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Per Westermark
02/29/12 10:01
Read: 602 times

#186281 - Some polarized capacitors survives AC but not reversed DC
Responding to: Erik Malund's previous message
When I talked about "20 years ago" I talked about the design with AC power just rectified and fed to long LED chains - not the use of two capacitors with reversed polarity.

Some capacitors (not all) can survive the polarity reversal as long as the polarity isn't permanently reversed. They fail quickly if they have a reversed DC potential. But they can handle AC because there will be current on both directions that protects the barrier from being reducted. Using two capacitors with reversed polarity is just to make the design symmetrical. If ignoring the symmetry, you could use a single capacitor.

But no - this is not a design to use. It's not all polarized capacitors that only fails for reversed DC but survives AC. And even when using a cap that survives AC, the capacitor is still used outside the manufacturer specification, which means that rated ageing, usage temperatures, ripple current etc do not apply anymore. When people do need a capacitor for AC, they should get a non-polarized capacitor.

List of 27 messages in thread
AC Powered LEDs + strobe      Chico Magalh„es      02/28/12 22:58      
   Isolated Switch      Jim Granville      02/28/12 23:39      
      Switching      Chico Magalh„es      03/15/12 00:41      
   Some basics      Oliver Sedlacek      02/29/12 00:54      
      Not a good design - and expensive        Per Westermark      02/29/12 03:00      
         Answers + need some explantion      Chico Magalh„es      02/29/12 23:55      
      Cap voltage      Chico Magalh„es      03/01/12 00:03      
         More than 280V      Oliver Sedlacek      03/02/12 04:30      
            Measurement      Chico Magalh„es      03/15/12 00:45      
               Measurement correct      Erik Malund      03/15/12 07:54      
   two caps polarity reversed on DC!! ?      Erik Malund      02/29/12 07:13      
      More common long time ago      Per Westermark      02/29/12 08:33      
         I was around then      Erik Malund      02/29/12 09:21      
            Some polarized capacitors survives AC but not reversed DC      Per Westermark      02/29/12 10:01      
               POOOOOOOOOF      Erik Malund      02/29/12 10:27      
                  Yes, very likely a failure      Per Westermark      02/29/12 11:10      
                     that need bold face      Erik Malund      02/29/12 11:44      
            Back to back connection...      Kai Klaas      03/02/12 17:05      
   And another thing...      Andy Neil      02/29/12 15:30      
      Pseudo-CC with resistor requires much % V over resistor      Per Westermark      02/29/12 16:52      
      chip      Chico Magalh„es      03/01/12 00:11      
         one of these      Erik Malund      03/01/12 08:00      
         Chips      Andy Neil      03/01/12 14:33      
   PC PSU      Chico Magalh„es      03/15/12 00:47      
      Definitely better with lower voltage design      Per Westermark      03/15/12 01:11      
         PSU      Chico Magalh„es      03/15/12 18:37      
            Some bricks can inform about rating      Per Westermark      03/16/12 02:21      

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