On Wed, 2004-11-24 at 10:36 -0600, PicDude wrote:
> I built the 12V stage, tested it under a load, and it worked fine.  Then I 
> built the 5V stage, and that worked fine under load.  When I built the final, 
> 3.3V stage and was testing it, the input capacitor for the 5V stage died.  
> This time though, the 5V stage was NOT under load.  I immediately thought 
> that the output capacitor must've died since it was not under load and 
> perhaps the output voltage went too high, but it was actually the input 
> capacitor (Nichicon 330uf, 35V, low-ESR electrolytic).
> 
> In more detail, the input voltage (which in this case was 14V from a switching 
> power supply) is split in 3 directions, each going to a separate SB540 
> schottky diode (for reverse-voltage protection), then to the switching 
> circuitry for each stage.  The 5V switcher is an OnSemi MC33167, with a 330uf 
> 35V input capacitor.  Because there are reverse-protection diodes on there, I 
> can't see how anything happening on either of the other stages could cause a 
> reverse voltage to the 330uf cap.  And since my input is 14V, that cap was 
> receiving a supply of ~13.5V (after the reverse-protection diode).

Well it looks like you've covered the bases, so it's simply possible you
got a bad cap. I've seen that from time to time.

FWIW electrolytic's can be both cranky and very forgiving at the same
time, and just because one blows at a certain time doesn't mean the
event that caused it to blow happened WHEN it blew.

For example, at work I was talking to a colleague about a few things
when out of the blue a cap when boom. Upon closer inspection it was
revealed that they had used a 6.3V cap on a 12V line, simple right?
Wrong. The board had been powered up for DAYS! :) It just took that long
for the cap to go, which surprised us. I guess that's a case where
buying a LOWER quality cap is a good idea, it'll withstand abuse for
less time! :)

I say replace the cap and keep going, if you're absolutely sure about
the voltages and polarity involved. TTYL
 
-----------------------------
Herbert's PIC Stuff:
http://repatch.dyndns.org:8383/pic_stuff/

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