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Kai Klaas
03/02/12 17:05
Read: 478 times

#186368 - Back to back connection...
Responding to: Erik Malund's previous message
Erik said:
can anyone explain
"Two caps in series will share the charge, how is one not getting reverse charged, does it short on reverse voltage? I have never understood this, but, since I have seen it used, it must work."

An elcap can be reverse charged unless the voltage exceeds about -2V. This is because the aluminium cathode foil also has a "natural" oxid layer due to the unavoidable oxidation of aluminium in the air, which corresponds to a "natural" formation voltage of about 2V for elcaps. So, a reverse voltage up to -2V doesn't matter.

DC case:

If you measure the leakage current of an elcap by increasing the reverse voltage from 0V on, you will find it rather high and becoming higher and higher the closer you come to -2V. The leakage current of a reverse charged elcap is considerably higher than the leakage current of a non reverse charged elcap. So, when charging two back to back connected elcaps, the two caps form a voltage divider, where the reverse charged cap is considerably lower impedant. So, only a small voltage drops across the reverse charged elcap. This mechanism keeps the reverse voltage well below the -2V limit.

AC situation:

Non-polar or back to back connected elcaps are often used in audio highpass filters. Here they also form a (frequency dependent) voltage divider in combination with a suited resistor. In usual RC-highpass filters the voltage that drops across the back to back connected elcaps does never reach 2V, even at the lowest frequencies and the highest amplitudes. So, none of the both is ever in danger to reach the -2V limit. But even if so, the same would take place as in the DC situation above.

The above is only true if the two back to back connected caps are not very different. So, it's wise to prefer the specially fabricated non-polar elcaps over simple back to back solutions.

Kai Klaas

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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