Re: gEDA-user: electrolytic capacitors

["Robas, Teodor" <teodor.robas@xxxxxxxxx>]

   Dylan Smith wrote:

On Sun, 28 Dec 2008, der Mouse wrote:

  

[Electrolytics] can explode pretty violently, so let me tell you a
story.  [...785V on a 450V cap...serious dent in plaster...]
      

My exploding cap story is not quite so impressive, but I know I
certainly found it convincing.
    

Neither is mine. When learning about switch mode (boost) supplies, 

   Yep, boost circuits can also do a lot o damage. At the company I
   worked
   one of the managers had the 'brilliant' idea to make a software
   regulated
   boost supply. I can not blame him, they made a plus of 30 cents per
   board.
   The circuit was rising voltage from 12V to 37V for an ASIC wireless
   transmission chip. The ASIC chip was conected on SPI to a big Motorola
   controller. The controller gave the control signal for boost
   controller on a
   software regulated PWM output. The problem was that that the ASIC draw
   a few Amps when transmitting and almost nothing when off.
   Now guess what happen when the software was too busy to take care of
   keeping the ASIC transmitting ? The ASIC would stop. The voltage to
   power
   the ASIC (rated 42V) grew up to ~80V. That voltage killed the ASIC and
   came
   back on the SPI lines to the controller. The resulting flame was
   always
   entertaining ! Sometimes the ASIC wanted to take all the punch and dig
   a
   hole the size of a finger in a 4 layer PCB. The capacitor, only
   220uF/50V,
   survived.


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