Sounds like there might have been some unwanted oscillation. Try it again 
with a bypass cap across the electrolytic.

John Ferrell
My Competition is not my enemy!
http://DixieNC.US

----- Original Message ----- 
From: "PicDude" <picdude2@narwani.org>
To: <piclist@mit.edu>
Sent: Wednesday, November 24, 2004 11:36 AM
Subject: [EE] Capacitor went Poof! ... why?


> Hi all,
>
> Late last night, while building a 3-output power supply, a capacitor 
> burned
> out (it just made a poof sound and the electrolyte leaked out) and it's
> totally baffling me.
>
> 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).
>
> I've not re-connected it for testing as I'd like to know what I should
> check/test for.  So far, I've verified that that cap and other components
> were soldered in with the correct polarity, etc.  Also, can/should I test
> with the existing (blown) cap in place, or should I replace it first?  I
> assume the latter.  I know that switchers need a minimum load, and I did 
> make
> provisions for this on the PCB, but do not have a min load resistor on 
> there
> yet.  How would I determine the min load required?  The datasheet does not
> indicate how to determine this.  Let me know if you need other 
> info/circuit
> diagrams, etc.
>
> Much thanks,
> -Neil.
>
>
>
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