[SI-LIST] Re: Reset problem with power supply

Greetings, Hal.  As you have noted, the Schmitt trigger properly handles
slow power risetimes.  A proper design will include an RC time constant
at the input so that it also handles fast risetimes by including a
minimum delay before reset releases.  As I mentioned, there are decent
ICs to do this that are lots more convenient than rolling your own
design.  Reset circuits are a specialty in their own right.  It's easy to
slap together a lousy one, tougher to design a bullet-proof one from
scratch.

The issue with electroytic caps isn't whether they are "modern" but
whether they are cheap.  There are decent Al electrolytics to be had for
a modest premium (OK, sometimes a big one), but the aluminum caps
actually used in consumer gear seem as crummy as ever.  I've had
motherboards fail after several years because the caps in the switching
supply weren't properly rated for the high frequency current and ran too
hot.  When it's really bad you get a brown tint to the circuit board in
the area around where the caps are mounted because the internal losses
increase as they age.  At the end they run hot as a pistol. 

The last time this happened I metered the caps and found that 100uF caps
had dropped to less than 2uF!  Replacing them did not bring the board
back from the dead because the excessive ripple voltage peaks from
inadequate filtering had already done their dirty work.  For machines you
really care about, checking the power supply caps once in awhile might be
a reasonable preventive maintenance procedure.  A finger touch while
under power for a quick temperature check is probably good enough (but
I'd touch the plastic insulation rather than the bare metal).  Tantalum
caps generally do not have the aging issue, though they still need to be
rated for the current demand.

Orin

On Sat, 06 Dec 2008 01:34:44 -0800 Hal Murray <hmurray@xxxxxxxxxxxxxxx>
writes:
> 
> How does a Schmitt trigger solve the problem?  I'd expect it to make
things cleaner if the input signal has a slow rise time, but that's not
going to solve the problem of reset going away before the power is
stable.
> 
> Are electrolytics getting old really a serious problem with modern
caps?  I've seen lots of credible reports of very old gear being brought
back to life by replacing all the old filter caps.  I hadn't thought much
about the  implications on gear I was designing.  How should I think
about modern filter caps getting old?

> > Cable inductance absolutely should not be an issue.  If it 
> actually
> > is, be grateful that you found a design flaw before you reached
> > production! This might be related to the power risetime.  Your POR
> > circuit needs to operate properly even if you slowly bring the 
> voltage
> > up manually.  Some circuits depend on fast power risetime to 
> operate
> > properly, and work OK... most of the time.  The worst ones 
> capacitor
> > couple the voltage bus into a transistor.  Not only do these fail 
> to
> > reset on a slow rise, if an aluminum electrolytic cap is used, the
> > value will drop with age as it dries out and eventually the 
> circuit
> > won't work even for normal supply risetime.  A Schmitt trigger is 
> a
> > good basis for a bullet-proof approach, as are any number of
> > supervisor ICs that also check for overvoltage, brown-out, and the
> > like. 
> 
____________________________________________________________
Take a break - you deserve it.  Click here to find a great vacation.
http://thirdpartyoffers.juno.com/TGL2141/fc/PnY6rw2gGUymLyXCUuqFPiRkgWMzRlFZD0pHTeyutmKSJLoJ92e4X/
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field


List technical documents are available at:
                http://www.si-list.net

List archives are viewable at:     
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu

Other related posts: