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Per Westermark
02/29/12 16:52
Read: 524 times

#186305 - Pseudo-CC with resistor requires much % V over resistor
Responding to: Andy Neil's previous message
Andy Neil said:
Apart from all the other truly horrible things about this, you can't just use a 470-ohm resistor with a 72-LED chain!!

When you use 1 resistor with 1 LED in a "normal" low-voltage circuit, the resistor has a significant effect on the current through the series combination; but one 470-ohm resistor in ~280V has negligible effect!!

Yes - a series resistor will only work as a "constant current generator", when a significant percent of the voltage is over the resistor compared to the voltage over the diodes. So a 5% change in diode voltages (say from component variations) will still not make much percentual change in voltage over the resistor and hence a small percentual change in current.

As the percentage of the voltage that is over the voltage is reduced, the amount of variation in LED voltage or in supply voltage you can handle also shrinks. There really is a big reason why there are so many constant-current solutions on the market. Active current regulation gives big advantages. Or actually huge advantages. And a real constant-current solution will manage to keep the current even if one LED should short. So you don't get an increased current resulting in an avalance effect through all the rest of the diodes. All you got is a potential for a higher voltage loss over the CC generator. And a good CC generator would contain a temperature protection making it deactivate in case too many LEDs have failed resulting in a too high amount of voltage for the CC generator to swallow.

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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