wrote:

Sylvia Else wrote:
Mike Tomlinson wrote:
Dave Platt writes

One manufacturer was citing a "50,000
hour" lifetime figure on the motherboard carton

Are you sure there isn't a microdot in the small print with the words
"electrolytic capacitors excepted" printed on it?


"lifetime" is a word of dubious interpretation. Does it mean that it
won't break in that time, that it can be repaired in that time if it
does break, or something else?

If they claimed an MTBF of 50,000 hours, that would be different.


AFAIUI, MTBF typically applies only to the flat part at the bottom of
teh bathtub curve. They conveniently hack off the infantile failures
at the left and the increasing failures as the useful life expires on
the right. IOW, a product can have a much higher MTFB than the time it
takes to wear out.

Also, 50,000 hours 24/7 is only 5.7 years, which is more-or-less what
you'd expect out of a motherboard.

And if you run a 105C cap at 40C you'll get ~ 85 times the datasheet
lifetime. So a 2,000 hr 105C cap would last over 19 years. Watch the ripple
current of course !

Interesting point there. One decoupling cap on a mobo of mine near the
graphics card slot was visibly bulged whereas others weren't. I imagine it
was hot air being blown onto it by the GPU fan.

Graham

I would imagine that it was a crappy capacitor, perhaps one of those
knock-off Taiwan caps. Sceptre LCD monitors had a rash of crappy
electrolytics which they used on the outputs of their switchers a few
years ago. Many computer motherboards also used icky caps. The two
Sceptre monitors that a friend had crapped out around the same time. I
replaced the high stress caps with United Chemicon caps.

For my pulse power amplifier applications, I stick to United Chemicon.

--
Mark